By late summer, the leaves of pumpkins and winter squash often appear more white than green as a result of powdery mildew infection. This disease is common on cucurbits, not just in the U.S. but around the world. All species of cucurbit crops are susceptible, although resistant varieties are available. In some years and locations the disease can cause major reductions in production and/or fruit quality, and thus, profitability.
Damage can be diverse. Not only can powdery mildew reduce yields because of decreased fruit size or number, but the premature senescence of infected leaves can also result in lower market quality if fruit become sunburned, or if they ripen prematurely. Winter squash that are infected with powdery mildew may not keep as long in storage. Infected melons may have low soluble solids and poor flavor. Pumpkins with powdery mildew infection often have poor rind color and shriveled handles, and speckling or other imperfections on their rinds. In addition, powdery mildew infection predisposes plants to other diseases, such as gummy stem blight (also known as black rot).
Powdery mildew is an aptly-named disease, because it is characterized by a pure white ‘fuzzy’ growth on the upper and lower leaf surface, petioles and stems. The white powder is actually made up fungal spores called conidia, which are spread by wind. Unlike many other foliar diseases, powdery mildew tends to develop and spread rapidly under dry conditions, without rainfall, although it does thrive in high relative humidity.
Have spore, will travel. Powdery mildew spreads via airborne conidia. It is believed that the disease moves up the east coast to new plantings from locations where cucurbit crops are grown earlier in the year. It’s also possible to have local sources of initial infection, from greenhouse-grown cucurbits and verbena planted prior to field production. In addition, successive cucurbit plantings can lead to the spread of infection from field to field on a farm so new plantings should be physically separated from older plantings to limit the movement of conidia.
Powdery mildew can spread quickly because it only takes 3 to 7 days from initial infection to the appearance of symptoms, and a large number of conidia can be produced in a short time. Favorable conditions for the disease include dense plant growth, low light intensity and high relative humidity. Infection can occur at 50 to 90 degrees F but 68 to 80 degrees F is ideal. If it gets very hot, over 100 degrees F, the disease does not spread.
Resistant varieties. An increasing number of cucurbit varieties are available that have some resistance to powdery mildew. The use of these varieties does not necessarily assure that powdery mildew will not become a problem, but rather, it can delay the onset of the disease or slow its development. Varieties that get their resistance from both parent lines (homozygous) are more effective than those that get resistance from just one parent (heterozygous). In catalogs these two types of varieties may be described as resistant and tolerant, respectively.
The best way to manage powdery mildew on organic farms is to use varieties with resistance to powdery mildew in combination with timely applications of an effective, organically approved fungicide.
Find it early. Once you can easily see signs of powdery mildew when walking through a field, it is too late to apply any treatments for control. Organically-approved fungicides (as well as conventional materials) work by preventing infection of healthy tissue, so starting treatment early is key to their effectiveness. Uncontrollable, established powdery mildew colonies [spots?] not only do not disappear when treated, they continue producing 100s of spores. These give the spots their powdery appearance. The more spores, the more opportunity that some will land by chance on leaf tissue lacking spray deposit.
Start by scouting the oldest cucurbit plantings once a week as soon as the first fruit start to enlarge. Examine 50 leaves by looking closely at the top and underside of 5 old crown leaves in 10 different locations in a field. The first signs of the disease are typically seen on shaded lower leaves, on the leaf undersides.
What to spray. There are quite a few materials that are labeled for control of powdery mildew that can be used by organic farmers. Many of these materials, including several bio-fungicides, can provide some suppression of the disease if applied in a timely fashion. However, only a few materials provide consistent levels of control, according to ongoing research by Dr. Meg McGrath at Cornell’s Long Island Horticultural Research and Extension Center. Her studies show that sulfur and ‘stylet’ oil are the most effective materials for managing powdery mildew. Fixed copper fungicides have overall been less effective, but can be highly effective, perhaps reflecting variation among formulations, Organic farmers should check with their certifying agent before applying any of these materials to be sure they use an approved formulation.