When a plant starts to look diseased, gardeners head to the store in search of a diagnosis and cure. The labels on the bottles explain what the bottle’s contents can treat, including black spots on leaves, shriveled stems, insect infestations and other problems that a sharp eye can diagnose.
Two years ago, Muskogee resident Phil Pratt retired, ending a 35-year career as an Oklahoma State University plant pathologist and County Extension Director. He agreed to provide a few basics that gardeners need to know before they buy anything to spray on their gardens.
According to Pratt, there are two kinds of plant problems: 1) Those caused by pathogenic organisms and 2) those not caused by pathogenic factors.
The pathogenic organisms that cause diseases include viruses, fungi, bacteria and nematodes. The plant problems not caused by non-pathogenic organisms, sometimes called abiotic, are caused by giving plants the incorrect fertilizer, water, light or temperature, poor planting site, and incorrect use of or exposure to herbicides.
“Most of the time when I made a residential visit to help a homeowner, 75% of the problems were abiotic, meaning not caused by disease or insects,” Pratt said. “When a plant pathologist attempts to diagnose a plant problem, one of the first things we do is eliminate the abiotic issues first.
A common mistake is overwatering. Automatic water systems, ideal for lawns, can easily over-water landscape ornamentals such as azaleas and newly planted trees. Their roots stay too wet which can lead to fungal root rot, and the plant’s eventual death.
“Of the biological pathogens (viruses, fungi, bacteria and nematodes), 85% of our plant problems are caused by some type of fungi bacteria,” Pratt said. “Even though most gardeners think their shrubs have a virus, the reality is there are very few cases of viral disease in landscape and garden plants.
When virus diseases occur they are often carried to plants by insects such as leafhoppers and aphids. There are no chemical treatments for plants infected with viruses but the good news is that they are rarely the cause of our problems.
“One way to prevent plant diseases, especially foliar and fruit diseases, is to spray a good, broad-spectrum fungicide such as Chlorothalonil,” said Pratt. “Be sure to read the label to see the list of plants it is safe to use it on.”
Chlorothalonil is sold as Daconil 2787, Liquid Lawn Disease Control and Multi-Purpose Fungicide. It is used for diseases on lawns, shrubs, trees, fruit, vegetables and flowers.
“One of the best things to do is to use good sanitation methods such as removing dead branches, and keeping leaves out from under trees and shrubs, and removing dead plants from flower beds. It is also a good idea to use disease-resistant plant varieties when they are available and adapted to growing in your area,” Pratt said.
Pratt said that it is time to spray herbicides for cool season, broadleaf, weed- control in lawns.
“I am not opposed to the use of chemicals to control weeds and insects,” said Pratt. “Just don’t use them unless it is justified and needed.”
Pratt said to seed fescue lawns in March and re-seed in September where they have become thin. When warm weather arrives, watch for problems such as dollar spot in Bermuda grass and brown patch in fescue. Spray lawns with the broad spectrum fungicide, Mancozeb, Dithane M45 or other fungicides labeled for use on Bermuda and fescue.
A problem homeowners can unwittingly cause is native Oak and Pine tree decline. Anything that alters the environment in which a tree is accustomed to growing such as building a flower bed around the trunk of an Oak tree is considered site disturbance.