Widespread QoI Fungicide Resistance Revealed Among Corynespora cassiicola Tomato Isolates in Florida

The mechanisms of target and non-target resistance to QoIs in Corynespora Cassiicola

Corynespora leaf spot, caused by Corynespora cassiicola, is a foliar disease in cucumber. While the application of quinone outside inhibitors (QoIs) is an effective measure for disease control, it carries the risk of resistance development. In our monitoring of trifloxystrobin resistance from 2008 to 2020, C. cassiicola isolates were categorized into three populations: sensitive isolates (S, 0.01 < EC50 < 0.83 μg/mL), moderately resistant isolates (MR, 1.18 < EC50 < 55.67 μg/mL), and highly resistant isolates (HR, EC50 > 56.98 μg/mL). The resistance frequency reached up to 90% during this period, with an increasing trend observed in the annual average EC50 values of all the isolates. Analysis of the CcCytb gene revealed that both MR and HR populations carried the G143A mutation. Additionally, we identified mitochondrial heterogeneity, with three isolates carrying both G143 and A143 in MR and HR populations. Interestingly, isolates with the G143A mutation (G143A-MR and G143A-HR) displayed differential sensitivity to QoIs. Further experiments involving gene knockout and complementation demonstrated that the major facilitator superfamily (MFS) transporter (CcMfs1) may contribute to the disparity in sensitivity to QoIs between the G143A-MR and G143A-HR populations. However, the difference in sensitivity caused by the CcMfs1 transporter is significantly lower than the differences observed between the two populations. This suggests additional mechanisms contributing to the variation in resistance levels among C. cassiicola isolates. Our study highlights the alarming level of trifloxystrobin resistance in C. cassiicola in China, emphasizing the need for strict prohibition of QoIs use. Furthermore, our findings shed light on the occurrence of both target and non-target resistance mechanisms associated with QoIs in C. cassiicola.

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.

Defining Fungicide Resistance Mechanisms in the Corynespora cassiicola Population from Mississippi Soybean

Target spot, caused by Corynespora cassiicola, is a common lower canopy soybean disease in the southern United States. Recently, target spot has resurged in importance especially following the identification of resistance to the quinone outside inhibitor (QoI) fungicides. As a result, a survey of C. cassiicola from soybean throughout Mississippi began in 2018. A total of 819 C. cassiicola monoconidial isolates were obtained from 228 fields in 75 counties. The molecular mechanism of QoI resistance was determined, which resulted from an amino acid substitution from glycine (G) to alanine (A) at position 143 using a PCR-RFLP method and comparing nucleotide sequences of the cytochrome b gene. Five previously defined geographic regions were used to present the distribution of the G143A substitution and included the Capital, Coast, Delta, Hills, and Pines. The Capital had the greatest proportion of G143A-containing isolates (95.0%), followed by the Coast (92.9%), Delta (89.8%), Pines (78.8%), and Hills (69.4%). In all, 85.8% of the C. cassiicola isolates carried the G143A substitution. In addition, the effective fungicide concentration (EC50) of randomly selected C. cassiicola isolates to azoxystrobin was used to characterize isolates as resistant (n = 14) (based on the presence of the G143A substitution and EC50 values >52 μg/ml) or sensitive (n = 11) (based on the absence of the G143A substitution and EC50 values <46 μg/ml). The EC50 values varied among isolates (P < 0.0001), with QoI-sensitive isolates exhibiting lower EC50 values than QoI-resistant isolates. The current study revealed that a reduction in sensitivity to QoI fungicides has likely resulted based on the percentage of C. cassiicola isolates containing the G143A substitution identified in Mississippi.

Microtiter plate test using liquid medium is an alternative method for monitoring metyltetraprole sensitivity in Cercospora beticola

BACKGROUND

Metyltetraprole is a new quinone outside inhibitor (QoI) fungicide showing potent activity against QoI-resistant fungi that possess the G143A cytochrome b mutation, which confers resistance to existing QoIs such as trifloxystrobin. For its sustainable use, monitoring of metyltetraprole sensitivity is necessary and the establishment of appropriate methodology is important in each pathogen species.

RESULTS

In Cercospora beticola, the causal agent of sugar beet leaf spot, some isolates were less sensitive to metyltetraprole (EC50  > 1 mg L-1 , higher than the saturated concentration) using the common agar plate method, even with 100 mg L-1 salicylhydroxamic acid, an alternative oxidase inhibitor. However, microtiter tests (EC50  < 0.01 mg L-1 ), conidial germination tests (EC50  < 0.01 mg L-1 ) and in planta tests (>80% control at 75 mg L-1 run-off spraying) confirmed that all tested isolates were highly sensitive to metyltetraprole. For trifloxystrobin, G143A mutants were clearly resistant upon microtiter plate tests (median EC50  > 2 mg L-1 ) and distinct from wild-type isolates (median EC50  < 0.01 mg L-1 ). Notably, mycelium fragments were usable for the microtiter plate tests and the test was applicable for isolates that do not form sufficient conidia. Our monitoring study by microtiter plate tests did not indicate the presence of metyltetraprole-resistant C. beticola isolates in populations in Hokkaido, Japan.

CONCLUSION

The microtiter tests were revealed to be useful for monitoring the sensitivity of C. beticola to metyltetraprole and trifloxystrobin. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.