Diseased zucchini plants may display similar symptoms to pumpkin or squash plants. They all belong to the gourd section of the cucumber family. Yellow leaves are just one of the outward warning signs to suggest a plant has a problem.
Cucumber Mosaic Virus
Zucchini and marrows are most susceptible to cucumber mosaic virus. The leaves are a mottled yellow with green patches. The leaf surface become puckered and distorted. There is no effective treatment for the virus and infected plants should be removed and destroyed. Hands should be thoroughly washed before touching other plants. Ideally a resistant variety should be grown.
The lower leaves turn yellow and the discolouration moves upwards when a zucchini is effected by verticillium wilt. Eventually all leaves become dry and wilted. Young plants in cold, wet conditions are most susceptible. Verticillium wilt can be verified by cutting the stem. Brown streaks will be found inside the stem tissue. It is a soil-borne fungus and infects the plant via the roots. Crops should be rotated to prevent infection.
The yellowing of leaves doesn't always indicate disease. If a soil is lacking in nutrients, it can cause leaves to turn yellow and can stunt growth. Iron deficiency does not always mean the soil is lacking in iron. It could mean that other nutrients are stopping the plant from absorbing iron. Using a well-balanced fertiliser can help.