White mold on squash leaves

Asked August 27, 2015, 3:03 PM EDT

Each year I get white mold on my squash, and the internet yields several remedies for this malady. Is it really just a natural progression of squash in the PNW, or do I need to pluck leaves and treat it? Thanks, Christina

Clackamas County Oregon powdery mildew horticulture

1 Response

Powdery mildew is a common disease on many types of plants. Two types attack squash plants in the PNW. Powdery mildews generally do not require moist conditions to establish and grow, and normally do well under warm conditions; so they have been active even in our drier summers.

Powdery mildew spores are carried by wind to new hosts. The humidity requirements for germination vary, but all can germinate without water. Moderate temperatures and shade are most favorable for growth. Since spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight planting in a sunny spot will minimize formation of powdery mildew.


The best method of control is prevention. Planting resistant vegetable varieties when available, or avoiding the very susceptible varieties, planting in the full sun, and following good cultural practices will adequately control powdery mildew in most cases. Unfortunately, very susceptible vegetables such as cucurbits (cucumber, melons, squash, and pumpkins) may require fungicide treatment. Several least-toxic fungicides are available but are most effective when applied no later than the first sign of disease.

Best Cultural practices

Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Overhead sprinkling can help reduce powdery mildew because spores are washed off the plant, but overhead sprinklers are not usually recommended as a control method in vegetables because their use may cause other pest problems.

Since there is a recurring problem, a good cultural practice would be to alternate squash to every other year or move the bed to another location. If resistant varieties can be found, they will slow but not stop the disease.

Sanitation is important to prevent the spores from overwintering to torment you next year. Keep dead leaves cleaned out, and once the plants are done for the year, promptly dispose of them in the garbage (do not try to compost).

Fungicide application

In some situations, especially with squash, fungicides may be needed. Fungicides function as protectants, eradicants, or both. Protectant fungicides prevent new infections from occurring while an eradicant can kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Once mildew growth is extensive, control with any fungicide becomes more difficult. The products listed here are for approved for home garden use.

Several least-toxic fungicides are available, including horticultural oils, neem oil, jojoba oil, and bicarbonate sprays. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also can have some protectant activity.

To eradicate mild to moderate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil, a bicarbonate spray such as Kaligreen or MilStop, or one of the plant-based oils such as neem oil or jojoba oil (e.g., E-rase). Be careful, however, to never apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer; always consult the fungicide label for any special precautions.

It is important to remember that as plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions are conducive to disease growth. Also important is that spray should cover the underside of leaves and the lower canopy.

Warning on the Use of Chemicals

Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. Consult the pesticide label to determine active ingredients and signal words.

Pesticides applied in your home and landscape can move and contaminate creeks, lakes, and rivers. Confine chemicals to the property being treated and never allow them to get into drains or creeks. Avoid drift onto neighboring properties, especially gardens containing fruits or vegetables ready to be picked.

Do not place containers containing pesticide in the trash or pour pesticides down sink, toilet, or outside drains. Either use the pesticide according to the label until the container is empty, or take unwanted pesticides to a Household Hazardous Waste Collection site. Contact your county agricultural commissioner for additional information on safe container disposal and for the location of the Hazardous Waste Collection site nearest you. Dispose of empty containers by following label directions. Never reuse or burn the containers or dispose of them in such a manner that they may contaminate water supplies or natural waterways.

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