Fungi, tiny microorganisms, cause the majority of diseases that afflict squash plants. The tiny fungi produce thousands of spores, which are wind driven. They will alight on the leaves of the plant. Once there, they absorb food and water from the squash plant. The foliage and vegetables of the plant often suffer severe damage from the infestations.
Squash plants often suffer from powdery mildew on the foliage caused by the fungus Erysiphe cichoracearum, Choanephora wet-rot caused by Choanephora cucurbitarum, black rot caused by Didymella bryoniae and pythium cottony leak caused by Pythium aphanidermatum. All the fungus disorders cause a gradual decline in the plant and possibly death. Fruit and foliage is often rendered unsightly.
Powdery mildew develops on the foliage of the squash plant when the weather is warm and moist, according to Penn State University. Pythium cottony leak affects the plant's fruit in areas where the squash has sustained damage. It will also affect the leaves if they touch the soil's surface. Choanephora wet-rot is spread rapidly by insects and the wind. The flowers are the first to be attacked by the fungus but it will also ruin the vegetable. Black rot lives within the soil during the winter. In the spring it spreads rapidly to the plant's foliage.
Powdery mildew appears on the plant's foliage as a white, furry covering. Pythium cottony leak causes soft spots on the vegetables and foliage. The area quickly begins to produce a white, cottony fungus. Choanephora wet-rot affects the flowers of the squash plant. It appears as tiny, black spots and prevents fruit production. Black rot causes darkened areas on the plant's foliage and vegetables. It progresses quickly and will ruin the entire squash crop within a few days.
Always practice crop rotation within the garden when planting squash. Numerous squash seed varieties are available that promise disease resistance. Avoid most fungus spread by planting the squash plants in well-draining soil. To help prevent the fungus from forming, the foliage of the plant should be kept dry. Watering in the morning helps the foliage dry out before nightfall and can prevent fungal infections. Keep weeds at bay because they can spread fungi to the squash plant. Avoid injuring squash still on the vine, because fungus tends to develop in damaged areas.
Squash plants suffering from widespread fungal infections should be pulled out and destroyed before the fungus moves on to neighboring garden plants. The squash plant should be regularly dusted with sulfur to help prevent and control most fungal infections. However, do not apply sulfur if the temperature exceeds 90 degrees Fahrenheit because foliage and vegetable damage may occur, according to North Dakota State University. Applying fungicides every seven to 10 days during the spring and summer helps control infections.