How can I control downy mildew on sugar pumpkins?
Answer: The good news is that only the leaves are infected. Unfortunately, this disease progresses quite rapidly, and can reduce crop yield, and because there is less leaf protection, sun scald on the pumpkin can become a problem. Symptoms appear four to 12 days after infection, so an apparently healthy crop can already be infected. The pathogen likes it cool and moist. Optimum conditions for sporulation are 59 degrees F with six to 12 hours of moisture present (usually in the form of morning dew). Copper sprays would likely be effective in halting the infestation.
Because this disease is carried to most fields on light winds, cultural practices like crop rotation and sanitation have a limited effect on the incidence of downy mildew. Still, there are several things that growers can do to suppress the disease. Growing vigorous plants, capable of withstanding or repelling disease onslaughts, is the first step. This involves careful irrigation and soil fertility management. You’ll find links to a host of useful ATTRA resources on soil management and related matters on the Soils & Compost section of our website.
Good soil fertility management can often be backed up with foliar fertilization, which some growers believe can assist in pest resistance.
Further cultural considerations include selecting growing sites with good air drainage, full sunlight, and low humidity. Using drip irrigation, or scheduling overhead irrigation to avoid excessive leaf wetness, will also reduce disease incidence. When detected early, disease spread might be slowed somewhat by removing and destroying infected plants, and by taking care not to transport the disease by hand or on infected tools and equipment.
Along with resistant varieties, fungicides are considered the principal means of downy mildew control in cucurbits. There are several alternatives to synthetic fungicides. Be certain to use all pesticides, synthetic or natural, according to label instructions. The listing below these descriptions was taken from ATTRA’s Ecological Pest Management Database.
Copper-based fungicides have traditionally been recommended for suppressing downy mildew in organic production systems. Caution is advised, however, as copper can be phytotoxic to cucurbits. Crop damage appears to be most common during periods of cool, wet weather—precisely the conditions in which downy mildew thrives. As a result, it is suggested that the most dilute application recommended for each product be followed.
The use of copper fungicides in organic production is somewhat controversial. Copper is a regulated material in organic production. Though an essential plant nutrient in small amounts, fungicidal levels of copper are directly toxic to some beneficial organisms such as earthworms and several soil microbes such as blue-green algae—an important nitrogen-fixer in many soils. Excessive use can also result in the buildup to toxic (crop-damaging) levels in the soil—particularly in climates where little to no leaching occurs. Thus, growers who use these sprays frequently must monitor soil copper levels through regular soil testing.
Neem oil is a botanical pesticide derived from the tree species Azadirachta indica. It is a multi-purpose insecticide, miticide, and fungicide labeled for control of both downy and powdery mildews on cucurbits.
Neem products, once considered largely benign to beneficial insects, have demonstrated some negative impacts. Washington State research has found neem to be toxic to ladybeetles, especially in their early larval stages. Being an oil formulation, neem can also harm bees and should be applied when they are not active in the field. Therefore, while neem oil is suitable for organic production, it should not be used without clear need and plenty of caution.
Serenade™, a fungicide based on the biocontrol agent Bacillus subtilis, is available in a wettable powder formulation that can be used for downy mildew control on vegetables. According to its manufacturer, Serenade turns on the plant’s natural immune system. It is also said to: “…stop plant pathogen spores from germinating, disrupt the germ tubes and mycelial growth and inhibit attachment of the plant pathogen to the leaf by producing a zone of inhibition restricting the growth of…disease causing pathogens.” For sources of the product, consult ATTRA’s Ecological Pest Management Database.
Note: The mention of specific brand names is for educational purposes only and does not constitute endorsement by NCAT, ATTRA, or USDA.