Powdery mildew is a major disease of cucurbits in greenhouse and field production. The symptom is the obvious white powdery appearance on the leaves.
Early stages of infection on the underside of a cantaloupe leaf. Shaded leaves, and underside of the leaves can be sites of first symptom development.
Even-though powdery mildew hasn't been prominent issue on Florida watermelon in the past, the disease is increasingly been observed in spring production in Florida.
Leaves that are severely infected with powdery mildew can have high levels of white talcum like growth on the under side and some on the upper side of the leaves.
Low light intensity conditions in combination with dense canopy cover are conducive conditions for disease onset and spread. The affected areas may turn necrotic leading to withering and death of the plant.
High relative humidity can favor survival and infection of the pathogen but disease can also occur under low relative humidity. Optimum temperatures for disease occurrence range from 50-90 F.
While the disease is rarely seen to affect watermelon fruits, we may see with white fluffy growth on fruits at times. This affects the marketability of fruits.
The disease can also affect cantaloupe fruits and symptoms are similar to that on watermelon fruits. In some cases, fruit symptoms are noticed in the absence of leaf symptoms.
A zucchini leaf with heavy infection of powdery mildew while the fruit shows no sign of infection. There are many pathotypes (host specificity) of the fungus that infects different types of cucurbits.
Cluster of long-chained white conidia (spores). Air-borne conidia are the primary cause for the spread of the disease. The pathogen may survive during winter months in cucurbit production in south Florida.
The pathogen can survive on volunteer plants and other host plants. Under ideal conditions, spores can easily spread to new production areas by air or close-by plants through splashing water and tools.