The frequent rains and high humidity of summer provide ideal conditions for foliage disease development in our lawns, landscapes and gardens.
These foliage diseases show up mostly as tan, brown of black colored spots varying in size and shape on plant leaves.
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.
Many fungal leaf spot diseases require a 12 to 14 hour period of uninterrupted wetness. A UF/IFAS Extension factsheet on gray leaf spot of St. Augustinegrass states, “Warm rainy spells from May through September commonly produce extended periods (12 hours and greater) of leaf wetness and relative humidity greater than 95%. During these periods, turfgrass leaf blades can remain wet and air temperatures often hover between 80 and 90 degrees Fahrenheit. Environmental conditions such as these are ideal for the pathogen growth, infection, and colonization of St. Augustinegrass.”
A Texas Cooperative Extension factsheet on Bipolaris and Exserohilum fungi states, “The severity of the disease increases with temperature and humidity. At 78°F a period of 8 to 10 hours with 100% relative humidity is all that is required for a high level of infection to develop.” These two fungi were once grouped under the name Helminthosporium and are common pathogens of bermudagrass and St. Augustinegrass.
Leaf spot diseases of trees and shrubs follow this same scenario. With higher humidity and frequent rains come diseased leaves. It’s common to find entomosporium leaf spot on red tip photinia and Indian Hawthorn and black spot on roses with these weather conditions.
This wet weather promotes foliage and fruit rot diseases on vegetables. It’s best to remove and dispose of diseased, worn-out vegetable plants as they succumb to summer’s heat, frequent rains and diseases.
Not all plants are equally susceptible to 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 here in Florida.
A fungicidal spray program can be used to prevent and reduce many of these diseases. But it needs to begin ahead of the symptoms in order to be effective. Frequent rains can make it a real challenge to apply a fungicide in a timely manner. There are also cultural practices that can be helpful in managing some of these diseases.
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.
Larry Williams, UF/IFAS Extension, Okaloosa County, June 21, 2017