White Spots on Yellow Squash Plants
updated: February 16, 2011
Immature fruit on a squash plant
Finding unsightly spots on your plants is certainly disconcerting, but often these issues are more than cosmetic. Spots are usually a sign that something is wrong, so they should never be ignored.
- While white spots are occasionally harmless dust or minor damage from pests, the most common cause is powdery mildew. Powdery mildew is a disease that affects many different types of plants. There are several different varieties of this disease, and each infects a different type of plant. Squash plants are typically affected by a variety known as Erysiphe cichoracearum.
- The most obvious sign of a powdery mildew infection is the presence of white, powdery spots on the surfaces of leaves and shoots. Sometimes the spots also appear on flowers and fruit. The spots typically grow in size over time, gradually spreading to other areas of the plants. The leaves of infected plants may turn yellow and fall off, exposing the fruit to enough sunlight to damage them.
- Powdery mildew does not usually kill plants entirely, but it can cause enough damage to them that yields are reduced, production times are shortened, and fruit is damaged or flavorless. If it is controlled, powdery mildew is unlikely to cause any serious damage, and the results are mostly cosmetic.
- Treatment of powdery mildew is not always required, but you can apply fungicide to squash plants if it is needed. Protectant fungicides will help ensure infection does not recur, and eradicants can kill existing infections. Be sure to use fungicide as soon as powdery mildew is suspected, because they become less effective as the infection spreads.
Some gardeners use horticultural oils, biological fungicides and sulfur dust. These products may help, but they should not be used together. Never use sulfur dust within 2 weeks of an oil spray, and ensure you use the products exactly as directed. Sulfur can irritate your skin and eyes, so use equipment to protect yourself if you use it. - Once you eradicate powdery mildew, take steps to prevent it from coming back. Use protectant fungicides, grow resistant varieties if available, and keep plants in an area with plenty of air circulation and sunlight. Avoid adding too much commercial fertilizer, as this can make conditions favorable for powdery mildew infections.
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