Zucchini plants turn yellow because of a disease of the plant known as the Zucchini Yellow Mosaic Virus. The Zucchini Yellow Mosaic Virus is carried by aphids and affects plants from the Cucurbitae family. This group includes several vegetables such as pumpkins, squashes, watermelons, melons, cucumbers and gherkins, apart from zucchini or courgette.
The virus causes the plant leaves to turn yellow and to suffer from the shoestring symptom of becoming deformed, twisted and withered. The fruit itself becomes stunted and twisted as well, and the deformity is characterized by raised protuberances. The virus strikes before harvest time and causes massive financial losses as the affected plant is not able to produce healthy fruit after a week or two of the infection.
Controlling the aphid population using insecticides or using virus protection inoculation can help fight against the virus. However, once the virus strikes, there is very little that can be done. Often, the virus is carried forward by a contaminated seed, so farmers and gardeners should be very particular about the seed they purchase in order to prevent the disease.
One of the reasons of the leaves of the zucchini plant turning yellow can be severe heat and dehydration. However, the plant should be observed closely to differentiate between harmless lack of enough water and serious symptoms of the Zucchini Yellow Mosaic Virus. In case of confirmation of the symptoms of the disease, one strategy is to separate the infected plants from others.
Apart from the virus and dehydration, sometimes the zucchini plant simply turns black at harvest end. This is known as harvest end rot and occurs in wet conditions and is caused by the failure of the plant to uptake enough calcium nutrition. This is not half as serious a problem as the virus, and removing the flowers and fruits can enable the plant to produce healthy fruit again.