Resistance to Pyraclostrobin and Boscalid in Populations of Botrytis cinerea from Stored Apples in Washington State

Methionine represses gray mold of tomato by keeping nitric oxide at an appropriate level via ethylene synthesis and signal transduction

Methionine (Met) can inhibit plant diseases caused by phytopathogens. However, the effect of Met on gray mold resulted from Botrytis cinerea in tomato is still unclear. This study showed 5 mM Met alleviated disease development of gray mold, enhanced chitinase (CHI) and β-1, 3-glucanase (GNS) activities and the expression of SlCHI, SlGNS, SlPR1 and SlNPR1 in tomatoes, rather than inhibited the growth of B. cinerea directly. Moreover, ethylene biosynthesis and signal transduction before pathogen inoculating were induced by 5 mM Met. Interestingly, Met reduced the nitrosylation levels of ACS4 and ACO6, enhanced the activities of nitric oxide synthase, nitrite reductase (NR) and S-nitrosoglutathione reductase (GSNOR) and the expression of SlNR and SlGSNOR. Tomatoes treated with aminoethoxyvinylglycine and carboxy-PTIO exhibited lower resistance to B. cinerea. These results indicate 5 mM Met promoted ethylene biosynthesis and signal transduction to facilitate NO synthesis and metabolism, enhancing the resistance of tomatoes to B. cinerea.

Varied sensitivity to boscalid among different Clarireedia species causing dollar spot in turfgrass

Dollar spot, a highly destructive turfgrasses disease worldwide, is caused by multiple species within the genus Clarireedia. Previous research indicated varying sensitivity to boscalid among Clarireedia populations not historically exposed to succinate dehydrogenase inhibitors (SDHIs). This study confirms that the differential sensitivity pattern is inherent among different Clarireedia spp., utilizing a combination of phylogenetic analyses, in vitro cross-resistance assays, and genetic transformation of target genes with different mutations. Furthermore, greenhouse inoculation experiments revealed that the differential boscalid sensitivity did not lead to pathogenicity issues or fitness penalties, thereby not resulting in control failure by boscalid. This research underscores the importance of continuous monitoring of fungicide sensitivity trends and highlights the complexity of chemical control of dollar spot due to the inherent variability in fungicide sensitivity among different Clarireedia spp.

Resistance of the Ginseng Gray Mold Pathogen, Botrytis cinerea, to Boscalid and Pyraclostrobin Fungicides in China

Gray mold caused by Botrytis cinerea severely threatens the yield of ginseng (Panax ginseng). Various categories of fungicides have been utilized to control gray mold on this crop. In this study, the resistance of 102 isolates of B. cinerea from 11 commercial ginseng-growing regions in China to fungicides was examined. A total of 32.4% were resistant to boscalid, with EC50 values that ranged from 12.26 to 235.87 μg/ml, and 94.1% were resistant to pyraclostrobin, with EC50 values that ranged from 5.88 to 487.72 μg/ml. Except for sdhA and sdhD, the amino acid substitutions of P225F, P225L, N230I, H272Y, and H272R in the sdhB subunit from 24 (4 sensitive [S] and 20 resistant [R]), 5 (1 S and 4 R), 1 (S), 1 (R), and 8 (4 S and 4 R) strains, respectively, and the concurrent amino acid substitutions of G85A + I93V + M158V + V168I in the sdhC subunit from 5 (4 S and 1 R) strains were identified. A G143A substitution in cytochrome b was identified in 96 isolates that were resistant to pyraclostrobin and three that were sensitive to it. The Bcbi-143/144 intron was identified in the other three isolates sensitive to pyraclostrobin, but it was absent in the isolates that harbored the G143A mutation. The results showed that the populations of B. cinerea on ginseng have developed strong resistance to pyraclostrobin. Therefore, it is not recommended to continue using this fungicide to control gray mold on P. ginseng. Boscalid is still effective against most isolates. However, to prevent fungicide resistance, it is recommended to use a mixture of boscalid with other categories of fungicides.

Colletotrichum acutatum: Causal Agent of Olive Anthracnose Isolation, Characterization, and Fungicide Susceptibility Screening in Punjab, Pakistan

Anthracnose of olive fruit caused by Colletotrichum acutatum was a severe epidemic disease in Pakistan that occurred in September 2020. The estimated disease incident was recorded as 59%. Anthracnose causes a significant reduction in yield and quality traits. Anthracnose has been found in several orchards. Agricultural practices, environmental factors, and disease aggressiveness vary between orchards. Therefore, we looked at spore size, cultural traits, morphological variation, growth pattern, and pathogenicity of different strains of C. acutatum from various orchards. Molecular and phylogenetic analysis confirmed the isolated strains as C. acutatum. In all, 15 C. acutatum isolates from olive orchards were tested for susceptibility to four commercial fungicides (P < 0.001). The examined isolates' in vitro fungicide sensitivity varied with fungicide concentration. The concentration at which conidial germination was hindered by 50% compared with the control values was observed for difenoconazole, tebuconazole, carbendazim, and cyprodinil, ranging from 0.12 to 2.69 g ml^-1. Based on the findings of the fungal growth inhibition studies, carbendazim has been found to be the only fungicide that effectively reduces (P < 0.001) anthracnose caused by C. acutatum strains. Additionally, results revealed that preharvest site treatments of different fungicides greatly decreased anthracnose infections on olive fruit (70 to 90%), and postharvest site applications significantly reduced disease prevalence and severity (75 to 95%). The fungicide carbendazim significantly decreased pre- and postharvest anthracnose infection on olive cultivars. This study suggests that the latter compound might be used to control olive anthracnose in Pakistan while lowering environmental impact and fungicide resistance.

Baseline sensitivity and control efficacy of a new QiI fungicide, florylpicoxamid, against Botrytis cinerea

BACKGROUND

Gray mold caused by Botrytis cinerea is an airborne plant pathogen with a necrotrophic lifestyle that infects more than 200 crops worldwide. Florylpicoxamid is a second-generation picolinamide fungicide inspired by a natural product. Florylpicoxamid targets the Qi site of the mitochondrial cytochrome bc1 complex and is currently being registered in China for the control of gray mold in a variety of crops. Although a broad spectrum of activity and attributes have been reported for florylpicoxamid, little is known about its effectiveness against gray mold or its protective and curative properties.

RESULTS

Florylpicoxamid exhibited substantial inhibitory activity against 12 tested species of plant-pathogenic fungi, with effective concentration for 50% growth inhibition (EC₅₀ ) values ranging from 0.017 to 2.096 μg ml^-1 . A total of 129 isolates of B. cinerea from ten regions were tested for their sensitivity to florylpicoxamid, and the mean EC₅₀ value was 0.04 ± 0.017 μg ml^-1 . Furthermore, florylpicoxamid was observed to substantially inhibit all developmental stages of B. cinerea, with mycelial development, sclerotium germination, germ tube elongation and conidial germination being restrained with an EC₅₀ value of 0.051 ± 0.0072, 0.012 ± 0.0069, 0.019 ± 0.0041 and 0.0062 ± 0.0007 μg ml^-1 , respectively. No cross-resistance was observed between florylpicoxamid and quinone outside inhibitor (QoI), methyl benzimidazole carbamates or succinate dehydrogenase inhibitor. Florylpicoxamid also exhibited protective and curative activity against the development of B. cinerea infection in tests on tomato fruits. At application rates of 90, 112.5 and 135 g a.i. ha^-1 , florylpicoxamid was also observed to provide more-effective control than boscalid (300 g a.i. ha^-1 ).

CONCLUSION

This study demonstrated that the novel fungicide florylpicoxamid exhibits strong inhibitory activity against B. cinerea, regardless of the resistance profiles of those isolates to tested fungicides with different modes of action. This makes florylpicoxamid a powerful new solution to optimize gray mold control and manage fungicide resistance. © 2022 Society of Chemical Industry.

Synergy of cystamine and pyraclostrobin against Fusarium graminearum involves membrane permeability mitigation and autophagy enhancement

The application of fungicide mixture is one of the most important measures to extend the service life of highly selective fungicides. Pyraclostrobin (PYR), which has been extensively used to control plant diseases by inhibiting mitochondrial respiration of pathogenic fungi, is at a high risk of resistance development. In this study, the potential of PYR alone or in combination with cystamine, an inhibitor of microbial transglutaminase, to suppress Fusarium graminearum was tested in vitro and in vivo. A synergistic effect of PYR/CYS mixture was observed both in vitro and when applied to etiolated wheat coleoptile. The control effect of PYR/CYS mixture on F. graminearum was better than that of PYR alone, which was reflected by the increased protection effect. The discrepancies of membrane permeability and the redox-physiological state were observed between PYR and PYR/CYS treatments, suggesting that an increased PYR availability in F. graminearum mycelia could be related with the observed synergistic action. Moreover, a synergistic profile was observed between PYR and CYS in regard of massive autophagosomes in mycelia, indicating that enhanced autophagy could be involved in the mode of action of PYR/CYS mixture. The differential content of mitochondrial metabolites between PYR and PYR/CYS treatments also provided evidence for CYS contribution to the fungicidal action of PYR/CYS mixture. The results provide insight into the synergistic mechanism of action of PYR/CYS mixture and an effective way to enhance the efficiency of PYR to combat F. graminearum.

Resistance to pydiflumetofen in Botrytis cinerea: risk assessment and detection of point mutations in sdh genes that confer resistance

BACKGROUND

Gray mold caused by Botrytis cinerea Pers. is one of the most significant airborne diseases. It can infest a wide range of crops, causing significant losses in yield and quality worldwide. Pydiflumetofen, a new generation succinate dehydrogenase inhibitor (SDHI), is currently being registered in China to control gray mold in a variety of crops. The baseline sensitivity, resistance risk, and resistance mechanism of Botrytis cinerea to pydiflumetofen were assessed in this study.

RESULTS

A total of 138 strains of B. cinerea from 10 different regions were tested for their sensitivity to pydiflumetofen, and the mean EC₅₀ value was 0.0056 μg mL^-1 . Eight mutants were obtained by fungicide adaption from five sensitive parental isolates, and the resistance factor (RF) ranged from 51 to 135. The mutants exhibited strong adaptive traits in conidial production, conidial germination, and pathogenicity. Positive cross-resistance was only observed between other SDHIs (i.e. boscalid, fluopyram, and isopyrazam). Two different types of pydiflumetofen-resistant mutants were identified: point mutation P225L in sdhB and double mutation G85A and I93V in sdhC. The in vivo control efficacy of pydiflumetofen on the resistant mutants carrying P225L in sdhB as well as G85A and I93V in sdhC was significantly decreased to 52.62% and 32.27%, respectively.

CONCLUSION

The fitness was significantly higher for all pydiflumetofen-resistant mutants than the corresponding parental. Two types of point mutations, sdhB-P225L and sdhC-G85A and I93V, might confer resistance to pydiflumetofen in B. cinerea. A precautionary resistance management strategy should be implemented. © 2021 Society of Chemical Industry.

Suppression of Ripe Rot on 'Zesy002' Kiwifruit with Commercial Agrochemicals

Ripe rot caused by Botryosphaeria dothidea is one of the serious diseases of postharvest kiwifruit. In order to control ripe rot on Actinidia chinensis cultivar 'Zesy002', several commercial agrofungicides were selected by an antifungal test on an artificial medium. Furthermore, disease suppression by the selected fungicides was evaluated on the kiwifruit by inoculation with a conidial suspension of B. dothidea. On the artificial media containing boscalid + fludioxonil was shown to be the most effective antifungal activity. However, in the bio-test pyraclostrobin + boscalid and iminoctadine-tris were the most effective agrochemicals on the fruit. On the other hand, the infection structures of B. dothidea on kiwifruit treated with pyraclostrobin + boscalid were observed with a fluorescent microscope. Most of the fungal conidia had not germinated on the kiwifruit treated with the agrochemicals whereas on the untreated fruit the fungal conidia had mostly germinated. Electron microscopy of the fine structures showed morphological changes to the conidia and branch of hyphae on the kiwifruit pre-treated with pyraclostrobin + boscalid, indicating its suppression effect on fungal growth. Based on this observation, it is suggested that ripe rot by B. dothidea may be suppressed through the inhibition of conidial germination on the kiwifruit treated with the agrochemicals.

Advances in Understanding Fungicide Resistance in Botrytis cinerea in China

Gray mold, caused by Botrytis cinerea, is a devastating disease that causes significant yield losses in various economically important plants. Fungicide application is one of the main strategies for management of gray mold; however, B. cinerea has developed resistance to various groups of fungicide. In China, benzimidazole-, dicarboximide-, and quinone outside inhibitor-resistant populations of B. cinerea have become dominant. Substitute mutations in fungicide target genes are responsible for resistance in B. cinerea. Based on known resistance mechanisms, molecular methods including loop-mediated isothermal amplification have been developed for rapid detection of resistant isolates of B. cinerea. Because B. cinerea is able to quickly develop resistance to various fungicides, various integrated strategies have been implemented in the last decade, including biological and agricultural practices, to manage fungicide resistance in B. cinerea.

Fungicidal Effect of Pyraclostrobin against Botrytis cinerea in Relation to Its Crystal Structure

Pyraclostrobin (PYR) is a commonly used strobilurin fungicide, which inhibits mitochondrial respiration at the ubiquinol oxidation center site of the cytochrome bc1 complex. Little information is available regarding the crystal structure of PYR on its fungicidal effect. In this study, the crystal structures of eight PYRs (PYR-A to H) from different sources are determined by using high-resolution X-ray powder diffraction (XRPD) and model construction with the Pawley refinement module. The effects of PYRs on mycelium growth, the kinetics of mycelial growth, conidial germination, and tube elongation of conidia of Botrytis cinerea from tomato are compared. The level of organic acids in the mitochondrial tricarboxylic acid cycle of PYR-treated B. cinerea is analyzed. The results show that PYR-A to PYR-H have their own unique character of XRPD patterns, but the crystal morphology of eight PYRs presents in the triclinic crystal system and space group P1̅. PYR-D with the eclipsed conformation and rational edge angles α (72.599°) and β (98.612°) in the crystal cell shows the highest inhibitory effect against mycelium growth with EC₅₀ as 3.383 μg mL^-1, the best time-dependent effects on the mycelium growth kinetics, and the strongest inhibition on tube elongation of conidia, whereas PYR-E with anticonformation is the worst. Moreover, a significant accumulation of fumarate, malate, and oxalate in the PYR-D-treated mycelium is observed. These findings reinforce the need for a definite crystal structure of PYR to limit usage and mitigate future selection pressure for gray mold management.

Activity of the Novel Succinate Dehydrogenase Inhibitor Fungicide Pydiflumetofen Against SDHI-Sensitive and SDHI-Resistant Isolates of Botrytis cinerea and Efficacy Against Gray Mold

Succinate dehydrogenase inhibitor (SDHI) fungicides are currently the most frequently used fungicides for controlling gray mold. However, isolates of Botrytis cinerea resistant to SDHI fungicides have emerged in the field. Pydiflumetofen is a new SDHI fungicide that can control a variety of fungal diseases, but its efficacy against gray mold and whether the activity of pydiflumetofen is affected by the current SDHI-resistant isolates is currently unknown. The sensitivity of 291 single-spore B. cinerea isolates collected from 2017 to 2019 to pydiflumetofen was determined by spore germination inhibition assays. The mean EC₅₀ value (fungicide concentration resulting in a 50% inhibition compared with that of the control) of pydiflumetofen was 0.06 ± 0.01, 0.07 ± 0.02, and 0.05 ± 0.02 mg/liter in 2017, 2018, and 2019, respectively. There was no significant difference in the sensitivity of B. cinerea to pydiflumetofen among the 3 years. Furthermore, pydiflumetofen at 300 mg/liter effectively controlled gray mold on cucumber leaves (80.9%), and its efficacy was superior to that of boscalid at 400 mg/liter (42.7%). The isolates carrying P225F, N230I, H272Y, and H272R mutations in the SdhB subunit were associated with the less sensitivity of B. cinerea to SDHI fungicides. After establishing the baseline sensitivity of B. cinerea to pydiflumetofen (EC₅₀ of 0.03 ± 0.003 mg/liter), we found that the P225F and H272Y mutant isolates showed low to moderate levels of resistance to pydiflumetofen, and the H272R and N230I mutant isolates showed low levels of resistance. The reduced sensitivity to pydiflumetofen resulted from the positive correlation of pydiflumetofen with the other four SDHI fungicides (i.e., boscalid, fluopyram, isopyrazam, and benzovindiflupyr). These results suggest that pydiflumetofen provides effective control for the management of gray mold but must be used with caution.

Effect of Application Rate and Timing on Residual Efficacy of Pyraclostrobin in the Control of Pepper Anthracnose

Anthracnose is a devastating disease that seriously affects pepper production worldwide. Anthracnose management is currently a major problem because of the widespread and long period of infection of this disease. Therefore, determination of the optimal fungicide application timing is important for controlling anthracnose in a timely manner . In vitro sensitivity tests showed no significant difference in the pyraclostrobin sensitivity of Colletotrichum scovillei collected from 2016 and 2017, with mean half maximal effective concentration values of 0.349 to 0.542 and 0.0475 to 0.0639 mg/liter for the inhibition of mycelial growth and spore germination, respectively. Fungicide application initiated at the full-bloom stage could significantly delay anthracnose disease onset, decrease anthracnose incidence and development (23.67 to 89.80%), and increase pepper yield by 10.7 to 29.2%. In addition, the application dosage was decreased by >50%. BF-500-3, the main metabolite of pyraclostrobin, was detected in pepper fruit and exhibited high inhibitory activity against C. scovillei. The final residues of all fungicides at different application timing were below maximum residue limits. Moreover, structural equation modeling indicated that application timing plays the most important role in anthracnose disease inhibition. The tank mixtures of pyraclostrobin with tebuconazole and fludioxonil showed more satisfactory efficacy (69.87 to 78.36%) against anthracnose than did pyraclostrobin alone under field conditions. This study is the first to determine the best fungicide application timing for anthracnose management. These results establish the basis for sustainable development of the pepper industry.

Mutations in the Membrane-Anchored SdhC Subunit Affect Fitness and Sensitivity to Succinate Dehydrogenase Inhibitors in Botrytis cinerea Populations from Multiple Hosts

Succinate dehydrogenase inhibitors (SDHIs) are an essential group of fungicides for managing gray mold, caused by Botrytis cinerea, in numerous crops. Resistance to boscalid, an early-generation SDHI, is widespread worldwide and was linked to mutations in the iron-sulfur protein encoding the SdhB subunit of the SDH complex. Herein, we report on four simultaneous dependent mutations at codons 85 (G85A), 93 (I93V), 158 (M158V), and 168 (V168I) of the membrane-anchored SdhC subunit of B. cinerea. Isolates without and with mutations in SdhC were referred to as C^- and C⁺ genotypes, respectively. The C⁺ genotype was found in all the five surveyed hosts from different U.S. regions but its frequency was higher, 25 to 40%, in the tree fruit isolates compared with 12 to 25% in the small fruit populations. The four SdhC mutations were found in isolates without mutations in SdhB or with mutations known to confer resistance to the SDHIs in SdhB. However, the frequency of C⁺ isolates was significantly higher in the SdhB wild-type isolates, which suggests that SDHI sprays may have played a role in selecting for the C^- over the C⁺ genotype. Field C⁺ isolates exhibited reduced sensitivity to fluopyram and increased sensitivity to boscalid and penthiopyrad in vitro and on detached fruit. Homology modeling confirmed the positioning of the four mutations in the ubiquinone-binding pocket. The SdhC^G85A is found in the proximal ubiquinone binding site and SdhC^M158V is positioned in the iron sulfur protein interface next to the [3Fe-4S] cluster, whereas SdhC^I93V is positioned next to the heme b with vital functions in the SDH enzyme. Beside the differential sensitivity to the SDHIs, these mutations caused a significant fitness cost in the C⁺ isolates including sporulation and increased sensitivity to reactive oxygen species. The presence of Botrytis populations differentially sensitive to the SDHIs suggests increased risks for resistance development but also opens up new perspective for future gray mold management using different SDHI fungicides.

Baseline Sensitivity and Control Efficacy of Pyraclostrobin Against Botryosphaeria dothidea Isolates in China

Botryosphaeria dothidea is an important fungal pathogen that causes apple ring rot, which can significantly reduce apple yield. Fungicide applications are the main control measure of apple ring rot worldwide. Pyraclostrobin is a quinone outside inhibitor (QoI) fungicide that has yet to be registered for control of B. dothidea in China. Baseline sensitivity of B. dothidea to pyraclostrobin (EC₅₀ of mycelial growth inhibition) was assessed for 97 isolates collected in Shandong Province. The EC₅₀ values ranged from 0.7010 to 7.1378 μg/ml with the mean value of 3.0870 μg/ml and displayed a unimodal frequency distribution. After cultured on fungicide-free PDA medium or on apples for multiple generations, the B. dothidea-resistant isolates (RS^T) remained resistant to pyraclostrobin, but exhibited similar virulence as the susceptible isolates (S^T). Cross-resistance investigation revealed that pyraclostrobin was not cross-resistant to tebuconazole, flusilazole, carbendazim, and iprodione. Field evolution showed that pyraclostrobin at 200 and 250 g a.i./ha provided greater than 80% control efficacy against apple ring rot disease when applied as a therapeutic or preventive fungicide. The efficacy was similar to fungicides that have been registered for apple.

Rapid Detection of SdhB P225F and SdhB H272R Mutations in Boscalid Resistant Botrytis cinerea Strains by ARMS-PCR

SdhB ^P225Fand SdhB ^H272R mutations have been found associated with boscalid resistance in Botrytis cinerea from strawberry in Shanghai, China. For rapid detection of two mutations, tetra-primers were designed and optimized to gain the relatively high accuracy and specificity based on the ARMS-PCR technique, by which resistance can be identified with different lengths of products on agarose gels. The tetra-primer ARMS-PCR systems for SdhB ^P225F and SdhB ^H272R were validated by 9 SdhB-squenced strains repeatedly. Then, sensitivity of 30 more strains were also tested by the methods, which were accordant with genotypes by sequencing and the sensitivity of conidial germination to boscalid by 100%. Thus, the methods developed in this study are proved to be rapid, inexpensive, accurate and practical for resistance detection of Botrytis cinerea caused by SdhB ^P225F and SdhB ^H272R mutations.

Occurrence, Distribution, and Characteristics of Boscalid-Resistant Corynespora cassiicola in China

Corynespora blight, caused by Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei, has become an important disease affecting cucumber in China. Its management mainly depends on fungicides; however, no research has been conducted to assess the sensitivity of C. cassiicola in China to boscalid, a succinate dehydrogenase inhibitor (SDHI). To facilitate boscalid resistance monitoring, baseline sensitivity was established. The EC₅₀ value (i.e., the concentration that results in 50% mycelial growth inhibition) frequency distribution was unimodal with a right-hand tail; with the means 0.95 ± 0.51 μg/ml and the range 0.03 to 2.85 μg/ml. We then assessed the sensitivity of C. cassiicola to boscalid using discriminatory doses and EC₅₀ values. In total, 27.8% of the 798 isolates were resistant, distributed in five provinces and two municipalities. Thirty-seven isolates with different resistance levels to boscalid were also evaluated for their sensitivity to carboxin, fluopyram, and penthiopyrad. Seven SDHI resistance patterns were observed (i.e., I: Bos^MRFluo^MRPen^LRCar^SS; II: Bos^VHRFluo^SSPen^MRCar^R; III: Bos^LRFluo^MRPen^LRCar^R; IV: Bos^MRFluo^MRPen^MRCar^R; V: Bos^HRFluo^MRPen^HRCar^R; VI: Bos^HRFluo^HRPen^HRCar^R; and VII: Bos^HRFluo^SSPen^LR Car^R, VHR: very highly resistant; HR: highly resistant; MR: moderately resistant; LR: low resistant; R: resistant; SS: supersensitive), corresponding to seven mutations in sdhB/C/D genes, respectively.

Thiophanate-methyl resistance in Sclerotinia homoeocarpa from golf courses in China

Sclerotinia homoeocarpa causes dollar spot disease on many turfgrass species and is a significant problem worldwide. Thiophanate-methyl (TM), a methyl benzimidazole carbamate (MBC) fungicide, has been used for over forty years to manage dollar spot. Here we describe genetic mutations linked to three distinct TM fungicide resistance phenotypes: sensitive (S), moderately resistant (MR) and highly resistant (HR). These were established using multiple doses of TM, compared to previous studies using single discriminatory doses. In total, 19 S, 3 MR and 22 HR isolates were detected. Analysis of the β-tubulin gene revealed the MR isolates had a point mutation from T to A at codon 200 changing phenylalanine (TTC) to tyrosine (TAC). Twenty HR isolates had a mutation at codon 198 changing glutamic acid (GAG) to alanine (GCG) and two HR isolates had a mutation at codon 198 changing glutamic acid (GAG) to lysine (AAG). Allele-specific PCR assays were developed for rapid detection of these mutations in isolates of S. homoeocarpa. In addition, our results suggest a two-dose system for in vitro screening provides useful information for monitoring the development of resistance.

Parasitic Fitness of Fungicide-Resistant and -Sensitive Isolates of Alternaria solani

Resistance to chemistries of the succinate dehydrogenase inhibiting (SDHI) and quinone outside inhibiting (QoI) fungicides has developed rapidly in populations of Alternaria solani, the cause of early blight of potato. Reduced sensitivity to the anilinopyrimidine (AP) fungicide pyrimethanil has also been identified recently, determining that resistance to three chemical classes of fungicides is present within the A. solani population. Although no mutations have been characterized to confer resistance to APs, in A. solani five point mutations on three AsSdh genes have been determined to convey resistance to SDHIs, and the substitution of phenylalanine with leucine at position 129 (F129L) in the cytb gene confers resistance to QoIs. The objective of this study was to investigate the parasitic fitness of A. solani isolates with resistance to one or more of these chemical classes. A total of 120 A. solani isolates collected from various geographical locations around the United States were chosen for in vitro assessment, and 60 of these isolates were further evaluated in vivo. Fitness parameters measured were (i) spore germination in vitro, (ii) mycelial expansion in vitro, and (iii) aggressiveness in vivo. No significant differences in spore germination or mycelial expansion (P = 0.44 and 0.51, respectively) were observed among wild-type and fungicide-resistant isolates in vitro. Only A. solani isolates possessing the D123E mutation were shown to be significantly more aggressive in vivo (P < 0.0001) compared with wild-type isolates. These results indicate that fungicide-resistant A. solani isolates have no significant fitness penalties compared with sensitive isolates under the parameters evaluated regardless of the presence or absence of reduced sensitivity to multiple chemical classes. Results of these studies suggest that A. solani isolates with multiple fungicide resistances may compete successfully with wild-type isolates under field conditions.

Evaluation of Quinone Outside and Succinate Dehydrogenase Inhibitors for Effectiveness Against Eastern Filbert Blight of Hazelnut

Most of the hazelnut production in Oregon, a value of $130 million in 2014, was based on eastern filbert blight (EFB) susceptible cultivars. On these cultivars, EFB management involves, among other tactics, fungicide treatment during bud break and early shoot growth. Many active ingredients have been shown to be effective against EFB. This report summarizes the evaluation of quinone outside (QoI, FRAC group 11) and succinate dehydrogenase (SDHI, FRAC group 7) inhibitors alone and in combination with each other or with demethylation-inhibiting (DMI, FRAC group 3) fungicides for management of EFB. Based on a meta-analysis, picoxystrobin, pyraclostrobin, or trifloxystrobin alone resulted in significant control over nontreated trees ranging between 64 and 74%. Fluoxastrobin was not as effective as other QoI fungicides with an average of 44% control and high variability. SDHI fungicides as a group were less useful for management of EFB with boscalid, fluopyram, and penthiopyrad ineffective while fluxapyroxad averaged 83% control against EFB. Prepackaged mixes of QoI materials with either SDHI or DMI fungicides were also significantly effective against EFB. Use of QoI fungicides and the SDHI material fluxapyroxad offers added flexibility and complexity within EFB management programs. Growers can incorporate any of five different modes of action in EFB management programs including FRAC groups M1, M5, 3, 7, and 11.

Sources of Primary Inoculum of Botrytis cinerea and Their Impact on Fungicide Resistance Development in Commercial Strawberry Fields

Strawberry transplants produced in nurseries across Canada, northern United States, and California are shipped annually to other strawberry-growing regions, including Florida. Botrytis cinerea, the causal agent of gray mold, causes latent infections on transplants which are suggested as a potential source of primary inoculum in strawberry fields. In this study, we investigated the survival of B. cinerea isolates over the summer in Florida, the presence of B. cinerea in transplants from 14 nurseries from Canada and the United States in 2011, 2012, and 2013, and the sensitivity of nursery population to several botryticides. Botrytis cinerea was detected on dead strawberry plants sampled from commercial strawberry fields between March and June but not in July and August, suggesting that the fungus does not over-summer in strawberry fields in Florida. Nursery transplants surveyed in 2011, 2012, and 2013 showed B. cinerea incidences of 20 to 37, 20 to 83, and 2.5 to 92.5%, respectively. In total, 409 isolates were tested for sensitivity to pyraclostrobin, boscalid, pyrimethanil, fenhexamid, iprodione, penthiopyrad, fluopyram, and fludioxonil. Overall, respective resistance frequencies were 91.7, 79.3, 33.2, 20.7, 2.4, 0.2, 0.2, and 0.0%. A majority of isolates tested were resistant to either 3 or 4 fungicides simultaneously. These findings reinforce the need for an integrated approach between strawberry nurseries and production fields to improve gray mold management and mitigate future risks of resistance development in B. cinerea.

Resistance to the SDHI Fungicides Boscalid, Fluopyram, Fluxapyroxad, and Penthiopyrad in Botrytis cinerea from Commercial Strawberry Fields in Spain

Gray mold, caused by the necrotrophic fungus Botrytis cinerea., is one of the most economically important diseases of strawberry. Gray mold control involves the application of fungicides throughout the strawberry growing season; however, B. cinerea isolates resistant to multiple classes of site-specific fungicides have been recently reported in the Spanish gray mold population. Succinate dehydrogenase inhibitors (SDHI) constitute a relatively novel class of fungicides registered for gray mold control representing new alternatives for strawberry growers. In the present study, 37 B. cinerea isolates previously characterized for their sensitivity to boscalid and amino acid changes in the SdhB protein were used to determine the effective concentration that reduces mycelial growth by 50% (EC₅₀) to fluopyram, fluxapyroxad, and penthiopyrad. The present study was also conducted to obtain discriminatory doses to monitor SDHI fungicide resistance in 580 B. cinerea isolates collected from 27 commercial fields in Spain during 2014, 2015, and 2016. The EC₅₀ values ranged from 0.01 to >100 μg/ml for fluopyram, <0.01 to 4.19 μg/ml for fluxapyroxad, and, finally, <0.01 to 59.65 μg/ml for penthiopyrad. Based on these results, as well as findings from a previous publication, the discriminatory doses chosen to examine sensitivities to boscalid, fluopyram, fluxapyroxad, and penthiopyrad were 100, 15, 1, and 6 μg/ml, respectively. Over the course of the 3-year monitoring period, the overall frequencies of resistance to the four SDHI were 56.9, 6.9, 12.9, and 24.6%, respectively. The frequency of boscalid-resistant isolates decreased from 73 to 41% over the years; however, the fluopyram-resistant isolates increased from 5 to 10% after 1 year of registration. Four SDHI resistance patterns were observed in our population, which included patterns I (30%; resistance to boscalid), II (13.8%; resistance to boscalid and penthiopyrad), III (5.7%; boscalid, fluxapyroxad, and penthiopyrad), and IV (7.9%; resistance to boscalid, fluopyram, fluxapyroxad, and penthiopyrad). Patterns I and II were associated with the amino acid substitutions H272R and H272Y; pattern III was associated only with the H272Y mutation; and, finally, pattern IV was associated with the N230I allele in the SdhB subunit. For gray mold management, it is suggested that the simultaneous use of boscalid and penthiopyrad should be limited to one application per season; however, fluxapyroxad and, especially, fluopyram could be used as valid SDHI alternatives for gray mold control, although they should be applied with caution.

Effectiveness of Different Classes of Fungicides on Botrytis cinerea Causing Gray Mold on Fruit and Vegetables

Botrytis cinerea is a necrotrophic pathogen causing a major problem in the export and post-harvest of strawberries. Inappropriate use of fungicides leads to resistance among fungal pathogens. Therefore, it is necessary to evaluate the sensitivity of B. cinerea to various classes of fungicide and to determine the effectiveness of different concentrations of commonly used fungicides. We thus evaluated the effectiveness of six classes of fungicide in inhibiting the growth and development of this pathogen, namely, fludioxonil, iprodione, pyrimethanil, tebuconazole, fenpyrazamine, and boscalid. Fludioxonil was the most effective (EC50 < 0.1 μg/ml), and pyrimethanil was the least effective (EC50 = 50 μg/ml), at inhibiting the mycelial growth of B. cinerea. Fenpyrazamine and pyrimethanil showed relatively low effectiveness in inhibiting the germination and conidial production of B. cinerea. Our results are useful for the management of B. cinerea and as a basis for monitoring the sensitivity of B. cinerea strains to fungicides.

Detection of sdhB Gene Mutations in SDHI-Resistant Isolates of Botrytis cinerea Using High Resolution Melting (HRM) Analysis

Botrytis cinerea, is a high risk pathogen for fungicide resistance development. Pathogen' resistance to SDHIs is associated with several mutations in sdh gene. The diversity of mutations and their differential effect on cross-resistance patterns among SDHIs and the fitness of resistant strains necessitate the availability of a tool for their rapid identification. This study was initiated to develop and validate a high-resolution melting (HRM) analysis for the identification of P225H/F/L//T, N230I, and H272L/R/Y mutations. Based on the sequence of sdhB subunit of resistant and sensitive isolates, a universal primer pair was designed. The specificity of the HRM analysis primers was verified to ensure against the cross-reaction with other fungal species and its sensitivity was evaluated using concentrations of known amounts of mutant's DNA. The melting curve analysis generated nine distinct curve profiles, enabling the discrimination of all the four mutations located at codon 225, the N230I mutation, the three mutations located in codon 272, and the non-mutated isolates (isolates of wild-type sensitivity). Similar results were obtained when DNA was extracted directly from artificially inoculated strawberry fruit. The method was validated by monitoring the presence of sdhB mutations in samples of naturally infected strawberry fruits and stone fruit rootstock seedling plants showing damping-off symptoms. HRM analysis data were compared with a standard PIRA-PCR technique and an absolute agreement was observed suggesting that in both populations the H272R mutation was the predominant one, while H272Y, N230I, and P225H were detected in lower frequencies. The results of the study suggest that HRM analysis can be a useful tool for sensate, accurate, and rapid identification of several sdhB mutations in B. cinerea and it is expected to contribute in routine fungicide resistance monitoring or assessments of the effectiveness of anti-resistance strategies implemented in crops heavily treated with botryticides.

Characterization of Resistance to Six Chemical Classes of Site-Specific Fungicides Registered for Gray Mold Control on Strawberry in Spain

Botrytis cinerea, causal agent of the gray mold disease, is one of the most economically important fungal pathogens of strawberry worldwide. In Spain, as in other parts of the world, management of gray mold control primarily involves the application of fungicides. To determine the fungicide resistance of the Spanish strawberry field population, 367 B. cinerea isolates were examined from one organic and 13 conventional strawberry fields in Huelva (Spain) in 2014 and 2015. The sensitivities of these isolates to six fungicides used for gray mold management in Spain were examined using a spore germination assay based on previously published discriminatory doses. The frequency of resistance to pyraclostrobin, boscalid, cyprodinil, fenhexamid, iprodione, and fludioxonil was 74.6, 64.8, 37.0, 23.7, 14.7, and 0.8%, respectively. The majority of isolates (35.1%) were resistant to three different fungicides classes. Within these isolates, the most prevalent resistance profile (55.8%) was resistance to pyraclostrobin, boscalid, and cyprodinil, followed by the resistance profile (30.2%) of resistance to pyraclostrobin, boscalid, and fenhexamid. One isolate collected in 2015 was resistant to all six fungicide classes. Resistance to boscalid, fenhexamid, iprodione, and pyraclostrobin was found to be caused by amino acid substitutions on target proteins, including H272R/Y in SdhB, F412I/S/V in Erg27, I365 N/S in Bos1, and G143A in Cytb, respectively. The presence of multifungicide resistance phenotypes in B. cinerea isolates from strawberry fields in Spain must be considered in the development of future resistance management practices.

Fungicide Resistance Profiling in Botrytis cinerea Populations from Blueberry in California and Washington and Their Impact on Control of Gray Mold

Gray mold caused by Botrytis cinerea is a major postharvest disease of blueberry grown in the Central Valley of California and western Washington State. Sensitivities to boscalid, cyprodinil, fenhexamid, fludioxonil, and pyraclostrobin, representing five different fungicide classes, were examined for 249 (California) and 106 (Washington) B. cinerea isolates recovered from decayed blueberry fruit or flowers. In California and Washington, 7 and 17 fungicide-resistant phenotypes, respectively, were detected: 66 and 49% of the isolates were resistant to boscalid, 20 and 29% were moderately resistant to cyprodinil, 29 and 29% were resistant to fenhexamid, and 66 and 55% were resistant to pyraclostrobin. All isolates from California were sensitive to fludioxonil, whereas 70% of the isolates from Washington showed reduced sensitivity to fludioxonil. In California, 26 and 30% of the isolates were resistant to two and three classes of fungicides, respectively. In Washington, 31, 14, 16, and 9% of the isolates were resistant to two, three, four, and five classes of fungicides, respectively. Inherent risk of the development of resistance to quinone outside inhibitor (QoI) fungicides was assessed by detecting the presence of the Bcbi-143/144 intron in gene cytb. The intron was detected in 11.8 and 40% of the isolates in California and Washington, respectively, suggesting that the risk of QoI resistance is higher in California than in Washington. On detached blueberry fruit inoculated with 11 isolates exhibiting different fungicide-resistant phenotypes, most fungicides failed to control gray mold on fruit inoculated with the respective resistant phenotypes but the mixture of cyprodinil and fludioxonil was effective against all fungicide-resistant phenotypes tested. Our findings would be useful in designing and implementing fungicide resistance management spray programs for control of gray mold in blueberry.

Population Structure, Fungicide Resistance Profile, and sdhB Mutation Frequency of Botrytis cinerea from Strawberry and Greenhouse-Grown Tomato in Greece

Botrytis cinerea is a pathogen with high genetic variability that has also shown high risk for fungicide resistance development. In total, 1,169 isolates obtained from strawberry (n = 297) and tomato (n = 872) in five geographic regions of Greece were tested for their sensitivity to several botryticides. A high frequency of isolates with multiple resistance to carbendazim, cyprodinil, pyraclostrobin, and boscalid was found in isolates from strawberry. In the isolates from tomato, the predominant phenotype was that of dual resistance to carbendazim and cyprodinil in the Crete island, of single resistance to carbendazim in the region of Preveza, and of sensitive isolates in the region of Kyparissia. None of the tested isolates was found to be fludioxonil resistant. High frequencies of boscalid-resistant phenotypes were observed in the strawberry isolates, while boscalid-resistance frequency in the tomato isolates was lower. H272R was the predominant sdhB mutation, associated with resistance to boscalid, in all the sampled isolates, while other sdhB mutations were found at low frequencies. B. cinerea group S, identified by the presence of a 21-bp insertion in the transcription factor mrr1 gene, was predominant within the tomato isolates obtained from all three sampled regions, with frequencies ranging from 62 to 75% of the isolates; whereas, within strawberry isolates, B. cinerea was predominant, with frequencies of 75 to 95%. Correlations of isolate genotype and fungicide resistance profile showed that B. cinerea sensu stricto isolates were more prone to the development of resistance to boscalid compared with the Botrytis group S isolates, which may explain the observed predominance of B. cinerea sensu stricto in strawberry fields.

Molecular characterisation and detection of resistance to succinate dehydrogenase inhibitor fungicides in Botryotinia fuckeliana (Botrytis cinerea)

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs), interfering with fungal respiration, are considered to be fungicides at medium to high risk of resistance. Boscalid was the first molecule belonging to the SDHIs that was introduced for the control of Botryotinia fuckeliana. A range of different target-site mutations leading to boscalid resistance have been found in field populations of the fungus. The different types of mutation confer different cross-resistance profiles towards novel SDHIs, such as the recently introduced fungicide fluopyram. This study combines the determination of cross-resistance profiles and the setting-up of methods for fast molecular detection of the mutations.

RESULTS

By means of in vitro tests, a range of SdhB mutations were characterised for resistance levels towards boscalid and fluopyram. SdhB mutations conferring P225L and P225F substitutions conferred high resistance to boscalid and high or moderate resistance to fluopyram respectively. Mutants carrying the N230I replacement were moderately resistant to both SDHIs. Substitutions at position H272 responsible for a high level of resistance to boscalid conferred sensitivity (H272R), hypersensitivity (H272Y) or moderate resistance (H272V) to fluopyram. Allele-specific (AS) PCR was developed and used for genotyping 135 B. fuckeliana isolates. The assay confirmed the strict association between resistance profiles and allelic variants of the SdhB gene. Real-time AS-PCR proved to be sensitive and specific for quantitative detection of different SDHI-resistant genotypes.

CONCLUSION

Fluopyram-resistant mutants are currently rarely detected in the field sprayed with boscalid, but this may change with intensive exposure of the fungal population to fluopyram. PCR assays/methods developed in the study provide tools for fast monitoring of field populations and observing possible changes in population composition following fluopyram introduction, useful for the setting-up of appropriate preventive measures.

Fungicide Resistance of Two Species of Alternaria from Potato in the Columbia Basin of Washington

Fungicides are extensively used in the Columbia Basin of Washington in an effort to manage early blight and brown spot on potato caused by Alternaria solani and A. alternata, respectively. Azoxystrobin and boscalid interfere with cellular respiration, and fungicide resistance is expected to occur in fungal populations due to the narrow modes of action dictated by each fungicide. The objective of this research was to assess A. solani and A. alternata populations in the Columbia Basin of Washington for azoxystrobin and boscalid resistance by initially screening isolates at a threshold concentration using mycelial growth and spore germination. Fifty isolates of A. solani and 58 of A. alternata were randomly selected for screening from 671 isolates collected from potato in the Columbia Basin from 2009 to 2011. Screening at a threshold concentration was done to estimate the range of fungicide resistance within the populations of A. solani and A. alternata in the region. Resistance of five isolates that differed in sensitivity to azoxystrobin and boscalid were then assessed for mycelial growth and spore germination at three fungicide concentrations in order to determine where rates were no longer effective. A. solani mycelial growth ratios and percent spore germination on azoxystrobin-amended media did not change from 2010 to 2011. A slight decrease in resistance of mycelial growth to azoxystrobin was exhibited among A. alternata isolates collected between 2010 and 2011. No change in sensitivity to azoxystrobin was observed in mycelial growth ratios among A. alternata isolates collected between 2010 and 2011. Resistance to boscalid by A. solani increased as indicated by a significant increase in mean mycelial growth ratio from 2010 to 2011, whereas A. alternata showed no change. Spore germination of A. solani was less at 100 μg/ml than at 1 or 10 μg/ml azoxystrobin. Spore germination of A. alternata did not differ among 1, 10, or 100 μg/ml azoxystrobin. Spore germination of A. solani and A. alternata did not differ among 0.5, 5, or 50 μg/ml boscalid. Resistance to azoxystrobin and boscalid in A. solani and A. alternata populations was shown to be present in the Columbia Basin and will likely become widespread with continued used of site-specific fungicides.

Fungicide resistance of Botrytis cinerea in tomato greenhouses in the Canary Islands and effectiveness of non-chemical treatments against gray mold

Tomato greenhouses in the Canary Islands, Spain, were surveyed to estimate frequencies of resistance to benzimidazoles, dicarboximides, anilinopyrimidines and N-phenylcarbamates in Botrytis cinerea. Resistance to carbendazim, iprodione, pyrimethanil and diethofencarb was found in 74.2, 86.4, 28.8 and 31.8% of isolates, respectively. Benzimidazole- and anilinopyrimide-resistant isolates were highly resistant, showing EC50 values above 500 µg/ml carbendazim and a mean EC50 value of 28.42 µg/ml pyrimethanil, respectively. By contrast, a low level of resistance was observed among dicarboximide-resistant isolates (mean EC50 value of 1.81 µg/ml iprodione). Phenotypes with double resistance to carbendazim and iprodione, and triple resistance to carbendazim, iprodione and pyrimethanil were the most common, occurring in 36.4 and 28.8% of isolates. The surveyed greenhouses had never been treated with fenhexamid and Signum™ (pre-packed mixture of boscalid and pyraclostrobin), and baseline sensitivities of B. cinerea isolates to these fungicides were determined. The EC50 values were within the range of 0.009-0.795 µg/ml fenhexamid and of 0.014-0.48 µg/ml Signum. In addition, available formulations based on elicitors of plant defense response and biocontrol agents were evaluated against B. cinerea in tomato plants under semi-controlled greenhouse conditions, the yeast Candida sake CPA-1 being able to reduce gray mold significantly when it was applied on petiole wounds and the plants were inoculated 24 h later. Likewise, C. sake was effective against B. cinerea in harvested tomato fruits, yeast-treated tomatoes showed a 70.66 and 30.31% reduction in the diameters of decay lesions compared with controls after 10 days of storage at 20 and 9 °C, respectively.

Fitness and competitive ability of Botrytis cinerea field isolates with dual resistance to SDHI and QoI fungicides, associated with several sdhB and the cytb G143A mutations

Respiration inhibitors such as the succinate dehydrogenase inhibitors (SDHIs) and the quinone outside inhibitors (QoIs) are fungicide classes with increasing relevance in gray mold control. However, recent studies have shown that dual resistance to both fungicide classes is a common trait in Botrytis cinerea populations from several hosts throughout the world. Resistance of B. cinerea to SDHIs is associated with several mutations in the sdhB, sdhC, and sdhD genes, while resistance to QoIs, in most cases, is associated with the G143A mutation in the cytb gene. The objective of the current study was to investigate the fitness and the competitive ability of B. cinerea field strains possessing one of the H272Y/R/L, N230I, or P225F sdhB substitutions and the G143A mutation of cytb. Fitness parameters measured were (i) mycelial growth and conidia germination in vitro, (ii) aggressiveness and sporulation capacity in vivo, (iii) sclerotia production in vitro and sclerotia viability under different storage conditions, and (iv) sensitivity to oxidative stress imposed by diquat treatments. The competitive ability of the resistant isolates was measured in the absence and presence of the SDHI fungicides boscalid and fluopyram selection pressure. The measurements of individual fitness components showed that the H272R/G143A isolates had the lower differences compared with the sensitive isolates. In contrast, the groups of H272Y/L/G143A, N230I/G143A, and P225F/G143A isolates showed reduced fitness values compared with the sensitive isolates. Isolates possessing only the cytb G143A substitution did not show any fitness cost. The competition experiments showed that, in the absence of fungicide selection pressure, after four disease cycles on apple fruit, the sensitive isolates dominated in the population in all the mixtures tested. In contrast, when the competition experiment was conducted under the selection pressure of boscalid, a gradual decrease in the frequency of sensitive isolates was observed, whereas the frequency of H272L and P225F isolates was increased. When the competition experiment was conducted in the presence of fluopyram, the sensitive isolates were eliminated even after the first disease cycle and the P225F mutants dominated in the population. Such results suggest that the sdhB mutations may have adverse effects on the mutants. The observed dominance of sensitive isolates in the competition experiments conducted in the absence of fungicides suggest that the application of SDHIs in alternation schemes may delay the selection or reduce the frequency of SDHI-resistant mutants.

Resistance to Fluopyram, Fluxapyroxad, and Penthiopyrad in Botrytis cinerea from Strawberry

Succinate dehydrogenase inhibitors (SDHIs) constitute a mainstay in management of gray mold caused by Botrytis cinerea in strawberry and several other crops. In this study, we investigated the risks of resistance development to three newer SDHIs (i.e., fluopyram, fluxapyroxad, and penthiopyrad) and their cross-resistance with the previously registered boscalid. We investigated the mutations in the SdhB subunit and evaluated their impact on microbial fitness in field populations of B. cinerea. Amino acid substitutions associated with resistance to SDHIs were detected at three codons of the SdhB subunit (B^H272R/Y/L, B^P225F, and B^N230I) in the succinate dehydrogenase gene of field isolates from Florida. The B^H272R, B^H272Y, B^H272L, B^P225F, and B^N230I mutations were detected at frequencies of 51.5, 28.0, 0.5, 2.5, and 4%, respectively. Strong cross-resistance patterns were evident between boscalid and fluxapyroxad and penthiopyrad but not with fluopyram, except in B^H272L, B^P225F, and B^N230I mutants. All five mutations conferred moderate to very high resistance to boscalid whereas the B^H272Y conferred resistance to fluxapyroxad and penthiopyrad. The B^H272L, B^N230I, and B^P225F mutations conferred high resistance to all four SDHIs tested. Resistance monitoring following the first use of penthiopyrad in strawberry fields in Florida in 2013 suggests potential for quick selection for highly resistant populations and warrants careful use of the newer SDHIs. No evidence of major fitness costs due to the mutations in the SdhB subunit was found, which indicates the potential ability of the mutants to survive and compete with wild-type isolates. Our study suggests high risks for rapid widespread occurrence of B. cinerea populations resistant to the novel SDHIs unless appropriate rotation strategies are implemented immediately upon registration.

Phenotypic Characterization of Multifungicide Resistance in Botrytis cinerea Isolates from Strawberry Fields in Florida

Chemical control has always been essential for the management of gray mold, caused by Botrytis cinerea, to ensure sustainable strawberry production. However, lack of knowledge about actual resistance development may have disastrous consequences and lead to severe epidemics such as the one that affected several strawberry fields in 2012 in Florida. In this study, we tested 392 isolates collected from Florida strawberry fields between 2010 and 2012 for their sensitivity to boscalid (Bosc), a succinate dehydrogenase inhibitor (SdhI); pyraclostrobin, a quinone outside inhibitor (QoI); boscalid + pyraclostrobin (Pristine); fenhexamid, a hydroxyanilide (Hyd); pyrimethanil and cyprodinil, anilinopyrimidines; fludioxonil, a phenylpyrrole; and fludioxonil + cyprodinil (Switch). The respective resistance frequencies for boscalid, pyraclostrobin, Pristine, fenhexamid, cyprodinil, and pyrimethanil were 85.4, 86.5, 86.0, 44.4, 52.7, and 59.5%. Overall, 17.8 and 19.8% of isolates showed reduced sensitivity to fludioxonil and Switch, respectively. All fungicides sprayed preventively on detached strawberry fruit failed to control isolates with high levels of resistance to each fungicide except for fludioxonil and Switch. Four phenotypes with multifungicide resistance (MFR) were detected in B. cinerea populations from Florida. Isolates resistant to one fungicide (FR1), two (MFR2), three (MFR3), and four (MFR4) fungicides from different chemical groups represented 5.9, 28.6, 41.8, and 23.7% of the total resistant population, respectively. The MFR3 isolates were predominant and contained two subpopulations, the Bosc-QoI-AP^R isolates (56.5%) and the Bosc-QoI-Hyd^R isolates (40.6%). In addition to reporting on very highly resistant populations to boscalid and QoI fungicides, we show evidence for a widespread multifungicide resistance to B. cinerea that warrants immediate implementation of novel management strategies to impede the development of more resistant populations.

Resistance to Pyraclostrobin and Boscalid in Botrytis cinerea Isolates from Strawberry Fields in the Carolinas

Botrytis cinerea, the causal agent of gray mold disease, is one of the most important plant-pathogenic fungi affecting strawberry. During the last decade, control of gray mold disease in the southeastern United States has largely been dependent on captan and the use of at-risk fungicides with single-site modes of action, including a combination of the quinone outside inhibitor (QoI) fungicide pyraclostrobin and succinate dehydrogenase inhibitor (SDHI) fungicide boscalid formulated as Pristine 38WG. Reports about loss of efficacy of Pristine in experimental fields in North Carolina prompted us to collect and examine 216 single-spore isolates from 10 conventional fields and 1 organic field in North Carolina and South Carolina in early summer 2011. Sensitivity to pyraclostrobin or boscalid was determined using a conidial germination assay with previously published discriminatory doses. Pyraclostrobin- and pyraclostrobin+boscalid-resistant isolates were found in all conventional fields (with some populations revealing no sensitive isolates) and in the organic field. Among the isolates collected, 66.7% were resistant to pyraclostrobin and 61.5% were resistant to both pyraclostrobin and boscalid. No isolates were identified that were resistant to boscalid but sensitive to pyraclostrobin, indicating that dual resistance may have derived from a QoI-resistant population. The molecular basis of QoI and SDHI fungicide resistance was determined in a subset of isolates. Polymerase chain reaction-restriction fragment length polymorphism analysis of the partial cytochrome b (CYTB) gene showed that pyraclostrobin-resistant isolates possessed the G143A mutation known to confer high levels of QoI fungicide resistance in fungi. Boscalid-resistant isolates revealed point mutations at codon 272 leading to the substitution of histidine to arginine (H272R) or tyrosine (H272Y), affecting the third Fe-S cluster region of the iron-sulfur protein (SdhB) target of SDHIs. The results of the study show that resistance to QoI fungicides and dual resistance to QoI and SDHI fungicides is common in B. cinerea from strawberry fields in the Carolinas. Resistant strains were more frequent in locations heavily sprayed with QoI and SDHI fungicides. However, resistance to both fungicides was also found in the unsprayed, organic field, indicating that some resistant strains may have been introduced from the nursery.

Biological activity of the succinate dehydrogenase inhibitor fluopyram against Botrytis cinerea and fungal baseline sensitivity

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) constitute a fungicide class with increasing relevance in crop protection. These fungicides could play a crucial role in successful management of grey mould disease. In the present study the effect of fluopyram, a novel SDHI fungicide, on several developmental stages of Botrytis cinerea was determined in vitro, and the protective and curative activity against the pathogen was determined on strawberry fruit. Furthermore, fungal baseline sensitivity was determined in a set of 192 pathogen isolates.

RESULTS

Inhibition of germ tube elongation was found to be the most sensitive growth stage affected by fluopyram, while mycelial growth was found to be the least sensitive growth stage. Fluopyram provided excellent protective activity against B. cinerea when applied at 100 µg mL(-1) 96, 48 or 24 h before the artificial inoculation of the strawberry fruit. Similarly, fluopyram showed a high curative activity when it was applied at 100 µg mL(-1) 24 h post-inoculation, but, when applications were conducted 48 or 96 h post-inoculation, disease control efficacy was modest or low. The measurement of baseline sensitivity showed that it was unimodal in all the populations tested. The individual EC(50) values for fluopyram ranged from 0.03 to 0.29 µg mL(-1). In addition, no correlation was found between sensitivity to fluopyram and sensitivity to other fungicides, including cyprodinil, fenhexamid, fludioxonil, iprodione, boscalid and pyraclostrobin.

CONCLUSIONS

The obtained biological activity, baseline sensitivity and cross-resistance relationship data suggest that fluopyram could play a key role in grey mould management in the near future and encourage its introduction into spray programmes.

Molecular characterization of pyraclostrobin resistance and structural diversity of the cytochrome b gene in Botrytis cinerea from apple

Botrytis cinerea isolates obtained from apple orchards were screened for resistance to the quinone outside inhibitor (QoI) pyraclostrobin. Of the 220 isolates tested, 43 (19.5%) were resistant to pyraclostrobin. Analysis of partial sequences of the cytochrome b gene (cyt b) in five pyraclostrobin-resistant (PR) and five pyraclostrobin-sensitive (PS) isolates showed that PR isolates harbored the point mutation leading to the substitution of glycine by alanine at codon position 143 in cyt b (G143A). Two pairs of allele-specific primers were designed based on this point mutation, and allele-specific polymerase chain reaction analysis with these primers showed that all 73 PR isolates (including 30 collected from decayed apple fruit) harbored the G143A mutation but PS isolates did not. Six pairs of primers were designed to analyze the presence of various introns in cyt b. There were six types (I to VI) of cyt b present in 247 isolates of B. cinerea collected from various apple-production areas in Washington State. Of the 247 isolates, 23 had type I cyt b containing all four introns (Bcbi-67/68, Bcbi-131/132, Bcbi-143/144, and Bcbi-164), 176 had type II cyt b containing three introns (Bcbi-67/68, Bcbi-131/132, and Bcbi-164), six had type III cyt b containing two introns (Bcbi-67/68 and Bcbi-131/132), one had type IV cyt b containing two introns (Bcbi-131/132 and Bcbi-164), one had type V cyt b containing only the Bcbi-131/132 intron, and 40 had type VI cyt b containing no introns. This is the first report of types III to VI cyt b present in B. cinerea. All 73 PR isolates did not carry the Bcbi-143/144 intron in cyt b. Of the 247 isolates tested, >90% did not carry the Bcbi-143/144 intron in cyt b, suggesting that B. cinerea populations from apple pose a high inherent risk for the development of resistance to QoIs because the presence of this intron in cyt b prevents the occurrence of G143A-mediated resistance. Analysis of genetic background based on three microsatellite primers showed that PR isolates originated from different lineages, and there was no correlation between cyt b types (I, II, and III) and the genetic background of the isolates; however, isolates carrying type VI cyt b might originate from the same lineage.

Stability and fitness of pyraclostrobin- and boscalid-resistant phenotypes in field isolates of Botrytis cinerea from apple

Phenotype stability, fitness, and competitive ability of pyraclostrobin- and boscalid-resistant isolates of Botrytis cinerea from apple were investigated. Stability of resistance was determined after consecutive transfers on potato dextrose agar (PDA) or being cycled on apple fruit. In vitro fitness components mycelial growth, osmotic sensitivity, conidial germination, and sporulation were evaluated on agar media. Pathogenicity, virulence and sporulation on apple fruit were evaluated at both 20 and 0°C. Competition between fungicide-resistant and -sensitive isolates on apple fruit also was evaluated. Resistance to the two fungicides was retained at levels similar to that of the initial generation after 20 and 10 transfers on PDA and five and three disease cycles on apple fruit at 20 and 0°C, respectively. Great variability in individual fitness components tested was observed among isolates within the same phenotype groups either sensitive or resistant to the fungicides but, when compared as phenotype groups, there were no significant differences in the mean values of these fitness components between resistant and sensitive phenotypes except that the phenotype resistant only to boscalid produced fewer conidia in vitro than sensitive isolates. Resistant isolates were as pathogenic and virulent on apple fruit as sensitive isolates. There was no significant correlation between the values of individual fitness components tested and the level of resistance to pyraclostrobin or boscalid, except that virulence at 20°C positively correlated with the level of resistance to the two fungicides. The final frequency of pyraclostrobin-resistant individuals in the populations was significantly decreased compared with the initial generation and no boscalid-resistant individuals were detected after four disease cycles on apple fruit inoculated with a pair mixture of a dual-sensitive isolate and one isolate each of the three phenotypes resistant to pyraclostrobin, boscalid, or both. The results suggest that resistance of B. cinerea to pyraclostrobin and boscalid was stable in the absence of the fungicides and that resistance to the two fungicides did not significantly impair individual fitness components tested. However, both pyraclostrobin- and boscalid-resistant isolates exhibited competitive disadvantage over the dual-sensitive isolate on apple fruit.

Detection and Molecular Characterization of Boscalid-Resistant Botrytis cinerea Isolates from Strawberry

Botrytis cinerea isolates (n = 122) were collected from strawberry fields located in northern Greece during a 3-year period (2008-10) and tested for their sensitivity to the succinate dehydrogenase inhibitor boscalid. Sensitivity measurements showed three distinct phenotypes consisting of isolates highly sensitive (fungicide concentration causing inhibition of germ tube growth by 50% [EC₅₀ values] of 0.05 to 0.21 μg ml^-1), moderately resistant (EC₅₀ values of 1.37 to 7.79 μg ml^-1), or highly resistant (EC₅₀ values of >50 μg ml^-1) to boscalid. Sequence analysis of the sdhB gene revealed five mutations leading to amino acid substitutions in the SdhB subunit in isolates moderately resistant and highly resistant to boscalid. Three moderately resistant isolates showed a nucleotide change from A to T at codon 230, resulting in an asparagine to isoleucine (N230I) substitution. Several moderately resistant isolates showed a nucleotide change from C to T at codon 272, resulting in a substitution from histidine to arginine (H272R) whereas, in another set of isolates, a nucleotide change from A to G was found at the same codon, leading to a substitution from histidine to tyrosine (H272Y). One highly resistant isolate had a nucleotide change from A to T at codon 272, leading to a substitution from histidine to leucine (H272L), whereas, in three other highly resistant isolates, a double nucleotide change from CC to TT was observed at codon 225, resulting in a substitution from proline to phenylalanine (P225F). To facilitate rapid detection of these mutations associated with resistance to boscalid, a primer-introduced restriction analysis polymerase chain reaction was developed. The method was successfully applied to the moderately and highly resistant subpopulations and showed that the H272R mutation was predominant with relative frequencies of 28.5, 37.5, and 30% during 2008, 2009, and 2010, respectively. In contrast, the H272L mutation was detected at a frequency of 2.5% only in the 2009 population, whereas the P225F mutation was detected at a frequency of 7.5% only in the 2010 population.

Resistance of Botrytis cinerea to Multiple Fungicides in Northern German Small-Fruit Production

During the vegetation period 2010, 353 isolates of Botrytis cinerea from 23 Northern German strawberry, raspberry, highbush blueberry, and redcurrant fields were examined for sensitivity to the benzimidazole derivative thiophanate-methyl and the dicarboximide iprodione, as well as five fungicides currently used against gray mold in Germany. Of all isolates, 40.5% were highly resistant to thiophanate-methyl, 64.0% to iprodione, 45.0% to fenhexamid, 76.8% to trifloxystrobin, 21.5% to boscalid, and 14.7% to cyprodinil. No high resistance to fludioxonil was observed but medium resistance was recorded to fludioxonil as well as cyprodinil (41.1 and 27.2% of all isolates, respectively). In all, 63 isolates were sensitive to all five of the currently registered botryticides whereas 43, 81, 94, 49, and 23 isolates were medium or highly resistant to one, two, three, four, and five fungicides, respectively. Isolates resistant to five fungicides in vitro were capable of causing fruit rot on wounded apple pretreated with any one of the three commercially available products containing fenhexamid, pyraclostrobin plus boscalid, or cyprodinil plus fludioxonil. These results question the sustainability of the current gray mold control strategy relying exclusively on fungicides with specific, single-site modes of action.

Evaluation of the incidence of the G143A mutation and cytb intron presence in the cytochrome bc-1 gene conferring QoI resistance in Botrytis cinerea populations from several hosts

BACKGROUND

Previous studies have shown that resistance of Botrytis cinerea to QoI fungicides has been attributed to the G143A mutation in the cytochrome b (cytb) gene, while, in a part of the fungal population, an intron has been detected at codon 143 of the gene, preventing QoI resistance. During 2005-2009, 304 grey mould isolates were collected from strawberry, tomato, grape, kiwifruit, cucumber and apple in Greece and screened for resistance to pyraclostrobin and for the presence of the cytb intron, using a novel real-time TaqMan PCR assay developed in the present study.

RESULTS

QoI-resistant phenotypes existed only within the population collected from strawberries. All resistant isolates possessed the G143A mutation. Differences were observed in the genotypic structure of cytb. Individuals possessing the intron were found at high incidence in apple fruit and greenhouse-grown tomato and cucumber populations, whereas in the strawberry population the intron frequency was lower. Cultivation of QoI-resistant and QoI-sensitive isolates for ten culture cycles on artificial nutrient medium in the presence or absence of fungicide selection showed that QoI resistance was stable.

CONCLUSIONS

The results of the study suggest that a high risk for selection of QoI-resistant strains exists in crops heavily treated with QoIs, in spite of the widespread occurrence of the cytb intron in B. cinerea populations. The developed real-time TaqMan PCR constitutes a powerful tool to streamline detection of the mutation by reducing pre- and post-amplification manipulations, and can be used for rapid screening and quantification of QoI resistance.

Molecular characterization of boscalid resistance in field isolates of Botrytis cinerea from apple

Botrytis cinerea isolates obtained from apple orchards were screened for resistance to boscalid. Boscalid-resistant (BosR) isolates were classified into four phenotypes based on the levels of the concentration that inhibited fungal growth by 50% relative to control. Of the 220 isolates tested, 42 were resistant to boscalid, with resistant phenotypes ranging from low to very high resistance. There was cross resistance between boscalid and carboxin. Analysis of partial sequences of the iron-sulfur subunit of succinate dehydrogenase gene in B. cinerea (BcSdhB) from 13 BosR and 9 boscalid-sensitive (BosS) isolates showed that point mutations in BcSdhB leading to amino acid substitutions at the codon position 272 from histidine to either tyrosine (H272Y) or arginine (H272R) were correlated with boscalid resistance. Allele-specific polymerase chain reaction (PCR) analysis of 66 BosR isolates (including 24 additional isolates obtained from decayed apple fruit) showed that 19 carried the point mutation H272Y and 46 had the point mutation H272R, but 1 BosR isolate gave no amplification product. Analysis of the BcSdhB sequence of this isolate revealed a different point mutation at codon 225, resulting in a substitution of proline (P) by phenylalanine (F) (P225F). The results indicated that H272R/Y in BcSdhB were the dominant genotypes of mutants in field BosR isolates from apple. A multiplex allele-specific PCR assay was developed to detect point mutations H272R/Y in a single PCR amplification. Levels of boscalid resistance ranged from low to very high within isolates carrying either the H272R or H272Y mutation, indicating that, among BosR isolates, different BosR phenotypes (levels of resistance) were not associated with particular types of point mutations (H272R versus H272Y) in BcSdhB. Analysis of genetic relationships between 39 BosR and 56 BosS isolates based on three microsatellite markers showed that 39 BosR isolates and 30 BosS isolates were clustered into two groups, and the third group consisted of only BosS isolates, suggesting that the development of resistance to boscalid in B. cinerea likely is not totally random, and resistant populations may come from specific genetic groups.