Powdery Mildew of Cucurbits

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Pest Management Fact Sheet #5085

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Powdery mildew of cucurbits is caused by either of two fungi. Sphaerotheca fuliginea (less common) is known to infect about 60 plant genera and Erysiphe cichoracearum is known to infect about 160 plant genera. All cucurbits are susceptible to powdery mildew although it does not commonly attack watermelon. Yield losses of up to 50% have been reported and are the result of defoliation caused by the fungi.

Environmental Conditions

Unlike most fungi, the spores of powdery mildew do not require free water for germination and are actually inhibited in its presence. High humidity is beneficial but not necessary for spore germination. Infection has been known to occur below a relative humidity of 50%, although the humidity at the surface of the leaf is undoubtedly higher. High humidity also increases the rate at which the fungus grows after infection occurs. Spores will germinate above 50°F with an optimum of around 80°F and an upper limit of 90°F.


The initial signs of the fungus show on the upper or lower surface of (especially) shaded leaves. The infection may appear first as a pale yellow spots but the fungus soon sporulates yielding a characteristic powdery-white appearance (brown in the case of Sphaerotheca). This powdery appearance can involve the entire leaf which eventually turns yellow then brown and dries to the point that it crumbles when crushed.

Survival and Dispersal

Overwintering of the fungi probably does not occur in Maine, although survival is possible on alternate hosts in the greenhouse. The initial infections are from spores carried by the wind from more southerly areas. Once the disease is present in the crop, abundant numbers of spores are produced which can spread the disease quite rapidly. Tens of millions of spores can be produced on a single infected leaf.


  1. Use resistant varieties.
  2. Plant in areas with good air drainage and allow for maximum air circulation.
  3. Crop rotation and fall plowing are of no benefit because the fungus does not survive in the field.
  4. The fungicides listed in the tables below are registered for powdery mildew control.

Traditional Fungicides for Powdery Mildew Control

Fungicide Apply when first observed Examples of Trade Names
Strobilurins: azoxystrobin trifloxystrobin Alternate with other chemicals. 5-7 day schedule Quadris, Flint
chlorothalonil Good coverage is critical. 7 day schedule Daconil, Bravo, Echo, Fungonil and others
myclobutanil Alternate with other chemicals Nova


Alternative Fungicides for Powdery Mildew Control

Fungicide Apply when first observed Examples of Trade Names
Bacillus subtilis strain QST 713 Coverage critical. Preventative 7 day schedule. Serenade
copper products Coverage critical. 5-7 day schedule. Kocide, Tenn-Cop, Basicop, and others
Neem oil Preventative 7-14 day schedule. 70% Neem Oil
Potassium bicarbonate 7-14 day schedule. Remedy Fungicide, Armicarb 100
Potassium salt of fatty acids 7-10 day schedule. M-pede
sulfur Coverage critical. 5-7 day schedule. Sulfur Dust, Wettable Sulfur, and others

When Using Pesticides


Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473-1295
1.800.287.0279 (in Maine)

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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