Wet weather promotes fungal diseases

Most fungal diseases

Most fungal diseases are dependent on moisture, especially foliage or leaf spot diseases like the cercospora leaf spot pictured on this hydrangea.

Photo by Susan Brown
Published: Thursday, August 7, 2014 at 01:23 PM.

Much of the Southeast has recently experienced the typical summer pattern of frequent late afternoon and evening thunderstorms. Heavy rainfall combined with high humidity creates a favorable environment for plant diseases.  Our wet, warm weather adds up to fungus “heaven” in our lawns, landscapes and gardens.

Keep a close watch for foliage diseases which are evident on plants by spots on plant leaves.  Most foliage diseases require wet conditions and as a result are more prevalent during wet weather. 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.

How do fungal diseases develop?

Foliar diseases occur on most species of landscape trees and shrubs as well as turf.  Most fungal diseases are dependent on moisture, especially foliage or leaf spot diseases. Many of these disease-causing fungi spread by microscopic airborne spores that require moisture to germinate, infect and colonize our plants.  Most fungal leaf spot diseases require a 12- to 14-hour period of uninterrupted wetness.  This is how moisture plays a role in disease development and why it’s important to irrigate on an as needed basis.  Typically these diseases create a problem only with the aesthetics of the plant.

Fungi are the primary cause of most leaf diseases.  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.

How can I prevent fungal diseases?

For the most part, diseases can be prevented by utilizing proper cultural practices such as variety selections, irrigation, plant and soil nutrition, pruning, and row spacing.  When there is inadequate circulation of air, poor water drainage, exorbitant irrigation, and too much dampness due to rainfall, the fungi can become a problem.  Not all plants are equally susceptible to these foliage diseases. It’s wise to learn the landscape and garden plants that are likely to experience disease problems as a result of our classic summer weather.  

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