Distribution and Fungicide Sensitivity of Colletotrichum Species Complexes from Rubber Tree in Hainan, China

Identifying and Controlling Anthracnose Caused by Colletotrichum Taxa of Welsh Onion in Sanxing, Taiwan

Leaves of Welsh onion (Allium fistulosum) are subject to various fungal diseases such as anthracnose (Colletotrichum species) and Stemphylium leaf blight (Stemphylium vesicarium). These diseases are the main biotic limitations to Welsh onion production in northern Taiwan. From 2018 to 2020, anthracnose symptoms were observed throughout Welsh onion fields in northern Taiwan, mainly the Sanxing area. In total, 33 strains of Colletotrichum species were isolated from diseased leaves, and major causative agents were identified based on a multilocus phylogenetic analysis using four genomic regions (act, tub2, gapdh, and internal transcribed spacer). Based on this phylogeny, Colletotrichum species causing anthracnose of Welsh onion were identified as C. spaethianum (C. spaethianum species complex) and C. circinans (C. dematium species complex) in the Sanxing area, northern Taiwan. To determine and compare the pathogenicity of each species, representative fungal strains of each species were inoculated on the cultivar 'Siao-Lyu' by spraying spore suspension onto the leaf surface. Welsh onion plants were susceptible to both species, but disease incidence and severity were higher in C. spaethianum. In total, 31 fungicides were tested to determine their efficacy in reducing mycelial growth and conidial germination of representative strains of C. spaethianum and C. circinans under laboratory conditions. Five fungicides-fluazinam, metiram, mancozeb, thiram, and dithianon-effectively reduced mycelial growth and spore germination in both C. spaethianum and C. circinans. In contrast, difenoconazole and trifloxystrobin + tebuconazole, which are commonly used in Welsh onion production in northern Taiwan, mainly the Sanxing area, were ineffective. These results serve as valuable insights for growers, enabling them to identify and address the emergence of anthracnose caused by C. spaethianum and C. circinans of Welsh onion, employing fungicides with diverse modes of action. The findings of this study support sustainable management of anthracnose in Sanxing, northern Taiwan, although further field tests of the fungicides are warranted.

Polymorphisms at amino acid positions 85 and 86 in succinate dehydrogenase subunit C of Colletotrichum siamense: Implications for fitness and intrinsic sensitivity to SDHI fungicides

Among succinate dehydrogenase inhibiter (SDHI) fungicides, penthiopyrad and benzovindiflupyr particularly inhibit Colletotrichum. Studying SDH amino acid polymorphism in Colletotrichum, along with its fungicide binding sites, is key to understanding their mechanisms of action. This study explores the SDH amino acid polymorphisms in Colletotrichum siamense strains from rubber trees in China and their interaction with SDHI fungicides, specifically penthiopyrad and benzovindiflupyr. Sequencing revealed most polymorphisms were in the SDHC subunit, particularly at positions 85 and 86, which are key to penthiopyrad resistance. Among 33 isolates, 33.3 % exhibited a substitution at position 85, and 9 % at position 86. A strain with W85L and T86N substitutions in SDHC showed reduced SDH activity, ATP content, mycelial growth, and virulence, and decreased sensitivity to penthiopyrad but not benzovindiflupyr. Molecular docking with Alphafold2 modeling suggested distinct binding modes of the two fungicides to C. siamense SDH. These findings underscore the importance of SDHC polymorphisms in C. siamense's fitness and sensitivity to SDHIs, enhancing our understanding of pathogen-SDHI interactions and aiding the development of novel SDHI fungicides.

Whole genome resequencing reveal patterns of genetic variation within Colletotrichum acutatum species complex from rubber trees in China

The Colletotrichum acutatum species complex possesses a diverse number of important traits, such as a wide host range and host preference, different modes of reproduction, and different strategies of host infection. Research using comparative genomics has attempted to find correlations between these traits. Here, we used multi-locus techniques and gene genealogical concordance analysis to investigate the phylogenetic relationships and taxonomic status of the Colletotrichum acutatum species complex using field isolates obtained from rubber trees. The results revealed that the dominant species was C. australisinense, followed by C. bannaense, while strain YNJH17109 was identified as C. laticiphilum. The taxonomic status of strains YNLC510 and YNLC511 was undetermined. Using whole-genome single nucleotide polymorphism data to analyze population structure, 18 strains of C. australisinense were subsequently divided into four populations, one of which was derived from an admixture of two populations. In addition, the strains LD1687, GD1628, and YNLC516, did not belong to any populations, and were considered to be admixtures of two or more populations. A split decomposition network analysis also provided evidence for genetic recombination within Colletotrichum acutatum species complex from rubber trees in China. Overall, a weak phylogeographic sub-structure was observed. Analysis also revealed significant differences in morphological characters and levels of virulence between populations.

The Colletotrichum siamense Hydrophobin CsHydr1 Interacts with the Lipid Droplet-Coating Protein CsCap20 and Regulates Lipid Metabolism and Virulence

Previous studies of the lipid droplet-coating protein Cap20 in Colletotrichum show that it plays a key role in appressorium development and virulence. In this study, the hydrophobin CsHydr1, which contains a signal peptide of 19 amino acids and a hydrophobic domain (HYDRO), was shown to interact with CsCap20 in Colletotrichum siamense. The CsHydr1 deletion mutant showed slightly enhanced mycelial growth, small conidia, slow spore germination and appressoria formation, cell wall integrity and virulence. Like CsCAP20, CsHydr1 is also localized on the lipid droplet surface of C. siamense. However, when CsCap20 was absent, some CsHydr1 was observed in other parts. Quantitative lipid determination showed that the absence of either CsHydr1 or CsCap20 reduced the content of lipids in mycelia and conidia, while the effect of CsCap20 was more obvious; these results suggest that an interaction protein CsHydr1 of CsCap20 is localized on the lipid droplet surface and involved in lipid metabolism, which affects appressorium formation and virulence in C. siamense.

Characterization of Colletotrichum Isolates Causing Colletotrichum Dieback of Citrus in California

Dieback caused by Colletotrichum spp. is an emerging disease in California citrus groves. A large-scale survey with emphasis on seasonal variations of latent infections was conducted throughout citrus orchards in Fresno, Kern, and Tulare counties in 2019 and 2020. Latent infections on citrus leaves and twigs varied markedly between years. Isolates of Colletotrichum spp. were obtained from asymptomatic tissue, and two groups were formed based on colony and spore morphology. The morphological groups were further identified based on multigene sequence analysis using the DNA regions ITS1-5.8S-ITS2, TUB2, and GAPDH. Results revealed that isolates belong to two phylogenetic species, C. gloeosporioides and C. karstii, being C. karstii more frequently isolated. Representative isolates of each species were further selected and characterized based on the response of physiological variables to temperature. Both species had similar optimum growth temperatures but differed in maximum growth rates, with C. gloeosporioides exhibiting a greater growth rate than that of C. karstii on media. Pathogenicity tests on citrus trees demonstrated the ability of C. gloeosporioides and C. karstii to cause lesions on twigs and no differences in aggressiveness. A fungicide screening performed in this study determined that the DMI fungicides were the most effective in reducing the mycelial growth of C. gloeosporioides and C. karstii. The QoI fungicides showed a remarkably inhibitory impact on spore germination of both species. On average, C. karstii was more sensitive to the DMI fungicides than C. gloeosporioides. The findings of this study provide new information to understand the Colletotrichum dieback of citrus.

Comparative Transcriptome Analyses Reveal Conserved and Distinct Mechanisms of the SDHI Fungicide Benzovindiflupyr Inhibiting Colletotrichum

Colletotrichum leaf disease (CLD) is an annual production concern for commercial growers worldwide. The succinate dehydrogenase inhibitor (SDHI) fungicide benzovindiflupyr shows higher bioactivity against CLD than other SDHIs. However, the mechanism underlying such difference remains unclear. In this study, benzovindiflupyr exhibits good inhibitory activity against Colletotrichum siamense and C. nymphaeae in vitro and in vivo. To reveal its mechanism for inhibiting Colletotrichum, we compared transcriptomes of C. siamense and C. nymphaeae under treatment with benzovindiflupyr and boscalid. Benzovindiflupyr exhibited higher inhibitory activity against SDH enzyme than boscalid, resulting in a greater reduction in the ATP content of Colletotrichum isolates. Most of the metabolic pathways induced in these fungicide-treated isolates were similar, indicating that benzovindiflupyr exhibited a conserved mechanism of SDHIs inhibiting Colletotrichum. At the same level of suppressive SDH activity, benzovindiflupyr activated more than three times greater gene numbers of Colletotrichum than boscalid, suggesting that benzovindiflupyr could activate distinct mechanisms against Colletotrichum. Membrane-related gene ontology terms, mainly including intrinsic components of membrane, were highly abundant for the benzovindiflupyr-treated isolates rather than boscalid-treated isolates. Only benzovindiflupyr increased the relative conductivities of hyphae, indicating that it could damage the cell membrane and increase mycelial electrolyte leakage. Thus, we proposed that the high bioactivity of benzovindiflupyr against Colletotrichum occurred by inhibiting SDH activity and damaging the cell membrane at the same time. The research improves our understanding the mode of action of SDHI fungicides against Colletotrichum.

CsAtf1, a bZIP transcription factor, is involved in fludioxonil sensitivity and virulence in the rubber tree anthracnose fungus Colletotrichum siamense

In phytopathogenic fungi, the HOG MAPK pathway has roles in osmoregulation, fungicide sensitivity, and other processes. The ATF1/CREB-activating transcription factor Atf1 is a regulator that functions downstream of the HOG MAPK pathway. Here, we identified a gene, designated CsAtf1, that encodes a bZIP transcription factor in Colletotrichum siamense, which is the main pathogen that causes Colletotrichum leaf fall disease in rubber trees in China. CsAtf1 localizes to the nucleus. Its mRNA expression correlates positively with that of CsPbs2 and CsHog1 in the HOG MAPK pathway in response to activator (anisomycin), inhibitor (SB203580) and fludioxonil treatments. The CsAtf1 deletion mutant showed slightly retarded mycelial growth, small conidia, slow spore germination, and abnormal appressorium formation. This mutant showed the increased spore germination rate after fludioxonil treatment and more resistance to the fungicide fludioxonil than did the wild-type fungus. However, unlike deletion of Pbs2 or Hog1, which resulted in greater sensitivity to osmotic stress, the CsAtf1 deletion induced slightly increased resistance to osmotic stress and the cell wall stress response. The ΔCsAtf1 strain also exhibited significantly reduced virulence on rubber tree leaves. These data revealed that CsAtf1 plays a key role in the regulation of fludioxonil sensitivity and in pathogenicity regulation in C. siamense.

CgNPG1 as a Novel Pathogenic Gene of Colletotrichum gloeosporioides From Hevea brasiliensis in Mycelial Growth, Conidiation, and the Invasive Structures Development

The rubber tree (Hevea brasiliensis) is a tropical perennial crop for the primary source of natural rubber. Colletotrichum gloeosporioides from Hevea brasiliensis (C. gloeosporioides Hb) and Colletotrichum acutatum from Hevea brasiliensis (C. acutatum Hb) are the causal agents of rubber tree anthracnose and lead to serious loss of natural rubber production. Inoculation tests showed that C. gloeosporioides Hb possessed higher pathogenicity than C. acutatum Hb to the rubber tree. Genomic analysis revealed that an unknown gene, named CgNPG1 (a Novel Pathogenic Gene 1), was presented in the genome of C. gloeosporioides Hb but not identified in C. acutatum Hb. CgNPG1 was predicted to encode a small secretory protein without any conserved domain. To investigate the functions of CgNPG1 in C. gloeosporioides Hb and in C. acutatum Hb, the gene deletion and overexpression mutants were generated. The phenotype analysis showed that deletion of CgNPG1 led to changed conidia morphology, decreased mycelial growth, conidiation, conidia germination rate, appressorium formation rate, and pathogenicity of C. gloeosporioides Hb to the rubber tree. Meanwhile, heterogeneous expression of CgNPG1 in C. acutatum Hb significantly changed the conidia morphology and improved the mycelial growth rate, conidiation, conidia germination rate, appressorium formation rate, and the pathogenicity of C. acutatum Hb to the rubber tree. Consistently, CgNPG1 increased the expression level of CaCRZ1 and CaCMK1 in C. acutatum Hb. These data suggested that CgNPG1 contributed to mycelial growth, conidiation, the development of invasive structures, and the pathogenicity of Colletotrichum to the rubber tree, which might be related to the modulation of CaCRZ1 and mitogen-activated protein kinase CMK1. Our results provided new insight into CgNPG1 in regulating growth and pathogenicity of the Colletotrichum spp.

Pathogenicity and Control of Phacidium lacerum, an Emerging Pome Fruit Pathogen in Washington State

Phacidium lacerum (anamorph Ceuthospora pinastri) is a recently reported quarantine fungal pathogen responsible for postharvest rot in apples and pears. Very little is known about its pathogenicity, epidemiology, and best management practices. We screened pathogenicity of P. lacerum on twigs from seven and fruit from nine major commercial apple cultivars. Among the nine cultivars tested, detached fruit of Honeycrisp and Gala cultivars were the most susceptible, whereas WA38 (Cosmic Crisp) was the least susceptible (P < 0.05). Effective concentrations to inhibit 50% growth (EC₅₀) were determined in 41 baseline P. lacerum isolates. The mean EC₅₀ values for four postharvest fungicides, i.e., fludioxonil (FDL), difenoconazole (DIF), thiabendazole (TBZ), and pyrimethanil (PYRI) were 0.16, 0.38, 0.54, and 0.72 µg/ml, respectively. The mean EC₅₀ values for four preharvest fungicides, i.e., pyraclostrobin (PYRA), fluxapyroxad (FLUX), boscalid (BOSC), and fluopyram (FLUP) were 0.96, 12.64, 16.54, and 44.46 µg/ml, respectively. In situ efficacy trials were conducted on detached Gala apples treated preventively and curatively with the aforementioned fungicides. After 6 months of storage at 1°C, FDL and DIF provided full control followed by TBZ and PYRI, whereas the other preharvest fungicides provided fair or low efficacies. Findings of this study shed light on pathogenicity of this emerging pathogen and provide necessary knowledge for effective management of Phacidium rot.

Characterization and Fungicide Sensitivity of Colletotrichum Species Causing Strawberry Anthracnose in Eastern China

Strawberry anthracnose caused by Colletotrichum spp. is one of the most serious diseases in the strawberry fields of China. In total, 196 isolates of Colletotrichum were obtained from leaves, stolons, and crowns of strawberry plants with anthracnose symptoms in eastern China and were characterized based on morphology, internal transcribed spacer (ITS), and β-tubulin (TUB2) gene sequences. All 196 isolates were identified as the Colletotrichum gloeosporioides species complex. In total, 62 strains were further identified at the species level by phylogenetic analyses of multilocus sequences of ITS, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), Apn2-Mat1-2 intergenic spacer and partial mating type (ApMat), calmodulin (CAL), and TUB2. Three species from the C. gloeosporioides species complex were identified: Colletotrichum siamense, C. fructicola, and C. aenigma. Isolates of C. siamense were tolerant to high temperatures, with a significantly larger colony diameter than the other two species when grown above 36°C. The inoculation of strawberry plants confirmed the pathogenicity of all three species. C. siamense isolates resulted in the highest disease severity. The in vitro sensitivities of C. siamense and C. fructicola isolates to azoxystrobin and three demethylation-inhibitor (DMI) fungicides (difenoconazole, tebuconazole, and prochloraz) were determined. Both species were sensitive to DMI fungicides but not to azoxystrobin. C. siamense isolates were more sensitive to prochloraz, while C. fructicola isolates were more sensitive to difenoconazole and tebuconazole. The present study provides valuable information for the effective management of strawberry anthracnose.

Identification, Virulence and Fungicide Sensitivity of Colletotrichum gloeosporioides s.s. Responsible for Walnut Anthracnose Disease in China

Walnut (Juglans regia L.) is an economically important woody nut and edible oil tree all over the world. However, walnut production is limited by walnut anthracnose, which is a disastrous disease that causes significant yield losses. Studying the etiology of anthracnose on walnut and the pathogens' virulence and sensitivities to fungicides would be beneficial for effective control. This study was conducted to identify the pathogen of walnut anthracnose and reveal the population diversity of pathogens through virulence, sensitivities to fungicides, and genetic variation. A total of 13 single-spore Colletotrichum isolates were collected from walnut anthracnose-diseased fruits and leaves from 13 walnut commercial orchards in Henan, Hubei, Shandong, and Shaanxi provinces in China. The isolates were identified as Colletotrichum gloeosporioides sensu stricto (s.s.) according to multilocus phylogenetic analyses (internal transcribed spacer, actin, glyceraldehyde-3-phosphate dehydrogenase, and chitin synthase), morphological as well as cultural characters, and pathogenicity. When the same walnut tissue was inoculated with different isolates, the disease lesion size was different. The results showed that the virulence of all isolates was considerably different, and the differences were not correlated with geographic origins. The virulence to walnut leaves and fruits inoculated with the same isolate was significantly different. Based on the virulence to walnut leaves and fruits, the 13 isolates were divided into three groups. Virulence of 69.2% of the isolates to walnut fruits was higher than that to leaves; 15.4% of isolates had no difference in pathogenicity, and the virulence to walnut leaves was higher for 15.4% of isolates. Tebuconazole, difenoconazole, flusilazole, and carbendazim inhibited the growth of fungal mycelia, and the concentration for 50% of maximal effect (EC₅₀) values were 0.4 to 20.5, 0.6 to 2.6, 0.2 to 1.6, and 0.002 to 0.2 µg/ml, respectively, with average values of 6.5 ± 6.9, 1.5 ± 0.6, 0.9 ± 0.4, and 0.1 ± 0.05 µg/ml, respectively. All isolates were more sensitive to difenoconazole, flusilazole, and carbendazim than tebuconazole (P < 0.01). Isolate sensitivities to the same fungicide were different. Isolates SL-31 and TS-09 were the least sensitive to carbendazim and tebuconazole, respectively, and the resistance ratios were 87.3 and 51.6, respectively. Sensitivities to difenoconazole and flusilazole were largely consistent among all isolates, and the resistance ratios were from 1 to 4.6 and from 1 to 7, respectively. Therefore, difenoconazole and flusilazole could be chosen for disease control. The differences of pathogenicity and fungicide sensitivity were not correlated with geographic regions. These results indicated that there was high intraspecific diversity of populations in C. gloeosporioides s.s. that caused walnut anthracnose. For effective management, the targeted control strategy should be implemented based on the different geographic regions.

Paralogous CYP51 Genes of Colletotrichum spp. Mediate Differential Sensitivity to Sterol Demethylation Inhibitors

Colletotrichum spp. isolates contain two paralogous CYP51 genes that encode sterol 14-demethylase enzymes; however, their role in sensitivity to demethylation inhibitor (DMI) fungicides is yet to be determined. In this study, each of the two genes from Colletotrichum fioriniae and C. nymphaeae was able to rescue the function of CYP51 in the yeast Saccharomyces cerevisiae, demonstrating their independent function. Deletion of CYP51A led to increased sensitivity to propiconazole, diniconazole, prothioconazole, cyproconazole, epoxiconazole, flutriafol, prochloraz, and difenoconazole in C. fioriniae, and to the same fungicides and tebuconazole in C. nymphaeae, with the exception of prochloraz. Deletion of CYP51B in C. fioriniae and CYP51B in C. nymphaeae made mutants increasingly sensitive to five of nine DMI fungicides tested. The results suggest species-specific, differential binding of DMI fungicides onto the two CYP51 enzymes. Pairing DMIs with different effects on CYP51A and -B deletion mutants resulted in synergistic effects, as determined in mycelial growth inhibition experiments. Deletion mutants showed no fitness penalty in terms of mycelial growth, sporulation, and virulence. Our study elucidates the effect of CYP51A and CYP51B of Colletotrichum spp. on DMI sensitivity, suggesting that using a mixture of DMIs may improve the efficacy for anthracnose management.

Mutations and Overexpression of CYP51 Associated with DMI-Resistance in Colletotrichum gloeosporioides from Chili

Chili anthracnose caused by Colletotrichum spp. is an annual production concern for growers in China. Sterol C14-demethylation inhibitors (DMIs, such as tebuconazole) have been widely used to control this disease for more than three decades. In the current study, of 48 isolates collected from commercial chili farms in Jiangsu Province of China during 2018 and 2019, 8 single-spore isolates were identified as Colletotrichum gloeosporioides and the rest were identified as C. acutatum. To determine whether the DMI resistance of isolates develops in the field, mycelial growth of the 48 isolates was measured in culture medium with and without tebuconazole. In all, 6 of the 8 C. gloeosporioides isolates were resistant to tebuconazole, but all 40 of the C. acutatum isolates were sensitive to tebuconazole. The fitness cost of resistance was low based on a comparison of fitness parameters between the sensitive and resistant isolates of C. gloeosporioides. Positive cross-resistance was observed between tebuconazole and difenconazole or propiconazole, but not prochloraz. Alignment results of the CgCYP51 amino acid sequences from the sensitive and resistant isolates indicated that mutations can be divided into three genotypes. Genotype I possessed four substitutions (V18F, L58V, S175P, and P341A) at the CgCYP51A gene but no substitutions at CgCYP51B, while genotype II had five substitutions (L58V, S175P, A340S, T379A, and N476T) at CgCYP51A, concomitant with three substitutions (D121N, T132A, and F391Y) at CgCYP51B. In addition, genotype III contained two substitutions (L58V and S175P) at CgCYP51A, concomitant with one substitution (T262A) at CgCYP51B. Molecular docking models illustrated that the affinity of tebuconazole to the binding site of the CgCYP51 protein from the resistant isolates was decreased when compared with binding site affinity of the sensitive isolates. Our findings provide not only novel insights into understanding the resistance mechanism to DMIs, but also some important references for resistance management of C. gloeosporioides on chili.

Three Colletotrichum Species, Including a New Species, are Associated to Leaf Anthracnose of Rubber Tree in Hainan, China

Colletotrichum gloeosporioides and C. acutatum have been reported to be causal agents of anthracnose disease of rubber tree. Recent investigations have shown that both C. gloeosporioides and C. acutatum are species complexes. The identities of Colletotrichum species causing anthracnose disease of rubber tree in Hainan, China, are unknown. In this study, 106 isolates obtained from rubber tree with symptoms of anthracnose were collected from 12 counties of Hainan and identified at the species complex level based on the ITS sequences and colony morphologies. Seventy-four isolates were identified as C. gloeosporioides species complex and the other 32 isolates as C. acutatum species complex. Forty-two isolates were selected for further multilocus phylogenetic analyses in order to identify the isolates to the species level. Twenty-six isolates from the C. gloeosporioides species complex were characterized for partial sequences of seven gene regions (ACT, TUB2, CHS-1, GAPDH, ITS, ApMat, and GS), and the other 16 isolates from the C. acutatum species complex for five gene regions (ACT, TUB2, CHS-1, GAPDH, and ITS). Three species were identified: C. siamense and C. fructicola from the C. gloeosporioides species complex, and a new species C. wanningense from the C. acutatum species complex. Artificial inoculation of rubber tree leaves confirmed the pathogenicity of the three species. The present study improves the understanding of species causing anthracnose on rubber tree and provides useful information for the effective control of the disease.

Selection Pressure Pathways and Mechanisms of Resistance to the Demethylation Inhibitor-Difenoconazole in Penicillium expansum

Penicillium expansum causes blue mold, the most economically important postharvest disease of pome fruit worldwide. Beside sanitation practices, the disease is managed through fungicide applications at harvest. Difenoconazole (DIF) is a new demethylation inhibitor (DMI) fungicide registered recently to manage postharvest diseases of pome fruit. Herein, we evaluated the sensitivity of 130 P. expansum baseline isolates never exposed to DIF and determined the effective concentration (EC50) necessary to inhibit 50% germination, germ tube length, and mycelial growth. The respective mean EC50 values of 0.32, 0.26, and 0.18 μg/ml indicate a high sensitivity of P. expansum baseline isolates to DIF. We also found full and extended control efficacy in vivo after 6 months of storage at 1°C. We conducted a risk assessment for DIF-resistance development using ultraviolet excitation combined with or without DIF-selection pressure to generate and characterize lab mutants. Fifteen DIF-resistant mutants were selected and showed EC50 values of 0.92 to 1.4 μg/ml and 1.7 to 3.8 μg/ml without and with a DIF selection pressure, respectively. Resistance to DIF was stable in vitro over a 10-week period without selection pressure. Alignment of the full CYP51 gene sequences from the three wild-type and 15 mutant isolates revealed a tyrosine to phenylalanine mutation at codon 126 (Y126F) in all of the 15 mutants but not in the wild-type parental isolates. Resistance factors increased 5 to 15-fold in the mutants compared to the wild-type-isolates. DIF-resistant mutants also displayed enhanced CYP51 expression by 2 to 14-fold and was positively correlated with the EC50 values (R 2 = 0.8264). Cross resistance between DIF and fludioxonil, the mixing-partner in the commercial product, was not observed. Our findings suggest P. expansum resistance to DIF is likely to emerge in commercial packinghouse when used frequently. Future studies will determine whether resistance to DIF is qualitative or quantitative which will be determinant in the speed at which resistance will develop and spread in commercial packinghouses and to develop appropriate strategies to extend the lifespan of this new fungicide.

Sensitivity of Phacidiopycnis spp. Isolates from Pome Fruit to Six Pre- and Postharvest Fungicides

Phacidiopycnis washingtonensis and P. pyri cause speck rot and Phacidiopycnis rot on apple and pear, respectively. Infection occurs in the orchard and remains latent, and symptoms appear after months of storage. Decay management relies on orchard sanitation and pre- and postharvest fungicides. In a 2017 survey, speck rot accounted for 6.4% of apple decay in central Washington, whereas Phacidiopycnis rot accounted for 3.9 and 6.7% of total pear decay in Washington and Oregon, respectively. Sensitivities of baseline populations of 110 P. washingtonensis and 76 P. pyri isolates collected between 2003 and 2005 to preharvest fungicides pyraclostrobin (PYRA) and boscalid (BOSC) and to postharvest fungicides thiabendazole (TBZ), fludioxonil (FDL), pyrimethanil (PYRI), and difenoconazole (DFC) were evaluated using a mycelial growth inhibition assay. Mean effective concentrations necessary to inhibit 50% growth (EC₅₀) of P. washingtonensis were 0.1, 0.3, 0.8, 1.8, 2.1, and 4.8 µg/ml for FDL, PYRI, TBZ, DFC, PYRA, and BOSC, respectively. Respective mean EC₅₀ values for P. pyri were 0.2, 0.6, 1.6, 1.1, 0.4, and 1.8 µg/ml. The sensitivity of exposed P. washingtonensis and P. pyri populations collected in 2017 revealed potential shifts toward BOSC and PYRA resistance. The efficacy of the six fungicides to control isolates of each pathogen with different in vitro sensitivity levels was evaluated on apple and pear fruit. FDL, DFC, and PYRI controlled both Phacidiopycnis spp. regardless of their EC₅₀ values after 5 months of storage at 0°C in a regular atmosphere. The consistent occurrence of Phacidiopycnis spp. will require continuous monitoring and development of disease management strategies based on fungicide phenotypes and efficacy of existing fungicides assessed herein.