Wet weather promotes foliage diseases in our landscapes, lawns and gardens. Many fungal diseases do well during extended wet weather. This includes heavy dews, high humidity and especially frequent late afternoon and evening showers. When wet weather occurs, keep a close watch for foliage diseases in your lawn, vegetable garden, fruit trees, landscape shrubs, etc. Most foliage diseases require wet conditions and as a result are more prevalent during wet weather.

Foliage diseases are evident on plants by spots on the plant’s leaves. Not all spots on plant leaves are caused by a foliage disease, though. So you have to be careful of the diagnosis. The main point is to be aware and watchful for disease development during wet weather.

About eighty five percent of plant diseases are caused by fungi. Many of the numerous species of fungi produce spores. Spores are tiny reproductive structures that allow the fungus to disperse and survive. Most fungi produce spores that are microscopic. These spores can be thought of as microscopic “seed” that can be carried for miles in the wind. Almost without exception, the spores that are produced by these fungi, regardless of the species, require wet conditions in order to infect the plant’s leaves, stems, flowers, fruit, etc. As the spores are blowing around in the wind, some will land on a susceptible plant species – it’s random chance. If the leaf (or other plant part) that the spore lands on is dry, the spore does not stick to the leaf, is blown off, can’t germinate, etc., because
there is not sufficient moisture. But, if the leaf or other plant part is wet (from rain, irrigation, dew, etc.), the spore can stick to the leaf, germinate and then penetrate the plant tissue. This is when infection occurs.

Most of these types of diseases require twelve to fourteen hours of uninterrupted moisture in order to germinate and infect the plant. This is how moisture plays a role in disease development and why it’s important to irrigate on an as needed basis and why diseases are more prevalent during rainy weather. Not all leaf spot diseases are serious. If you need help identifying or controlling a foliage disease, contact your local University of Florida Extension Office.

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