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Zucchini Plant Diseases

By Rachel Asher ; Updated September 21, 2017
Zucchini Plant Diseases

Zucchini plants are easy to grow in a home garden, offer a fruitful bounty and are a tasty addition to any summer meal. Zucchini plants are susceptible to a few diseases, however. If you're planning to grow zucchini in the future, or are growing it now and looking to troubleshoot a problem with your plant, this guide to common zucchini plant diseases should help.

Bacterial Wilt

Bacterial wilt is caused by the bacteria Erwinia tracheiphila and is spread by cucumber beetles. The symptom is a green patch on the leaf, usually appearing a week after the bacteria-infected beetle makes a feeding wound in the leaf. The disease can spread throughout the whole leaf so quickly that the initial patch may not even be detected. Unfortunately, a slimy discharge can cause the plant to wilt, and the plant will not survive once it's reached this stage. The most effective control is beetle control, and one option is to grow an initial crop of zucchini to attract the beetles before spraying. However, this can lead to an increase in beetle numbers if the spraying is not thorough. Small plantings can be protected through a process called "screening," which involves covering them with cheesecloth. Other preventative measures include frequent weeding, keeping plants away from wooded areas and storing only zucchini produced from healthy vines.

Downy Mildew

Downy Mildew is caused by the fungus Pseudoperonospora cubensis, which flourishes in humidity and foggy, wet weather. It can survive hot days, although a prolonged heat wave will diminish the capacity of the disease. The fungus is easily carried through farm machinery, rain splash or by wind. The disease is most common on late summer plantings. Signs of the disease could be light green spots, yellow spots or fur underneath the leaves. Like bacterial wilt, the best form of treatment for downy mildew is control. If not using a resistant variety such as Zucchini Select, then grow on a site with good sunlight, irrigation and low humidity. If the disease is detected early, pull out the infected zucchini plants and destroy them so they don't infect the other zucchini plants.

Zucchini Yellow Mosaic Virus

Zucchini yellow mosaic virus was discovered in 1981. It is transmitted from infected plants to healthy plants by aphids: a small, winged insect. Aphids are infected with the disease after landing on a sick plant, and then infect healthy zucchini plants simply by landing on them. To diagnose zucchini yellow mosaic virus, inspect the edges of the leaves on the plant for a yellow color, or wrinkling. Avoiding this virus largely involves avoiding aphids--so if you know aphids are infecting a field or garden nearby, don't plant your zucchini until the aphids pass or keep the plants in a well-sealed greenhouse. If you've been infected already, fill a spray bottle with mineral oil and spray the infected plants. This mixture has proved to be a natural insecticide.

Gray Mold

Botrytis blight, or gray mold, is a mold that can affect zucchini in wet or humid weather. If you see brown spotting on the plant and particularly silver spores on dead parts of the plant, then your zucchini may be infected with gray mold. A good way to avoid the disease is to check the plants regularly for signs of infection. If you see gray mold, remove that plant from the rest of your zucchini plants. If you've planted during a wet summer, keep your zucchini plants spaced out from one another to ensure good air circulation. Fungicides can also be useful to prevent infections, particularly during rainy days.

Squash Blossom End Rot

While squash end rot is not actually a disease, it is a common problem with zucchini plants, and it can be mistaken for a disease. Squash end rot is caused by a calcium deficiency in the soil. While it isn't poisonous or dangerous to eat infected zucchinis, a calcium deficiency can cause the zucchini to mature early and it will lack flavor. End rot cannot be treated, so the best way to avoid it is through prevention. Watering the crop evenly is the best way to prevent squash end rot.