Downy mildew, caused by a fungal organism, and is most destructive to cucumber and cantaloupe, though all cucurbits are susceptible. Symptoms first appear as pale green areas on the upper leaf surfaces. These change to yellow angular spots. A fine white-to-grayish downy growth soon appears on the lower leaf surface. Infected leaves generally die but may remain erect while the edges of the leaf blades curl inward. Usually, the leaves near the center of a hill or row are infected first. The infected area spreads outward, causing defoliation, stunted growth, and poor fruit development. The entire plant may eventually be killed.
The fungus is easily carried by wind currents, rain splash, farm implements, or the hands and clothes of farm workers. It is favored by cool to moderately warm temperatures, but tolerates hot days, although long periods of dry hot weather can stifle the spread of the disease. Unlike powdery mildew, it requires humidity to flourish. Therefore, downy mildew is most aggressive when heavy dews, fog, and frequent rains occur.
Downy mildew does not overwinter beyond Mexico and the southernmost tier of U.S. states, where it survives on cultivated and wild cucurbit plants. Spores are blown northward each season as favorable seasonal conditions advance. As a result, the disease is most common on late summer plantings and is infrequently seen on spring cucurbits.
Keeping abreast of when, and how severely, downy mildew is occurring in your area can help you determine the proper time to treat it. The North American Plant Disease Forecast Center is an online forecasting network that tracks outbreaks of downy mildew from March through the end of the growing season.
Data is posted twice weekly. Growers can use the website to identify areas where an outbreak is reported, as well as spore movement in that area. The site also offers information on control measures, photos, and more. Because the website relies on growers and others to report the outbreaks, it isn’t comprehensive or foolproof. It is, however, a useful monitoring tool.
One of the principal means of managing downy mildew in cantaloupe and cucumber is the use of genetically resistant cultivars. Resistance has not been developed in other cucurbits, though some squash varieties like Super Select and Zucchini Select are considered to be tolerant, as are cucumber varieties like ‘Poinsett’ and ‘Galaxie’. The Virginia Extension publication Downy Mildew of Cucurbits identifies other resistant cucumber cultivars. Gardeners are advised to contact Cooperative Extension and local seed suppliers for assistance in selecting resistant varieties that also perform well in their location.
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.
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.
Neem oil is a botanical fungicide; 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 lady beetles, 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.
Peroxides Organic growers and others in alternative agriculture have often mentioned hydrogen peroxide (H2O2) as a disease preventive for crops. While documentation on the use of food- and/or pharmacy-grade peroxide in managing plant diseases is sketchy, BioSafe Systems has recently released a peroxigen formulation under the name of OxiDate™, which is labeled as a broad-spectrum bactericide and fungicide. Downy and powdery mildews of cucurbits are among the diseases it is said to control. Among the listed benefits are biodegradability, little to no phytotoxicity, and the ability to kill fungal spores on contact.
Although the Organic Materials Review Institute (OMRI) had previously approved OxiDate for organic production, it removed the product from its listing in spring 2002 because of non-compliance with federal regulations. If reformulated, it may be approved again in the future.
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