On the underside of the leaf, the spots initially appear water soaked. Then under very humid conditions, lesions sporulate, giving them a purplish gray color (distinguishing them from angular leaf spot). On other cucurbit hosts like pumpkin, the lesions may not be as distinct and delineated by the leaf margins; however, sporulating lesions will develop the same purplish gray color on the underside of the leaf.
Fortunately for many growers in Northern climates (or climates that experience a hard freeze), the cucurbit downy mildew pathogen does not survive over winter. Therefore, unless moved by transplanting infected seedlings from infested sources, the spores move from season-long production regions in the south, northward along long-range wind trajectories. Based on this principle, a disease forecasting system is being used to predict the movement of spores up the East coast using forecasted wind trajectories from infected fields, which form disease sources.
Risk of downy mildew developing in other areas as a result of this spore movement is determined based on:
• disease severity at each known source;
• how favorable the weather conditions are predicted to be for spore dispersal from these sources;
• the likelihood that spores will be protected by clouds from sunlight and remain viable in the wind current; and
• how favorable weather conditions might be for infection in the field where the spores could be deposited.
If an area is considered high risk, then conditions are favorable for spore dispersal, movement, deposition, and infection in susceptible crops along that wind trajectory. In a low risk area, the weather may be favorable for spore dispersal from infected fields, but unfavorable for their transport and deposition onto susceptible crops or vice versa.
In response to the recent devastating outbreaks of downy mildew since 2004, grower organizations and 23 land grant universities in the eastern U.S. came together with financial support from USDA and commodity stakeholders to initiate the cucurbit downy mildew ipmPIPE project in 2008. Eighty-three sentinel plots were established along the East coast from Florida to Massachusetts, and as far west as Texas to Wisconsin to monitor the movement and outbreak of downy mildew.
Each plot was planted with six cucurbit types including cucumber, cantaloupe, pumpkin, butternut squash, acorn squash, and watermelon and scouted regularly. Information from confirmed reports of downy mildew from these plots as well as commercial fields was used to develop forecasts and assign risk levels that define the potential for disease development under a given set of conditions. During the growing season, current forecasts can be found at a Cucurbit ipmPIPE website. In 2008, a total of 325 confirmed cases of downy mildew were reported through the North Carolina State University website from sentinel plots, research plots, commercial fields, and backyard gardens in 130 counties located mostly east of the Mississippi River. Approximately 50% of the reports were from cucumber distantly followed by cantaloupe (10%), butternut squash (10%), pumpkin (10%), and watermelon (6%).
Fungicides are one of the most important options for managing downy mildew. Broad-spectrum, protectant fungicides, such as chlorothalonil and mancozeb, will provide some degree of protection when used alone. However, when a moderate to high risk of downy mildew is forecast for a production area, fungicide programs should be shifted to include more effective, systemic products that will penetrate the top leaf surface and translocate to the lower leaf surface.
There are several effective mobile fungicide products available for managing cucurbit downy mildew. These fungicides include: Ranman (cyazofamid, FMC Agricultural Products, FRAC code 21), Presidio (fluopicolide, Valent U.S.A. Corp., 43), Previcur Flex (propomocarb HCL, Bayer CropScience, 28), Curzate (cymoxanil, DuPont Crop Protection, 27), Tanos (cymoxanil and famoxadone, DuPont, 27 + 11), and Gavel (mancozeb and zoxamide, Dow AgroSciences, M3 + 22).
However, there is a risk of the pathogen developing resistance to any one of these single-site mode of action chemistries, as it has already done to QoI (code 11) and Ridomil (4) fungicides. In order for the remaining products to continue to be effective, it is important to integrate them into resistance fungicide management programs that include tank mixing with a broad-spectrum protectant and alternating between fungicides with different FRAC codes to reduce the chances for resistance development.
Note that Curzate and Tanos have the same effective component (cymoxanil) and thus should not be used in alternation. Importantly, growers should use the downy mildew forecasting system to help time the incorporation of systemic fungicides into fungicide programs and make more informed management decisions on cucurbit downy mildew control.