Cucurbit downy mildew is caused by two types of the oomycyte Pseudoperonosora cubensis. The first type, known as Clade I, infects pumpkins, watermelon and squash. Clade II infects cucumbers and cantaloupe. Clade I is not seen as frequently as Clade II and shouldn’t be treated until it is detected in your fields or your region.

Cucumbers are highly susceptible to downy mildew from the Clade II type of pathogen, and growers may be able to easily recognize the telltale symptoms – angular brown and yellow lesions restrained by the leaf veins, so they resemble blocks. Tissue necrosis occurs, and spores are visible as darkened areas located on the undersides of leaves. Plant yield is reduced, and plant death occurs if left untreated. But downy mildew infections from Clade I pathogens don’t look quite the same.

“Downy mildew looks strikingly different than in does in cucumbers” when found on other cucurbit crops, Dr. Doug Higgins, Michigan State University, said during the virtual Great Lakes Expo. “We get more of an irregular lesion pattern.”

When found in other cucurbits, downy mildew lesions are more irregular than those seen in cucumbers, and there is some variation in color and appearance of the lesions, with some looking purplish. Lesions are not contained to the leaf veins as in cucumbers. In pumpkin, it looks like brown and yellow flecks on the green leaves. The infection ultimately causes yellowing and necrosis and can quickly defoliate plants.

Environmental factors needed for downy mildew to survive include high relative humidity levels and a film of moisture on the plant leaves. The pathogen favors cool to moderate temperatures, which is why it’s often most prevalent during the second half of the growing season, when temperatures begin to cool down.

High humidity “helps the pathogens to produce spores, and helps the spores survive,” Higgins said. As an airborne pathogen, the spores of downy mildew are loosened from their stalks and blown by the wind from plant to plant. Luckily, P. cubensis can’t survive without a living host. In cold regions, it does not overwinter and must be re-introduced each season. In the greenhouse environment, or in warmer growing regions where cucurbits can be grown year-round, the pathogen can accumulate.

For Clade II, no treatment is needed in fields unless spores are found. Spore detection units are being used at MSU to capture spores from the air and test for the presence of downy mildew and other pathogens, via light microscopy or molecular techniques. Weekly reports are posted so Michigan growers can be alerted to the arrival of the pathogen in their region.

According to Cornell University, early planting can be one strategy to protect against the disease, as some cucurbits could be harvested before the disease arrives, which is typically sometime between July 1 and Aug. 31 in New York State.

Clade II infections are known to have resistance to some fungicides, including those from at least four different FRAC codes. But products with other modes of action are effective. Rotation of fungicides using different modes of action, and scouting for any signs of resistance in the field, are necessary.

Powdery Mildew

Powdery mildew is a true fungal pathogen. It forms spores that appear as powdery substances, which typically first appear on the undersides of leaves, and which eventually will cover the entire leaf surface and cause a decrease in photosynthesis, reducing overall yield. Leaf necrosis will occur.

The infection can also move into the stem of cucurbits, and for pumpkin growers, this alone can make the crop unmarketable. The disease also predisposes stems to other diseases.

According to information from Cornell, fruit infections in cucumber and watermelon are rare, as the disease primarily infects leaves. Fruit quality and quantity is decreased due to defoliation, with incomplete fruiting, ripening, reduced size and sunscald also occurring.

Powdery mildew in cucurbits is caused by Podosphaera xanthii, which is airborne. The spores, like with downy mildew, are windblown from plant to plant. Unlike downy mildew, primary infections favor warm and moist (not cool) conditions. But once the initial inoculum infects the plant, the environmental conditions aren’t really important, and a wide range of temperatures can favor propagation and spread. This leads to rapid development of disease symptoms. The pathogen does not require moisture on the leaf surface, as does downy mildew.

“There’s a lot of races of powdery mildew,” Higgins said, but some newer cultivars have some resistance to the disease. Modern cucumber cultivars often have resistance. Pumpkins and squash have some modern cultivars with moderate resistance, with the best option being those with resistance from both parents (homozygous resistance). In melons, resistance to race 1 and race 2 powdery mildew pathogens is of primary importance, particularly in the eastern U.S.

Early signs of powdery mildew should trigger fungicide applications. Tank mixes with fungicides having different modes of action are recommended, as controlling for resistance is important. Tank mixes should contain a systemic product and protectant fungicide in combination. FRAC codes for systemic products should be alternated. Protectant fungicides include potassium bicarbonate, sulfur and copper hydroxide. These need to be applied in large volumes to provide complete coverage to the crop.

“Scouting is still important here,” Higgins emphasized. “You’re going to want to catch this disease very early.”

According to information from Cornell, another difference between cucurbit powdery mildew and cucurbit downy mildew is powdery mildew’s production of sexual reproductive structures which do not need a living host to survive, meaning that at least theoretically, they can overwinter, possibly using alternative hosts. The role of these structures in the spread of powdery mildew is not well understood, although both mating types needed for reproduction have been found regularly. However, powdery mildew infections are thought to primarily occur through inoculation with the airborne asexually produced spores, which are obligate parasites.

Properly identifying the first signs of downy or powdery mildew is crucial for successful cucurbit production. Both diseases can spread rapidly from plant to plant and cause significant loss of yield and quality. Just as important is a scouting and treatment protocol which accounts for resistance management. Best practices include the use of resistant cultivars whenever practical.