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Managing Powdery Mildew on Cucurbits

By Dorothy Noble


Powdery mildews have been affecting crops for over 150 years. Fruit trees, grasses, ornamentals, vegetables - virtually every type of plant, including weeds, is susceptible to the fungus.

In vegetable production, cucurbits in particular demand management. Powdery mildew is the most common disease for this group, attacking all melons, cucumbers, pumpkins, zucchinis, and summer and winter squashes. However, several products and techniques are available to help manage powdery mildew.

The fungus typically first appears as powdery white spots, which are mostly spores, on the plant's leaves. As wind spreads the spores, the fluffy white fungal growth coats the upper and lower sides of leaves, stems and vines. The infected leaves usually wither and die. With severe fungus feeding, the entire plant may become stunted, turn yellow and possibly die.

Premature loss of leaves reduces yield and quality. The disease can inhibit the plant's normal fruit ripening. Premature fruit has less sugar content, which compromises flavor and storability. With leaf loss, the fruits suffer sunscald. The fruit rind may be speckled, poorly colored or both, and handles may be shriveled or even rotten. The number of fruits may be reduced, and fruit size may be diminished. Malformed fruit can result. Plus, the ailing, infected plant is more likely to be invaded by other diseases as well. Powdery mildew expert Dr. Margaret T. McGrath, a Cornell pathologist who has conducted extensive cucurbit research in Long Island, N.Y., notes that gummy stem blight especially can follow powdery mildew.

Conditions favoring development of the disease include dry weather, moderately warm temperatures, fertile soil, and dense plantings with lush growth and poor air circulation, as well as low light intensity. However, it also prefers humidity, which irrigation and plant canopies can easily supply.

Tom Turini, University of California Extension advisor in Fresno, Calif., observes that in desert melon fields, the melon canopy is dense enough to provide 45 percent or higher humidity. "The dry desert doesn't stop powdery mildew from being a problem," he says.

Judson Reid, Cornell Cooperative Extension vegetable specialist in Yates County, N.Y., comments that the disease typically appears in the humid environment of high tunnels sooner than in the field.

According to McGrath, powdery mildew does not need free water to infect. "In contrast with other foliar fungal pathogens, rain is actually unfavorable to the pathogen," she says.

University of Georgia plant pathologist Dr. David Langston observes, "We don't see as much powdery mildew in wet years."

The powdery mildew pathogens affecting cucurbits are host-specific; the powdery mildew on nearby ornamentals will not infect the vegetable crop.

Cultural controls that can assist with powdery mildew management include avoiding planting in low, shady locations; avoiding nitrogen fertilizers late in the crop season; using drip rather than overhead irrigation; pruning overcrowded plantings; and removing and destroying infected plant parts. Also, avoid stressing plants. McGrath has noticed powdery mildew symptoms on young field-grown plants that were stressed due to delayed transplanting or surrounded by tall weeds.

McGrath's research indicates that both resistant varieties and fungicides are essential to manage powdery mildew. "An integrated program with both management tools is needed to achieve effective control, because the pathogen is adept at evolving new strains resistant to individual tools," she explains. "It is more difficult for new pathogen strains to develop when an integrated program is used. Thus, effective control is more likely."

Researchers recommend planting resistant varieties of cucurbits, more of which are being released each year. Mark Willis, vegetable seed product manager for Harris Seeds, reports that the newer vine crops have genetic resistance to powdery mildew. He says, "Most released in the past five or six years have some resistance."

Kevin Skaling, president of DP Seeds, agrees: "More resistance is being built into cucurbits, and work is continuing. This is why more money is being spent on breeding and research."

Since pathogens have a history of developing resistance to fungicides, it's imperative to rotate applications of fungicides with different modes of action. Fungicides that share similar modes of action may have similar risks for resistance development. In addition, some fungicides have been identified as having a high risk for resistance, and overusing them leads to resistance. The rotation concept cannot be understated; a few fungicides have become ineffective because resistance developed within a year of introduction. In fact, powdery mildew is the prime example of swift resistance. The modes of action are detailed by the Fungicide Resistance Action Committee (FRAC) and designated by FRAC codes, which are found on fungicide labels.

McGrath reports that a key to effective powdery mildew management is using mobile fungicides targeted to the particular disease. (Additional fungicides would be needed to manage other diseases, such as downy mildew and Phytophthora blight.) Mobile products are needed for control on the undersides of leaves. Plus, the application must alternate among the targeted mobile fungicides and be incorporated into a tank mix with protectant fungicides.

McGrath says, "The goal is to delay development of resistance, not manage resistant strains afterward."

Vegetable varieties that were once resistant to powdery mildew sometimes succumb in a season. The powdery mildew fungus, like many other pathogens, has the capacity to evolve new strains. Plus, the resistance of many cucurbits is race-specific. A different race or strain limits the ability of the plant to withstand the pathogen. These factors reinforce the need for effective resistance management.

Since various regions and localities report differences in resistances and performance among cucurbit varieties, plus differences in effectiveness and availability of fungicides, growers need to consult their local extension offices for product recommendations. McGrath reminds growers that the fungicide label supersedes any conflicts in usage directions.

For maximum control in susceptible areas, some researchers advise initiating controls before powdery mildew signs become visible. Others counsel applying preventive fungicides after the cucurbit runners appear. In any event, scouting is critical to early detection of powdery mildew. McGrath reports that the plants are most susceptible in the field after fruiting begins, but are susceptible at any age when grown in a greenhouse.

McGrath's scouting protocol entails weekly examination of both leaf surfaces of five old-crown leaves in at least 10 locations throughout a field. Start applications when powdery mildew is found on at least one of the 50 examined leaves.

If started early and repeated about weekly, many protective fungicides approved for organic production can be effective. These include mineral and botanical oils, sulfur, copper, potassium bicarbonate and biofungicides. Copper also helps control several other diseases, and more biopesticides are being developed.

McGrath says, "Powdery mildew is a relatively easy disease to control organically." She emphasizes timing - starting at the very first sign - and using resistant varieties, as well as getting good spray coverage, including the leaf underside.

As mentioned, county extension offices keep abreast of new developments in fungicides and resistance. Also, regional alliances produce vegetable guidelines. Twelve land-grant universities collaborate on a southeastern vegetable guide. California and other states have recommendations specific to their conditions. For example, for the Northeastern IPM Center, Rutgers vegetable pathologist Dr. Andy Wyenandt led the production of "Fungicide Resistance Management Guidelines for Vegetable Crops Grown in the Mid-Atlantic Region, 2014." General fungicide resistance guidelines can be accessed through the state universities. For example, the link for the FRAC guide in Delaware is http://extension.udel.edu/ag/vegetable-fruit-resources.

McGrath updates her research at http://www.longislandhort.cornell.edu/vegpath.

The author is a writer and researcher specializing in agriculture.