White, dying leaves on zucchini are discouraging, but there is a solution. The white coating is caused by two common diseases that attack plants in the cucurbit family: powdery mildew and downy mildew. Both of these diseases can be present on a zucchini plant at the same time. Powdery mildew is a fungus, while downy mildew is more closely related to algae. These diseases have caused heavy economic losses in agricultural crops. In July of 2004, the remnants of a hurricane carried downy mildew spores up the eastern seaboard from North Carolina to New York, causing severe crop devastation.
Downy mildew spores are a purplish-gray and are only found on the undersides of the leaves, often first appearing as a water-soaked area. The symptoms of downy mildew only occur on the leaves; green leaf petioles often still hold the dead leaf tissue upright. Angular spots start out pale green and turn yellow, eventually causing tissue death. Several spots often join into a group.
Identification can be challenging, since spores are not always present. Downy mildew spores cannot survive extreme cold temperatures, and they are only found when conditions are favorable during the warmer months.
Powdery mildew is easily recognizable by its white talcum-powder appearance. Powdery mildew is a host-specific fungus, meaning the pathogen that attacks one plant won't attack another. The fungus that attacks zucchini can also infect other members of the cucurbit family.
The white, powdery growth appears on both the bottom and top of leaves and on petioles and stems. It first strikes the crown leaves, shaded lower leaves and the undersides of leaves of older plants. Yellow spots appear, which quickly spread and cause leaf death.
Spores of powdery mildew are carried by water, wind and rain. They routinely travel long distances carried by storm systems that affect wide areas. In the home garden, the spores are often carried on clothing or spread by handling of infected plants. Overhead watering spreads spores by splashing them from one plant to another.
Prevention and Control
Planting resistant varieties of zucchini is always the first and best means of preventing these diseases.
Downy and powdery mildew thrive in warm, moist areas with poor air circulation. Plant zucchini plants far enough apart to allow air to circulate freely, and water plants in the early morning hours to allow the leaves to dry quickly. Drip irrigation is beneficial for controlling water-splashed spread of the spores.
Regular spraying every seven to ten days with broad spectrum fungicides before or at the first sign of infection provides some measure of control. Liquid copper and a solution of baking soda and water are organic alternatives. Always spray plants early in the morning and cover all areas of the plant with the fungicide solution. Fungi cannot thrive in an alkaline environment, so a baking soda solution sprayed regularly may prevent infection.