Does Powdery Mildew Stay in Garden Soil for a Long Time?

Powdery mildew is not a soilborne disease. Unlike soilborne plant diseases such as verticillium or fusarium wilt, which stay in garden soil for years, powdery mildew fungi remain associated with infected plant material. Powdery mildew disease is produced by a large number of fungi such as Erysiphe spp., Sphaerotheca spp., Unicinula spp. and Microsphaea spp., which often target only specific plants or plant groups. Spores produced by the powdery mildew fungus float in air, stay attached to plants and may rest on the top layer of garden soil. Don't confuse powdery mildew with downy mildew (Plasmopara spp.), which is a different disease with somewhat similar symptoms.

Powdery mildew can affect almost any kind of plant. The fungus covers the surface of leaves with a white or grayish, powdery growth. Often, stems, buds, flowers and fruit also become infected. Leaves may turn yellow and fall and buds may not open. Young, succulent plant growth is more susceptible than older growth. Although the fungus is unsightly, it rarely causes severe damage or kills plants.

The fungi that cause powdery mildew require the plant species that they infect to survive. Powdery mildew fungi don't invade their host, they produce fungal threads (mycelium) on the outside surfaces of leaves or stems. They feed on the plants by sending rootlike structures (haustoria) into the top cell layer of the plant. The powdery mildew fungi overwinters by clinging to plant debris. In spring, fruiting structures of the fungi produce spores, which can be carried by wind or splashed by rain onto new host plants.

Shade favors powdery mildew, so grow susceptible plants in sunny areas. Avoid excess or late summer application of fertilizer, because it may produce succulent growth favored by powdery mildew. A slow-release fertilizer is a better option. Improve air circulation by pruning or spacing plants farther apart. Poor air circulation increases humidity and rate of fungal infection. Removing infected plant material and plant debris in fall will help reduce powdery mildew infections.

Several horticultural oils are effective in preventing and eradicating powdery mildew. Sulfur sprays are best used as a preventative. A nontoxic spray made of baking soda and lightweight horticultural oil (1 tablespoon baking soda, 2.5 tablespoons oil in 1 gallon of water) was found to effectively combat powdery mildew by researchers at the University of Rhode Island.