Published on 04/30/03

Avoid overwatering to prevent plant diseases

By Cat Holmes
University of Georgia

Despite last year's drought, the most commonly diagnosed plant disease in Georgia was caused by excess water, say University of Georgia experts.

"Out of all the samples of diseased plants we got from homeowners, nurseries, landscapers and greenhouses, root rot was the problem more than 40 percent of the time," said Alfredo Martinez, a UGA plant pathologist. "We saw this on a full spectrum of plants, including roses, marigolds, verbenas, hollies, box woods, azaleas and rhododendrons."

The problem with root rot is that the symptoms are confusing. "People see plants that are wilted and yellowing, with stunted growth, and they think the problem is lack of water," he said. "So they water more."

Causes of root rot

Unfortunately, the causes of root rot -- the fungi pythium and rhizoctonia -- are very aggressive pathogens that love wet soil, Martinez said.

"If you have root rot disease, it's primarily a water problem," Martinez said. "Chances are, either the plant has been watered too much or soil drainage is poor.

"We have a lot of soils that contain clay in Georgia," he said, "And clay retains moisture and doesn't drain well."

He recommends this test to see how well your soil drains: Dig a hole a foot or so deep and a few inches wide. Fill it with water. After the water has drained, fill the hole a second time. The water should drain out in 24 hours or less. If it takes more than that, you need to add sand or some other amendment.

Soil samples

Take soil samples to the county extension agent to determine what amendments, if any, would be helpful and what sort of fertilizer would help, Martinez said.

"It is much easier to improve the characteristics of the soil than to treat disease that has set in," he said.

One good watering each week is enough for most plants. "Avoid light watering that gets the top layer of the soil wet but doesn't penetrate the 2 to 3 inches plants really need," Martinez said. "Often people overwater simply out of habit or because the top layer of soil is dry."

It's important to check the soil from time to time to see how well it's draining and whether plants are getting moisture. To do that, he said, dig about 6 inches down to see how much moisture the soil contains.

Dig around roots

"Don't dig in the root systems of plants," he said. "Dig around them. But make sure you get down below the root zone -- about 6 inches, in most cases. If it's dry and powdery that far down, it needs to be watered. Well-watered soil will stick together when it's pressed into a ball."

Another key to preventing root rot is to carefully check new plants before introducing them to the garden, Martinez said. Contaminated soil is another way the pathogen can be introduced.

Take one or two plants out of a flat of bedding plants, Martinez said, and rinse the roots.

"Roots should be white or silvery," he said. "If they're brownish, soft or sparse, then the plant is probably infected with a root rot-causing pathogen. Don't introduce sick plants to the growing site."

The treatment

If root rot is diagnosed, fungicides are on the market that, if wisely chosen, can reduce or alleviate the problem, Martinez said.

However, he said, the best thing to do is to correct the real problem: overly wet soil. After all, the root of the problem, he said, is in the roots.

(Cat Holmes is a science writer with the University of Georgia College of Agricultural and Environmental Sciences.)

Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.

Cat Holmes was a science writer with the University of Georgia College of Agricultural and Environmental Sciences.