Extension-based Fungicide Resistance Monitoring for Cucurbit Downy Mildew

Downy mildew is a disease that has potential to devastate cucurbit crops in a matter of days during rainy production seasons. This disease affects cucumbers, summer and winter squash, melons, and pumpkin. Effective management of cucurbit downy mildew requires fungicides due to rapid disease development and high inoculum potential. Downy mildew fungicides are among the most expensive agrichemicals KY specialty crop producers use, with per-application costs approaching $50 / acre solely for chemical. As with all fungicides, these are most effective when used preventatively, yet excessively premature treatment results in poorly utilized or unnecessary sprays. The pathogen that causes cucurbit downy mildew does not overwinter in KY but is rather blown into the state from areas with less challenging winter conditions. The cucurbit downy mildew IPM-pipe ( cdm.ipmpipe.org ) is a free disease forecasting resource that has helped growers throughout the eastern U.S. better utilize chemical fungicides. By entering their farm location, growers receive alerts when their individual crops come under risk of downy mildew based on proximity of positive identifications of the disease and projected weather patterns. Using this information, growers may fine-tune the timing of their expensive fungicide applications to periods when their crops are at greatest risk of disease. This refinement of their spray program can result in environmental and economic savings, but only if the chemicals used are effective in managing the disease. Potential resistance to various downy mildew-specific fungicides has been suggested by many researchers working with cucurbits in the past. These reports can vary by region and by cropping season. Recently, a peer-reviewed journal article detailed a robust method for evaluating fungicide resistance in-season using treated live plants (Keinath, 2016; Pl. Dis. 100(2) 490-499). With KY's somewhat centralized location in the eastern U.S., there is potential for disease-causing inoculum to be introduced from southern, western, and northern locations. Inoculum from different locations may have differing sensitivities to frequently-used fungicides, depending on the crop of interest and where it is produced (greenhouse vs. field). In the past, KY's inoculum source was generally assumed to be the southeastern U.S., however, yearly weather dynamics may influence this. The geographical east-west breadth of the commonwealth also could result in different areas of KY having different inoculum sources, with far western KY receiving inoculum from a Texas source, while eastern KY receives inoculum as assumed from the southeastern source. Through this project, our objective is to implement an in-season fungicide resistance monitoring network for cucurbit producers to help inform fungicide recommendations on a regional level and with respect to local downy mildew pathogen populations. Network sites would be located at five geographically separated county extension offices and would be monitored by extension agents. For the 2017 season, the Kentucky Vegetable Growers Association provided support for four initial sites, and this SCBGP request would allow for an additional six sites as well as maintenance of the original four for the 2018-2020 field seasons. This project has the potential to reduce expensive fungicide applications while maintaining a high level of disease management. Educational and awareness efforts will include timely reporting of results to stakeholders in an effort to help inform their fungicide choices.