PCR-RFLP analysis for detecting potential QoI and CAA fungicide resistance in onion and lettuce downy mildews

Downy mildew of onion and lettuce, caused by Peronospora destructor and Bremia lactucae, respectively, is distributed worldwide. Control of these serious diseases largely relies on oomycete fungicides including quinone-outside inhibitors (QoIs) and carboxylic acid amides (CAAs), both of which bear the risk of resistance development in pathogens. Resistance to QoI and CAA fungicides has not been confirmed in field isolates of onion and lettuce downy mildews to date, and monitoring using biological methods is challenging as both pathogens are obligate biotrophs and cannot be grown in agar culture. In this study, the RFLP-method using PCR-amplified gene fragments encoding the fungicide targets cytochrome b (cytb) and cellulose synthase 3 (CesA3) has been developed for detecting potential QoI and CAA resistance in P. destructor and B. lactucae, as this is likely to develop in the future. This method will make it possible to diagnose resistance prior to efficacy reduction of fungicides contributing to sustainable disease control.

Automated summary

Downy mildew of onion and lettuce, caused by Peronospora destructor and Bremia lactucae, respectively, is a worldwide disease. Current control methods rely on oomycete fungicides, but the development of resistance in the pathogens is a risk. In this study, a method using RFLP and PCR-amplified gene fragments has been developed for detecting potential resistance to fungicides, which will contribute to sustainable disease control.