Downy Mildew is a type of foliar plant disease that spreads under cool, wet conditions and affects many different crops. Ornamental flowers, grapes, onions, basil, lettuce and cucumber all get Downy Mildew, a parasitic pathogen in the Peronosporaceae family. Although they all come from the same “big, happy family”, different species of Downy Mildew attack different crops. For example, Bremia lactucae affects lettuce, Pseudoperonospora cubensis affects cucurbits and Peronospora belbahrii affects basil. (Can you imagine calling that family to dinner?)
Each species of Downy Mildew favors slightly different temperature conditions, but overall, cool and damp is what they all dig. Spores are spread by wind or splashing water. Once a leaf is infected with Downy Mildew, the presence of moisture will result in the germination of more spores, which then travel to unaffected leaves through any available film of water. Rain, morning dew that is slow to evaporate, or even overhead irrigation for prolonged periods or on an overcast day can all spell trouble when it comes to Downy Mildew.
Downy Mildew rarely causes the death of a plant, but it will cause foliar losses. For some crops, like lettuce and basil, this can result in loss of the marketable crop. For other crops, like melons and pumpkins, it can result in loss in productivity, fruit size, and fruit quantity. Leaves infected with Downy Mildew can also lead to secondary infections and diseases.
Downy Mildew in Lettuce
Bremia lactucae, Downy Mildew in lettuce, thrives at cool temperatures and under damp, overcast or low-light conditions. Sounds like fall growing conditions on the east coast or winter high tunnel growing conditions! The disease is also a problem in the cool, coastal regions of California, where conditions are often foggy or dew-dense.
Older leaves show symptoms first: yellow, angular spotting on the upper side of the leaves with white, cotton-y (“downy”) patches on the underside indicating sporulation. Eventually, the patches turn necrotic, dried up and brown.
DM in Spinach
Peronospora farinosa f. sp. Spinaceae, Downy Mildew in spinach has similar symptoms and thrives under similar environmental conditions as Downy Mildew in lettuce. Yellow spots form on the leaves; these spots later enlarge and become tan and dry. The underside of the leaf will show purple fuzz indicating growth of the sporangia and sporangiospores. If disease development is extensive, leaves can appear blighted. Because of the layered growth habit of spinach and the density of plants in most fields, conditions may remain conducive for disease development even during seemingly dry conditions.
DM in Cucurbits
Pseudoperonospora cubensis, Downy Mildew in cucurbits, is a late season disease. It doesn’t overwinter in the north, and it comes up from the south by degrees, carried on storms, arriving usually in July and August. It can be spread by storms, wind and cucumber beetles. The disease is favored by high humidity but doesn’t need temperatures to be quite as cool as other strains of Downy Mildew, so those late summer storms and humid days can offer perfect conditions for spreading disease. Downy Mildew is less of a problem on squashes and pumpkins because they get the disease later and are affected less. Melons affected by Downy Mildew will continue to produce fruit but it will be poorer quality. Downy Mildew is most serious for cucumbers, which can drop dead in a week and stop producing any fruit to harvest. Of all the cucurbit crops, cucumbers are most severely and widely affected. Downy Mildew exists as different pathotypes that differ in their ability to infect various crops, but cucumber is susceptible to all the different pathotypes.
Like with lettuce, the disease starts with angular yellow lesions on the top of the leaf that are delineated within the veins of the leaf (since the pathogen cannot grow past the veins). As the disease progresses, the yellow lesions turn brown and corresponding fuzzy lesions develop on the underside of the leaf (sporulation phase). Eventually areas of the leaf or the entire leaf become necrotic (dried and dead). Uniquely, in watermelons, the yellow lesions may be non-angular, going past the leaf veins, and the presence of Downy Mildew causes the leaves to curl upwards.
So What’s a(n Organic) Grower to Do?
For organic growers, there aren’t a lot of options for treating Downy Mildew. According to the Resource Guide for Organic Pest and Disease Management, copper has shown mixed results in treating different species of Downy Mildew. So prevention is the best – and pretty much only – option.
Since Downy Mildew needs moisture to spread, it follows that cultural practices promoting good air circulation (i.e. pruning, adequate plant spacing), and therefore allowing more thorough drying of the leaves, helps to prevent the spread of Downy Mildew. Try to select planting sites with good air movement and avoid overhead irrigation.
Choosing resistant varieties can offer an important line of defense against Downy Mildew in your crops, and even for conventional growers, relying on genetic resistance can greatly reduce the amount of fungicidal chemical treatments required.
Resistance in Lettuce: Breeding for Downy Mildew resistance in lettuce is very up-to-date (due to the significance of the California baby leaf market) and there is a wide selection of varieties with various levels of resistances. In fact, there exists a sort of arms race between commercial lettuce breeders and the latest strain of Downy Mildew, which is always evolving to overcome genetic resistances. At this point, the highest level of Downy Mildew resistance is to race 28. However, it is unknown whether, and probably unlikely, that race 28 is present on the East Coast. Which Downy Mildew races are present in the eastern part of the country isn’t known because there isn’t testing done like there is in California. We carry many lettuce varieties that are Downy Mildew resistant – be sure to check out our lettuce comparison chart, listing all disease resistance types.
Resistance in Spinach: Similar to lettuce, Downy Mildew resistance in spinach is up-to-date and fast-evolving. The latest strain of Downy Mildew in spinach, race 14, was identified in late 2012. Before 1990, there were only 3 races identified of Downy Mildew in spinach, and new races have been emerging at a faster rate in the last decade. This is in California, where there is significant spinach production. In the East and Midwest, Downy Mildew in spinach has not been a major problem, according to both our organic spinach breeders/wholesalers and to Cornell’s plant pathologist and plant-microbe biologist, Meg Tuttle McGrath. McGrath posits that the low occurrence of Downy Mildew in the eastern U.S. reflects cleaner seed, effective resistant varieties, and appropriate cultural practices used by growers. She advises growers in the northeast to choose resistant varieties; however, since the disease has not been occurring routinely, this may not be as critical as with some of the other crop diseases. All but three of the spinach varieties we carry have Downy Mildew resistance; check out our spinach comparison chart for more details.
Resistance in Cucumber: Up until 2004, there were good options for Downy Mildew resistant cultivars in cucumbers. However, in 2004 the pathogen adapted and overcame the genetic resistance and since then has been a major problem in cucumbers. Varieties which had previously demonstrated resistance, like Marketmore 76 and H-19 Little Leaf, became susceptible. Adam F1 gherkin shows intermediate resistance to Downy Mildew. Adam F1 will get Downy Mildew, but will develop symptoms later than in the most susceptible varieties, and will not get completely defoliated either. You can live with the DM resistance level in Adam F1, as long as the disease pressure is not too high. Cornell University is working on breeding new genetic resistance in cucumbers, and has made some promising selections in recent years. So, stay tuned for more disease resistance varieties in the future.