Sentinel crops provide downy mildew alerts

A system of sentinel plots, similar in design to the soybean rust defense system, is providing adequate warning for upper Southeast vegetable growers to protect their crop from downy mildew.

Dinwiddie, Va., is one of three sites, along with Virginia Beach and the Eastern Shore, in the state where sentinel plots are checked for disease development. The Virginia system is part of a national vegetable disease detection and management system that runs from Florida to Canada.

Despite back to back dry growing seasons in southern Virginia, Extension Area Vegetable Specialist Janet Spencer says cucurbit crops, and watermelons in particular, have been susceptible to diseases because of typical early morning heavy dews from July-September. Combined with typical high humidity, the environment is right, she notes, for downy mildew to form.

The sentinel plots are critical to provide growers with ample time to spray their crops to protect them from mildew problems. Even with ample warning, a limited number of fungicides and diseases that can rapidly develop new races make it tough for growers to stay ahead of diseases.

In the upper Southeast, downy mildew is the worst disease problem faced by cucurbit growers in the fall, according to Virginia Tech Extension Vegetable Specialist Steve Rideout.

The Virginia Extension specialist explains that downy mildew, caused by the fungal organism Pseudoperonospora cubensis, is most destructive to cucumbers and cantaloupes, though all cucurbits are susceptible. In Virginia, where watermelon production has increased since the tobacco buyout, the disease can severely restrict production of high quality, high value melons.

Symptoms of downy mildew 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.

The disease often mimics other diseases. Infected leaves generally die but may remain erect while the edges of the leaf blades curl inward, often leading growers to think they are dealing with weather related problems.

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. In severe cases, in which plants are not treated with fungicides, the entire plant may die, and in most severe cases, large areas of plants may die.

Rideout explains that the organism which causes downy mildew cannot survive as far north as southern Virginia and usually not much farther north than Florida. The sentinel plots are invaluable in tracking the south to north movement of the disease.

In the past a number of fungicides had good activity on downy mildew, but due to resistance problems, the options now are few. “We still have two materials, Ranman and Folicur that will control downy mildew, but these should be rotated with mancozeb and chlorothalanil,” Rideout says.

Fall cucurbit crops are particularly vulnerable to the disease. Historically, it builds up to the south and affects cucurbits in the northern half of North Carolina and on into Virginia, starting in August.

In some years the disease organism appears as early as mid-July. Once it gets started in a crop, downy mildew can do a lot of damage in a short time. There are also different races — the earliest of these tends to have more impact on cucumbers, Rideout says.

By staggering the planting dates, the sentinel plots provide valuable information about the different races of downy mildew. Though not specifically designed to track specific races of the disease, by altering planting dates, scientist can monitor the movement of some different races of downy, Rideout says.

“Once we see downy appear in a sentinel plot in North Carolina, we can put out information to growers. Given a few weeks advance notice, growers have time to take action — though controlling it is still a challenge,” the Virginia vegetable specialist says.

The different races have differences in what they can and cannot affect. Some have more heat tolerance and others have different impact on different crops, he explains.

Some cucurbit varieties, especially cucumbers do have some resistance to the disease. The resistance isn’t complete, but these varieties usually give a grower a couple of weeks from susceptible to resistant, which can be valuable in recognizing and treating the disease.

A sister disease, powdery mildew, is often mistaken for downy mildew. It has a distinctive white, cottony growth on top of leaves. The problem, Rideout stresses, is that it takes a completely different fungicide to manage powdery, versus downy mildew. There is no fungicide available that has significant activity on both disease organisms, he notes.

Over time, powdery mildew will suffocate the plant and cause it to collapse over time. The end result — the plant dies — may mimic downy mildew, but how it gets to the point of death is different.

The best materials are Nova, Pristine, and Endura for controlling powdery mildew. As with downy mildew, it is a good insurance bet to add chlorothalanil and/or zinc-based fungicides to the mix.

Several of the strobilurin fungicides that worked on both disease organisms have developed resistance problems. Though some resistance problems with the triazole fungicides, including Nova, have been reported from researchers in Georgia, in general triazole fungicides are still working in the upper Southeast for powdery mildew on cucurbits.

As growers enter the fall vegetable production season, Rideout urges them to monitor the downy mildew monitoring system and to increase their awareness of the different diseases that can be late season problems.

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TAGS: Vegetables
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