Powdery mildew can be a serious problem on foliage and stems of cucumbers, melons, squash, pumpkins and ornamental gourds. The disease is widespread and can occur wherever these crops are grown.

Symptoms

Small, discrete colonies composed of fine white thread spreading across upper surfaces of infected leaves appear in the late July on most crops in Ohio. These colonies coalesce and eventually cover stems and foliage with a white, powdery masses of spores. In severe infections, leaves will yellow and die. Petioles, stems and Rarely fruit will become infected. Early death of leaves can decrease the total amount of fruits formed as well as reduce fruit size. Loss of foliage hastens maturity of fruit and increases sunburning. Stems infected with Powdery Mildew become brown. This can spoil the appearance of “handles” on pumpkin which lowers value of the fruit.

Causal Organisms

Two fungi cause powdery mildew on vine crops, Sphaerotheca fuliginea and Erysiphe cichoracearum. S. fuliginea is most common in Ohio. Powdery mildew fungi thrive under conditions of high relative humidity, warm temperatures, low light, high fertility and succulent plant growth. Unlike bacteria and many fungi, free moisture on leaf surfaces actually inhibits infection by these pathogens, although very high relative humidity is required for spore germination. Powdery mildew fungi grow only on living host plants. They survive the winter as dormant mycelium on perennial plants or as spores in thick walled fruiting structures. Powdery mildew spores can be blown into Ohio in the spring from warmer southern areas. Mature foliage is most readily infected, with very young leaves being nearly immune.

Control

1. A number of fungicides effective against powdery mildew are available. (See Bulletin 672, OH Vegetable Production Guide)

2. Resistant cultivars of cucumbers and muskmelons are available. Pumpkin Varieties vary greatly in susceptibility to this disease.

3. Use cultural practices that avoid excessive succulence, overcrowding, shading, overwatering or excess fertilization.

4. Avoid making new planting of cucurbits in the vicinity of older plantings, especially if mildew is already present.