Inherent tolerance of Colletotrichum gloeosporioides to fludioxonil

Utilizing Disease Prediction Models to Time Fungicide Applications for Controlling Ripe Rot, Caused by Colletotrichum spp., in Maryland Vineyards

Two previously published ripe rot prediction models, DF2-NN and GH2-DT, were evaluated for fungicide application timing efficacy in Maryland vineyards. Both models utilize leaf wetness duration (LWD), temperature, and grape cluster phenological stages as model parameters. These three parameters were tracked throughout the 2021 to 2023 seasons in three vineyards. The fungicide efficacy trials started at the veraison phenological stage and included a nontreated control, a 12-day interval treatment, and two model-triggered treatments when risk predicted by the models crossed a threshold. The severity of ripe rot on the clusters in each treatment was assessed when the fruit were mature. Ripe rot severity in the nontreated controls was higher during seasons with more LWD and more precipitation. Days in which the models were triggered by the environmental conditions primarily coincided with precipitation events and lengthy LWDs. The model-triggered treatments never had significantly higher ripe rot severity than the 12-day interval treatment but had significantly lower severities than the nontreated control in most trials which had high ripe rot pressure. Furthermore, the model-triggered treatments resulted in fewer fungicide applications than the 12-day interval treatment on average. The DF2-NN model appeared to be more accurate and useful for ripe rot prediction and treatment than the GH2-DT model because it triggered fewer fungicide applications while reducing ripe rot. This model may be useful for improving or maintaining ripe rot control with fewer fungicide inputs, which may be beneficial for the environment and the reduction of fungicide resistance selection.

Identification and Fungicide Screening of Fungal Species Associated with Walnut Anthracnose in Shaanxi and Liaoning Provinces, China

Walnut is cultivated around the world for its precious woody nut and edible oil. Recently, walnut infected by Colletotrichum spp. resulted in a great yield and quality loss. In August and September 2014, walnut fruits with anthracnose were sampled from two commercial orchards in Shaanxi and Liaoning provinces, and five representative isolates were used in this study. To identify the pathogen properly, four genes per region (internal transcribed spacer, glyceraldehyde-3-phosphate dehydrogenase, actin, and chitin synthase) were sequenced and used in phylogenetic studies. Based on multilocus phylogenetic analysis, five isolates clustered with Colletotrichum fioriniae, including its ex-type, with 100% bootstrap support. The results of multilocus phylogenetic analyses, morphology, and pathogenicity confirmed that C. fioriniae was one of the walnut anthracnose pathogens in China. All 13 fungicides tested inhibited mycelial growth and spore germination. Flusilazole, fluazinam, prochloraz, and pyraclostrobin showed the strongest suppressive effects on the mycelial growth than the others, the average EC50 values ranged from 0.09 to 0.40 μg/ml, and there was not any significant difference (P < 0.05). Pyraclostrobin, thiram, and azoxystrobin were the most effective fungicides on spore germination (P < 0.05), and the EC50 values ranged from 0.01 to 0.44 μg/ml. Pyraclostrobin, azoxystrobin, fluazinam, flusilazole, mancozeb, thiram, and prochloraz exhibited a good control effect on walnut anthracnose caused by C. fioriniae, and preventive activities were greater than curative activities. Pyraclostrobin at 250 a.i. μg/ml and fluazinam at 500 a.i. μg/ml provided the highest preventive and curative efficacy, and the values ranged from 81.3 to 82.2% and from 72.9 to 73.6%, respectively. As a consequence, mancozeb and thiram could be used at the preinfection stage, and pyraclostrobin, azoxystrobin, flusilazole, fluazinam, and prochloraz could be used at the early stage for effective prevention and control of walnut anthracnose caused by C. fioriniae. The results will provide more significant instructions for controlling the disease effectively in northern China.

Sensitivity to 12 Fungicides and Resistance Mechanism to Trifloxystrobin, Carbendazim, and Succinate Dehydrogenase Inhibitors in Cucumber Corynespora Leaf Spot (Corynespora cassiicola)

Corynespora cassiicola is the causal agent of cucumber Corynespora leaf spot, which affects many economically important plant species. Chemical control of this disease is hampered by the common development of fungicide resistance. In this study, 100 isolates from Liaoning Province were collected, and their sensitivity to 12 fungicides was determined. All the isolates (100%) were resistant to trifloxystrobin and carbendazim, and 98% were resistant to fluopyram, boscalid, pydiflumetofen, isopyrazam, and fluxapyroxad. However, none were resistant to propiconazole, prochloraz, tebuconazole, difenoconazole, and fludioxonil. The Cytb gene of trifloxystrobin-resistant isolates encoded the G143A mutation, whereas the β-tubulin gene of carbendazim-resistant isolates encoded the E198A and E198A and M163I mutations. Mutations in SdhB-I280V, SdhC-S73P, SdhC-H134R, SdhD-D95E, and SdhD-G109V were associated with resistance to the succinate dehydrogenase inhibitors (SDHIs). Trifloxystrobin, carbendazim, and fluopyram were barely effective on the resistant isolates, whereas fludioxonil and prochloraz were effective on the isolates that were resistant to the quinone outside inhibitors (QoIs), SDHIs, and benzimidazoles. Ultimately, this study demonstrates that fungicide resistance seriously threatens the effective control of Corynespora leaf spot.

Putrescine (1,4-Diaminobutane) enhances antifungal activity in postharvest mango fruit against Colletotrichum gloeosporioides through direct fungicidal and induced resistance mechanisms

Anthracnose decay caused by Colletotrichum gloeosporioides greatly shortens the shelf life and commercial quality of mango fruit. Putrescine (1,4-Diaminobutane) is involved in modulating plant defense to various environmental stresses. In this research, in vivo and in vitro tests were used to explore the antifungal activity and the underlying mechanism of putrescine against C. gloeosporioides in mango fruit after harvested. In vivo tests suggested that putrescine markedly delayed the occurrence of disease and limited the spots expansion on inoculated mango fruit. Further analysis exhibited that putrescine treatment enhanced disease resistance, along with enhanced activities of chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-coumarate coenzyme A ligase (4CL), polyphenol oxidase (PPO) and the accumulation of lignin, flavonoid, phenolics, and anthocyanin in infected mango fruit. In addition, in vitro tests showed that putrescine exerted strongly antifungal activity against C. gloeosporioides. Putrescine induced the production of reactive oxygen species (ROS) and severe lipid peroxidation damage in C. gloeosporioides mycelia, resulting in the leakage of soluble protein, soluble sugar, nucleic acids, K+ and Ca2+ of C. gloeosporioides mycelia. The mycelium treated with putrescine showed severe deformity and shrinkage, and even cracking. Taken together, putrescine could effectively reduce the incidence rate and severity of anthracnose disease possibly through direct fungicidal effect and indirect induced resistance mechanism, thus showing great potential to be applied to disease control.

Sensitivity and resistance risk of Botryosphaeria dothidea causing Chinese hickory trunk canker to fludioxonil

Hickory trunk canker (HTC), primarily caused by Botryosphaeria dothidea, is an aggravating disease that threatens an important regional economic tree species of Chinese hickory and few information is available in the control of this disease. Here, the sensitivity of 93 isolates to fludioxonil and the resistance risk were investigated. All the isolates tested were sensitive to fludioxonil and the EC50 ranged from 0.0028 to 0.0569 μg/mL. The tamed fludioxonil-resistant mutants remained highly resistant to fludioxonil even after 10 consecutive transfers to fludioxonil-free PDA plates. As for fitness penalty, the fludioxonil-resistant mutants demonstrated a reduction in conidia production and virulence as well as increased sensitivity to high osmotic stress. While, variations in mycelial growth and responses to SDS and H2O2 were not detected in all the resistant mutants. In addition, the resistant mutants demonstrated positive cross-resistance to iprodione but not to fungicides of other modes of action. Sequential analysis of BdNik1 showed that premature stop codon occurred in all the resistant mutants despite of point mutation (BD16-22R9 and BD16-22R20) or frameshift mutation (BD16-22R8, BD16-22R11 and BD16-22R18). Our study suggested that fludioxonil exhibited excellent inhibition activity on mycelial growth of B. dothidea in vitro, the resistance risk of B. dothidea to fludioxonil should be low to moderate and fludioxonil would be a nice candidate in controlling HTC caused by B. dothidea.

Identification and Fungicide Sensitivity of Colletotrichum spp. from Apple Flowers and Fruitlets in Brazil

Glomerella leaf spot (GLS) and bitter rot (BR), caused by Colletotrichum spp., are major diseases on apple in southern Brazil. Among integrated pest management tools for disease management in commercial orchards, fungicides remain an important component. This study aimed to identify Colletotrichum spp. from cultivar Eva in Paraná state orchards; evaluate their in vitro sensitivity to cyprodinil, tebuconazole, iprodione, and fluazinam; and determine the baseline in vitro sensitivity of these isolates to benzovindiflupyr and natamycin. Most isolates belonged to Colletotrichum melonis and C. nymphaeae of the C. acutatum species complex. The two species varied in sensitivity to fluazinam and tebuconazole, but no variability was found for any other fungicide. The lowest 50% effective concentration (EC₅₀) values of Colletotrichum spp. were observed for cyprodinil (mean EC₅₀ < 0.02) and benzovindiflupyr (mean EC₅₀ < 0.05); EC₅₀ values were intermediate for fluazinam (mean EC₅₀ < 0.33) and tebuconazole (mean EC₅₀ < 0.14), and they were highest for natamycin (mean EC₅₀ < 5.56) and iprodione (mean EC₅₀ > 12). Cyprodinil and fluazinam are registered for use in Brazil for apple disease management but not specifically for GLS and BR. Tebuconazole is one of the few products registered for Colletotrichum spp. control in apples. In conclusion, flowers and fruitlets can serve as sources of inoculum for GLS and BR disease; C. acutatum was the predominant species complex in these tissues; cyprodinil and fluazinam applications may suppress GLS and BR; and benzovindiflupyr and natamycin warrant further investigation for GLS and BR disease control of apple due to comparably high in vitro sensitivity.

Detection of fungicide resistance to fludioxonil and tebuconazole in Fusarium pseudograminearum, the causal agent of Fusarium crown rot in wheat

Fusarium crown rot (FCR) on wheat is a soil-borne disease that affects the yield and quality of the produce. In 2020, 297 Fusarium pseudograminearum isolates were isolated from diseased FCR wheat samples from eight regional areas across Hebei Province in China. Baseline sensitivity of F. pseudograminearum to fludioxonil (0.0613 ± 0.0347 μg/mL) and tebuconazole (0.2328 ± 0.0840 μg/mL) were constructed based on the in vitro tests of 71 and 83 isolates, respectively. The resistance index analysis showed no resistance isolate to fludioxonil but two low-resistance isolates to tebuconazole in 2020. There was an increased frequency of resistant isolates from 2021 to 2022 based on the baseline sensitivity for tebuconazole. There was no cross-resistance between fludioxonil and tebuconazole. This study provides a significant theoretical and practical basis for monitoring the resistance of F. pseudograminearum to fungicides, especially the control of FCR.

Sensitivity of Colletotrichum acutatum Species Complex from Strawberry to Fungicide Alternatives to Quinone-Outside Inhibitors

Colletotrichum acutatum is a species complex that causes anthracnose fruit rot and root necrosis on strawberry. The major and minor species within the complex that affect strawberry production are C. nymphaeae and C. fioriniae, respectively. The disease can significantly reduce yield under conducive weather, and its management has greatly relied on quinone-outside inhibitor fungicides (QoI). However, due to the emergence of resistant isolates, such products are no longer effective. Therefore, alternative fungicides were investigated. C. nymphaeae and C. fioriniae isolates were collected from multiple strawberry fields in the United States from 1995 to 2017. The sensitivity of benzovindiflupyr, penthiopyrad, pydiflumetofen, fluazinam, fludioxonil, and cyprodinil was assessed by in vitro and in vivo assays. Both Colletotrichum species were sensitive to benzovindiflupyr, penthiopyrad, fluazinam, and fludioxonil based on mycelial growth assays. Interestingly, of these products, only penthiopyrad did not inhibit conidial germination at 100 µg/ml. For cyprodinil, C. nymphaeae was sensitive based on the mycelial growth, whereas C. fioriniae was not inhibited. Neither species was inhibited by pydiflumetofen in mycelial growth, conidial germination, nor detached fruit assays. The prepackaged mixtures fludioxonil + cyprodinil and fludioxonil + pydiflumetofen were effective in a field trial; however, their use should be carefully considered because of the lack of efficacy of one of the compounds in the mixture. This study sheds light on the potential registration of products alternative to QoIs, such as benzovindiflupyr and fluazinam, which could improve the management of strawberry anthracnose.

Evaluation of the Antifungal Activities of Photorhabdus akhurstii and Its Secondary Metabolites against Phytopathogenic Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a phytopathogenic fungus that causes devastating losses in strawberries without effective countermeasures. Members of the genus Photorhabdus exhibit antimicrobial capability and have been found to have the potential for use as biocontrol agents against C. gloeosporioides. Photorhabdus species exhibit two phase variations with a differentiated composition of secondary metabolites designated to each phase. In this study, Photorhabdus akhurstii sp. nov. 0813-124 exhibited phase I (PL1) and phase II (PL2); however, only PL1 displayed distinct inhibition of C. gloeosporioides in the confrontation assay. We identified the bioactive ingredients of P. akhurstii sp. nov. 0813-124 to be glidobactin A and cepafungin I, with MIC values lower than 1.5 and 2.0 µg/mL, respectively. Furthermore, we revealed the biosynthetic gene cluster (BGC) of corresponding bioactive molecules through genomics analysis and determined its expression level in PL1 and PL2. The expression of glidobactin BGC in PL1 increased rapidly within 24 h, while PL2 was eventually stimulated after 60 h. In summary, we demonstrated that P. akhurstii sp. nov. 0813-124 could potentially be used as a biocontrol agent or part of a natural product repertoire for combating C. gloeosporioides.

Colletotrichum Species Associated with Peaches in China

Colletotrichum is regarded as one of the 10 most important genera of plant pathogens in the world. It causes diseases in a wide range of economically important plants, including peaches. China is the largest producer of peaches in the world but little is known about the Colletotrichum spp. affecting the crop. In 2017 and 2018, a total of 286 Colletotrichum isolates were isolated from symptomatic fruit and leaves in 11 peach production provinces of China. Based on multilocus phylogenetic analyses (ITS, ACT, CAL, CHS-1, GAPDH, TUB2, and HIS3) and morphological characterization, the isolates were identified to be C. nymphaeae, C. fioriniae, and C. godetiae of the C. acutatum species complex, C. fructicola and C. siamense of the C. gloeosporioides species complex, C. karsti of the C. boninense species complex, and one newly identified species, C. folicola sp. nov. This study is the first report of C. karsti and C. godetiae in peaches, and the first report of C. nymphaeae, C. fioriniae, C. fructicola, and C. siamense in peaches in China. C. nymphaeae is the most prevalent species of Colletotrichum in peaches in China, which may be the result of fungicide selection. Pathogenicity tests revealed that all species found in this study were pathogenic on both the leaves and fruit of peaches, except for C. folicola, which only infected the leaves. The present study substantially improves our understanding of the causal agents of anthracnose on peaches in China.

Sensitivity to fungicides in isolates of Colletotrichum gloeosporioides and C. acutatum species complexes and efficacy against anthracnose diseases

Colletotrichum species cause diseases on many plants and are among the 'top 10' fungal plant pathogens. Species of the C. gloeosporioides and C. acutatum complexes are particularly important because they infect temperate fruit crops, but their control relies largely on chemical fungicides. In this study, differences in intrinsic fungicide sensitivity were determined in vitro using isolates of the C. gloeosporioides sp. complex (C. fructicola, C. siamense, and C. tropicale) and the C. acutatum sp. complex (C. fioriniae and C. nymphaeae), which had never been exposed to fungicides. Mycelial growth of all isolates was sensitive to the QoI azoxystrobin, the SDHI benzovindiflupyr, and the new DMI fungicide mefentrifluconazole. The isolates of C. nymphaeae were highly sensitive to the phenylpyrrole fungicide fludioxonil. The isolates of C. gloeosporioides sp. complex were sensitive to the bis-guanidine fungicide iminoctadine-albesilate, whereas those of C. acutatum sp. complex were inherently insensitive. These results are valuable when sensitivity of field populations is monitored in resistance management. Although SDHI fungicides are largely not effective against diseases caused by Colletotrichum species, benzovindiflupyr controlled anthracnose disease of various crops such as kidney bean, garland chrysanthemum, and strawberry, caused by C. lindemuthianum, C. chrysanthemi, and C. siamense, respectively, demonstrating this fungicide to be unique among SDHIs and having a broad control spectrum against anthracnose. To help understanding the reason for differential activity of benzovindiflupyr and boscalid, sdhB gene sequences were analyzed but those of C. lindemuthianum, C. chrysanthemi, and C. scovillei revealed no known mutations reported to be responsible for SDHI resistance in other fungi, indicating that other mechanism(s) than target-site modification may be involved in differential sensitivity to benzovindiflupyr and boscalid, found in Colletotrichum species.

Sensitivity of Colletotrichum nymphaeae to Six Fungicides and Characterization of Fludioxonil-Resistant Isolates in China

Colletotrichum nymphaeae is the dominant species causing anthracnose disease of peach in China. In this study, 140 isolates of C. nymphaeae were assessed for their sensitivity to six fungicides. It was found that C. nymphaeae was highly resistant to carbendazim, procymidone, and boscalid but sensitive to pyraclostrobin and prochloraz. For fludioxonil, the fungus exhibited differential sensitivities (i.e., approximately 14% of isolates were resistant to fludioxonil and the resistance was stable). Fludioxonil-resistant isolates had a mean EC₅₀ value of 2.2380 µg/ml, whereas the mean EC₅₀ value was 0.0194 µg/ml in fludioxonil-sensitive isolates. The mean EC₅₀ values of C. nymphaeae for pyraclostrobin and prochloraz were 0.0083 µg/ml and 0.002 µg/ml, respectively. No cross-resistance was observed between fungicides from different groups. Mycelial growth rate, control efficacy, and osmotic stress responses were significantly different (P < 0.05) between fludioxonil-sensitive (Flu^S) and -resistant (Flu^R) isolates, but no significant difference was observed (P > 0.05) in virulence and sporulation between Flu^S and Flu^R isolates. No mutation was detected in coding regions of the CnOs-1, Cal, Hk1, Hog1, TPI, and Mrr1 genes. Interestingly, with fludioxonil treatment, the expression of ABC transporter gene atrB was significantly overexpressed in some resistant isolates. However, overexpression of the atrB gene was not detected in one moderately and one highly resistant isolate, indicating that other unknown mechanisms may be involved. Current findings uncovered several effective chemicals and provided the foundation for designing management strategies to practically control peach anthracnose with the most effective demethylation inhibitor fungicides and quinone outside inhibitor fungicides.

Sensitivity of Colletotrichum fructicola and Colletotrichum siamense of Peach in China to Multiple Classes of Fungicides and Characterization of Pyraclostrobin-Resistant Isolates

Anthracnose, mainly caused by Colletotrichum gloeosporioides species complex including Colletotrichum fructicola and Colletotrichum siamense, is a devastating disease of peach. Chemical control has been widely used for years, but management failures have increased with the commonly used fungicides. Therefore, screening of sensitivity of Colletotrichum spp. to fungicides with different modes of action is needed to make proper management strategies for peach anthracnose. In this study, the sensitivity of 80 isolates of C. fructicola and C. siamense was screened for pyraclostrobin, procymidone, prochloraz, and fludioxonil based on mycelial growth inhibition at discriminatory doses. Results showed that C. fructicola and C. siamense isolates were highly resistant to procymidone and fludioxonil with 100% resistance frequencies to both fungicides, but sensitive to prochloraz, i.e., no resistant isolates were found. For pyraclostrobin, 74% of C. fructicola isolates showed high resistance, 26% showed low resistance, and all of the C. siamense isolates showed low resistance. No positive cross-resistance was observed between pyraclostrobin and azoxystrobin even when they are members of the same quinone outside inhibitor (QoI) fungicide group or between pyraclostrobin and non-QoIs. Resistant isolates to QoI fungicides were evaluated for the fitness penalty. Results showed that no significant differences except for the mycelial growth rates that were detected between high- and low-resistance isolates of C. fructicola. Molecular characterization of the Cyt b gene revealed that the G143A point mutation was the determinant of the high resistance in C. fructicola. This study demonstrated the resistance status of C. fructicola and C. siamense to different fungicides and briefly discussed implications of that resistance. Demethylation inhibitor fungicides were found to be the best option among the different chemicals studied here, to control peach anthracnose in China.

A Lignocellulolytic Colletotrichum sp. OH with Broad-Spectrum Tolerance to Lignocellulosic Pretreatment Compounds and Derivatives and the Efficiency to Produce Hydrogen Peroxide and 5-Hydroxymethylfurfural Tolerant Cellulases

Fungal endophytes are an emerging source of novel traits and biomolecules suitable for lignocellulosic biomass treatment. This work documents the toxicity tolerance of Colletotrichum sp. OH toward various lignocellulosic pretreatment-derived inhibitors. The effects of aldehydes (vanillin, p-hydroxybenzaldehyde, furfural, 5-hydroxymethylfurfural; HMF), acids (gallic, formic, levulinic, and p-hydroxybenzoic acid), phenolics (hydroquinone, p-coumaric acid), and two pretreatment chemicals (hydrogen peroxide and ionic liquid), on the mycelium growth, biomass accumulation, and lignocellulolytic enzyme activities, were tested. The reported Colletotrichum sp. OH was naturally tolerant to high concentrations of single inhibitors like HMF (IC50; 17.5 mM), levulinic acid (IC50; 29.7 mM), hydroquinone (IC50; 10.76 mM), and H2O2 (IC50; 50 mM). The lignocellulolytic enzymes displayed a wide range of single and mixed inhibitor tolerance profiles. The enzymes β-glucosidase and endoglucanase showed H2O2- and HMF-dependent activity enhancements. The enzyme β-glucosidase activity was 34% higher in 75 mM and retained 20% activity in 125 mM H2O2. Further, β-glucosidase activity increased to 24 and 32% in the presence of 17.76 and 8.8 mM HMF. This research suggests that the Colletotrichum sp. OH, or its enzymes, can be used to pretreat plant biomass, hydrolyze it, and remove inhibitory by-products.