Figure 1.  Powdery mildew

Powdery mildew is the most common pumpkin disease in Kansas and can result in yield losses exceeding 30 percent if left untreated. The disease is most damaging if it appears three to four weeks before harvest.

SYMPTOMS

Plants infected with mildew initially develop white, powdery spots or blotches on the lower and upper leaf surfaces. Early infection may be difficult to detect since the small fungal colonies may be hidden on lower leaves or stems. As the disease progresses, the whole plant may become covered with the white dusty spores of the fungus. Diseased leaves prematurely turn yellow and die. Though the fruit is not attacked by the fungus, it may become malformed or develop sunscald because of a lack of leaf coverage.

DISEASE CYCLE

Two different powdery mildew fungi called Erysiphe cichoracearum and Sphaerotheca fuliginea can infect pumpkin. The Sphaerotheca fungus is the dominant species in Kansas.

Powdery mildew infection starts in early August and is favored by warm daytime temperatures, cool nights and high relatively humidity. The fungus does not require a film of moisture on the leaf surface for infection and can be a problem even during summer periods of little or no rain.

CONTROL

Most of the current pumpkin varieties do not have resistance to powdery mildew. However, several varieties with resistance to mildew will be released in the next several years. These new varieties will become mildewed, but the rate of mildew development is slower than on susceptible varieties. This prevents early defoliation and helps retain good fruit set and quality.

Fungicides are necessary to prevent powdery mildew infection on susceptible cultivars. For effective control, make the first fungicide application in late July or early August before symptoms appear. Fungicides are less effective when applied after mildew develops. Make a second application for mildew control in mid-August. The objective is to maintain adequate leaf coverage through August and the early part of September. The fungicides benomyl, thiophanate methyl, trifloxstrobin and azoxstrobin are currently labeled for powdery mildew control.

Some University studies indicate that ultrafine oils and baking soda will reduce powdery mildew severity. These products might fit into an organic production system. However, these products will need to be applied more frequently or alternated with synthetic fungicides for effective control.

Thiophanate methyl: Topsin

Homeowner: benomyl, Fertilome Halt

Web updated 9/01/06
Web comments to jpierz@ksu.edu