Feb 24, 2012
Managing downy mildew in pumpkins

Cucurbit downy mildew is most serious on cucumbers, but can damage other vine crops, including pumpkins. Changes in the strain types (more aggressive and overcoming cultivar resistance) and early appearance in the northern parts of Ohio (usually in late June) combine to increase the risk of economic loss due to downy mildew.

The causal agent of downy mildew, Pseudoperonospora cubensis, is a plant pathogen in the water mold family, whose survival is only associated with living cucurbit tissue. Therefore, it does not survive the winter in the Midwest and must be re-introduced each year. Counties in northern Ohio and southeast Michigan appear to bear the brunt of early introductions from the north (possibly from northern cucumber production greenhouses, which may serve as a “green bridge” for the pathogen), while areas further south generally don’t see the disease until late July or August. This may be due to spread from northern areas as well as introduction from the southern United States.

There are several pathotypes of the downy mildew pathogen that affect vine crops differently. For example, several do not attack watermelon, pumpkins and squash, while others attack all vine crops. The types we have been seeing in the early introductions appear to be consistently aggressive on cucumber and less so on melons, pumpkins, squash and watermelon. Later introductions from the south may be more aggressive on pumpkins, squash and melon than those from the north.

Sporangia, the reproductive as well as windborne transport structures, are produced on the undersides of the leaves when conditions are humid and night-time temperatures are between 55˚ F and 75˚ F. The transport and survival of these sporangia is highly dependent on weather conditions. Cloudiness is especially important, as direct sunlight can cause the sporangia to dry out in transport. Rainfall can wash sporangia out of the air and deposit them in production fields. Upon deposition of the sporangia on a leaf surface, the absence of free moisture on the leaf may prevent infection, though only two to six hours of free moisture are required. Likewise, temperatures outside of the acceptable range for infection (41˚ to 82˚ F) may also inhibit infection.

In cucumber, water-soaked lesions on the underside of the leaf are often observed first. Yellow, irregularly shaped lesions confined by the small leaf veins appear soon after on the top of the leaf. These lesions then turn brown and may drop out of the leaf. The “checkerboard” arrangement of lesions is characteristic of cucumber downy mildew. Symptoms normally appear four to 12 days after infection.
On cantaloupe, the somewhat angular lesions tend to have a yellow halo around them. On watermelon, the spots may or may not be angular, normally turning brown or black with the leaf developing an upward curl.

On pumpkins and winter squash, the symptoms may resemble powdery mildew, causing yellow spotting that tends to brown out. As the lesions age, they usually become necrotic on all types of cucurbits and the leaves often senesce. This dieback is normally first noticed on the oldest leaves near the center of the plant. Regardless of the variability in appearance of the leaf lesions among the different cucurbits, the one similarity and diagnostic sign is the presence of purplish-gray sporangia on the bottom side of the leaf within the lesions. These are most readily observed when conditions are cool and moist, with or without the aid of a hand lens. They may also appear when an infected leaf is placed in a closed plastic bag with a damp paper towel for 12 to 24 hours. The leaves are the only portion of the plant directly affected by downy mildew, though the resulting loss in leaf surface can cause loss of yield, misshapen fruit and sunscald.

Managing downy mildew in pumpkins

While some degrees of partial resistance to downy mildew can be found in cucumbers, downy mildew-resistant pumpkin cultivars are not available. To fight downy mildew in pumpkins, select growing sites with good drainage and airflow, full sunlight and low humidity. Avoid overhead irrigation to prevent leaf wetness. Insure adequate, but not excessive fertility. Monitor the crop frequently – scouting is one of the most important things you can do to combat downy mildew economically.

Fungicides are required to control downy mildew. When to start applying them and which products to use will depend on your location and the weather conditions. Warm, bright sunny days are not conducive to downy mildew, as the sporangia are killed by UV light.

In areas in which downy mildew typically arrives early, start scouting pumpkin fields for downy mildew by late June, and apply protectant (contact) fungicides containing chlorthalanil (e.g. Bravo, Echo, Equus) or mancozeb on a seven to 10-day schedule. Use the shorter schedule under cool, wet conditions. Once downy mildew has been found nearby, add the more effective (but more expensive) fungicides such as Presidio, Ranman, Tanos and Previcur Flex to the program.

Remember to tank mix the latter fungicides with a contact type fungicide and alternate products with different modes of action.

Growers in areas where downy mildew typically appears in August or September should also scout regularly, but may be able to delay application of fungicides until early August.

By Sally A. Miller, Ohio State University

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