POWDERY MILDEW OF CUCURBITS

Introduction

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^oF with an optimum of around 80^oF and an upper limit of 90^oF.

Symptoms

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.

Control

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.

Bacillus subtilis strain QST 713

When Using Pesticides  ALWAYS FOLLOW  LABEL DIRECTIONS! Bruce A. Watt, Extension Plant Pathologist 2004 Where trade names are used, no discrimination  is intended and no endorsement by Cooperative  Extension is implied.

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