Downy Mildew is once again in the forefront of concerns for Sampson cucumber growers. This disease attacks both the long green and pickling cucumbers. Fall cucumbers are at risk from the beginning of the crop.
As you may recall in 2004, North Carolina experienced one of the most severe epidemics ever of downy mildew in cucumber. Yield losses due to the disease ranged from 30 to 100 percent averaging around 40 percent statewide for spring and fall crops combined. Downy Mildew has remained aggressive on cucumber since then.
Downy mildew is a common disease on all cucurbits (except watermelon) in North Carolina. In North Carolina prior to 2004, the disease did not normally appear until late July and was usually not severe enough on cucumber to warrant fungicide applications. Since 2004, downy mildew has been present at least by the first week of July and very aggressive.
The Cucurbit Downy Mildew Forecasting System is up and going. The forecast system can be found at http://www.ces.ncsu.edu/depts/pp/cucurbit/. Forecasts are posted three times per week on Monday, Wednesday, and Friday. Each forecast gives a risk assessment or likelihood of disease development for areas along a projected pathway of spore movement. In this way, growers can avoid applying costly fungicides before it is necessary. The forecasting system will not dictate when to spray, but will help growers make a more informed decision based on where the disease is and where it is likely to move in the future. Although this system has a much greater benefit in the spring prior to local disease appearance, there is still a wealth of information available from the website.
How should cucumber growers treat the fall season? Plan to treat consistently with appropriate fungicides. Although we cannot predict what will happen, it would be wise to be prepared. Good disease control starts with selecting disease-resistant cultivars. Although cultivar resistance is not sufficient to control the disease, genetic resistance does help. Resistance to downy mildew should be carefully considered along with other horticultural characteristics. Pretty much all of the current varieties have genetic resistance. Growers should decide what fungicides they will use. From our trial work (2004-2010), the following fungicides worked well: Presidio, Ranman, Previcur Flex, Tanos, Curzate, and Gavel. Presidio and Ranman have performed the best in recent trials. Most fungicides will need to be tank mixed with a protectant (e.g., chlorothalonil or mancozeb) and rotated with another product with a different mode of action. Follow label instructions for rates and tank mix partners. Also, pre-harvest intervals should be considered when choosing fungicides as harvest draws near. Spray interval should be five to seven days. Because there are several products that work well, there is not one single “best program” but rather, several good programs.
Downy mildew is active in the fall crop as this is the normal timing for this disease. By integrating the use of disease-resistant cultivars, effective fungicides, and frequent applications, growers can control downy mildew. The main key to success with downy mildew is to be ahead of it with preventative treatments.