Downy mildew strains threaten Southeast vegetables

Downy mildew is a common problem for Southeastern vegetable producers and a new strain that can be devastating to cucurbits has been found in areas of coastal South Carolina, prompting vegetable specialists to encourage growers to pay special attention to managing the disease prior to planting 2008 crops.

The disease started showing up in vegetable fields and monitoring plots in south Florida in March, progressed slowly from south to north, and despite severe drought throughout most of the 2007 growing season, caused sporadic problems for vegetable growers throughout the Southeast.

Downy mildew, caused by the fungus Pseudoperonospora cubensis, is found annually on squash, cucumbers, pumpkins, and muskmelons grown in all areas. Although downy mildew of all cucurbits is caused by the same species, strains of P. cubensis exist. For example, it is not uncommon to see squash, cantaloupe, and cucumber severely diseased by downy mildew, whereas watermelons, nearby, show no signs of the disease. Strains like the one infecting cucurbits in the Carolinas is not uncommon.

Downy mildew causes severe vine and leaf problems, which reduces the number and size of fruit. Typically, the disease does not cause fruit rot, as is the case with many cucurbit diseases, but it does reduce sugar content, especially in watermelons and cantaloupe.

In South Carolina, a particularly troublesome strain of downy mildew struck cucurbits in the early fall. Clemson University Vegetable Pathologist Tony Keinath says the disease damaged cantaloupe, cucumber, summer and winter squash, pumpkin, watermelons and bottle gourd.

In late September Keinath says downy mildew was found in Charleston County near the South Carolina coast on various crops, including watermelon. The watermelon planting is a 60 acre field that is estimated to be 10 percent infected. Keinath says sporulation on the leaves was very heavy, which doesn’t bode well for susceptible crops in the area.

Downy mildew infections were confirmed on cucumbers as early as June 26, in eastern North Carolina. The 25 acre farm, on which the initial infection was detected, was over 90 percent infected by the time reports were filed, suggesting some levels of infection may have been present as early as mid-May. Of particular concern to vegetable specialists is both the earliness of the infection and that it occurred in an area that was in the early stages of a drought and not prone to irrigation.

Heat and drought from southern Virginia to South Carolina appear to have blunted spread of the disease, though sporadic outbreaks were reported in eastern North Carolina and southern Virginia in late August. The disease was confirmed on cucumbers in two eastern North Carolina counties, but at low (5 percent) infection levels. It was also confirmed in late August on a 35 acre field of cucumbers near Cape Charles, Va.

In South Carolina, Keinath says the disease was widespread on cucumber and squash in the middle part of the state. Keinath, who works at Clemson’s Coastal Education and Research Center in Charleston, says downy mildew disease spores are usually found on the bottom of the leaf, although spores may be formed on top of the leaf in severe infections.

The best time to look for spores is in the morning before dew has dried. The spores are brownish-purple and the mold growth is white to colorless.

Keinath says symptoms on watermelon are different than symptoms on other cucurbits. Leaf spots on watermelon are dark brown and irregular in shape. Slight yellowing may be seen around the edges of the spots or in small patches in other parts of the leaf. Leaves that are infected curl inward as the leaves die.

Gummy stem blight is often confused with downy mildew damage on watermelon. For growers not familiar with the two diseases, Keinath suggests looking at the size, shape and position of leaf spots. Leaf spots on plants infected with gummy stem blight are much larger individual spots of downy mildew, he explains.

Growers who find downey mildew in their crops are faced with some difficult challenges. The best approach, according to Keinath is to alternate available fungicides and to mix the newer more efficacious materials with chlorothalonil or mancozeb, which have been used to manage fruit and vegetable diseases for many years.

Mobile (systemic, translaminar) fungicides with an active ingredient that specifically targets oomycete fungi are recommended beginning when downy mildew is forecast to occur in the area or symptoms have just started to develop.

These materials can be applied every 5-7 days, depending on disease severity. Fungicide resistance is a concern with downy mildew and with these fungicides due to their specific mode of action. Alternating among systemic fungicides in different chemical classes and tank-mixing with protectant fungicides when the systemic is not formulated with a protectant are highly recommended, as is planting resistant varieties when possible.

Fortunately growers have several systemic fungicides available to mange this rapidly changing disease.

Both downy mildew and powdery mildew have shown an ability to adapt to fungicides. In 2005, researchers in Georgia and North Carolina documented resistance of these diseases to strobilurin fungicides, which had provided easy and relatively inexpensive management for a number of years. Fortunately a number of good fungicides remain available to growers.

Of the fungicides available, Tanos and Previcur Flex have provided consistent protection from the newer strains of downy mildew.

Tanos, which is marketed by DuPont, is labeled for use in commercial and/or farm plantings on cucurbits (including cantaloupe, cucumber, honeydew melon, muskmelon, watermelon, pumpkin, summer squash, and other cucurbits), head lettuce, peppers, potatoes and tomatoes.

Tanos contains 25 percent cymoxanil and 25 percent famoxadone. Famoxadone is an oxazolidinedione contact fungicide and affects susceptible fungal pathogens through inhibition of mitochondrial respiration. The maximum application rate is 72 ounces of product per acre per year (1.1 pound per acre/year of cymoxanil and 1.1 pound per acre per year of famoxadone).

Previcur Flex, marketed by Bayer Crop Protection, is a fully systemic fungicide that penetrates the leaf and stem surface and moves throughout the plant to protect new growth. A long-time favorite of potato and vegetable growers, this fungicide has flexible use rates and application timing can be used with a variety of tankmix partners to control early blight, late blight and downy mildew.

Broad-spectrum contact fungicides (Bravo, Maneb, Dithane, copper), used to protect against infection, provide some downy mildew control.

Researchers in North Carolina regularly conducting fungicide efficacy trials for downy mildew rated chlorothalonil better than mancozeb and maneb, both of which were rated higher than copper.

Forum (dimethomorph), is a new 2006) formulation labeled for use at 6 ounces per acre tank-mixed with a protectant fungicide, such as chlorthalonil, on a 5-10 day schedule for a maximum of five times with no more than two sequential applications.

Gavel (mancozeb and zoxamide) can be used on cucumber, melon, summer squash, and watermelon. Gavel is labeled for use at 1.5 to 2.0 pounds per acre and can be applied every 7-10 days, or when conditions are favorable for disease for a maximum of eight applications.

New phosphorus acid fungicides, including Phostrol, ProPhyt, and Fosphite) can be applied to cucurbits at 2.5-5 pints per acre on a 7-14 day interval up to 6-7 times per crop per season. Phosphite ion, the active ingredient for these fungicides, effects fungal pathogens directly and promotes the plant’s defense system.

Ranman is labeled for use at 2.1-2.75 fluid ounces per acre on a 7-10 day schedule for a maximum of six applications per season. This fungicide has some specific use limitations and should be used in a tank-mix with a protectant fungicide.

Cucurbit downy mildew updates are available from the North American Plant Disease Forecast Center at or by contacting vegetable specialists at any Land-Grant University.

e-mail: [email protected]

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