Avoid overwatering to prevent plant diseases
by Louise Estabrook
May 31, 2013 12:00 AM | 1572 views | 0 0 comments | 5 5 recommendations | email to a friend | print
Despite recent droughts, one of the most commonly diagnosed plant diseases in Georgia is caused by excess soil water. Dr. Alfredo Martinez, a UGA plant pathologist based in Griffin at the Georgia Experiment Station told me that, “Out of all the samples of diseased plants that we get from homeowners, nurseries, landscapers and greenhouses every year, root rot is the problem more than 40 percent of the time. Dr. Martinez also told me that he noted root rot 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 often confusing. People see plants that are wilted and yellowing, with stunted growth, and they naturally think the problem is lack of water — so they water more. Unfortunately, the causes of root rot — pythium and rhizoctonia fungi — are both very aggressive pathogens that thrive in wet soil.

When root rot attacks a plant, the roots die and stop transporting water within the plant. Leaves wither and start to turn brown along their margins. Even the support stems will shrivel when the water pressure within the plant is reduced. The exact same symptoms occur when the plant doesn’t have enough water. It’s hard to tell sometimes by just looking at a leaf — you really need to inspect the roots.

Good drainage

If you have root rot disease, it’s primarily a water problem. Chances are, either the plant has been watered too much or the soil drainage is poor or both. Most soils in the Cherokee County area contain lots of clay and, of course, soils high in clay retain moisture and don’t drain well. To find out how well your soil drains, try this test before you plant. Dig a hole a foot or so deep and about a foot wide. Then 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 24 hours, you need to add topsoil, organic matter or some other amendment.

Also, bring your soil to the Cooperative Extension office to have a soil test done to determine what type and amounts of fertilizers are needed to boost plant growth. It’s much easier to improve the characteristics of the soil before planting than to treat diseases that might set in later.

Avoid Overwatering

One good watering each week is enough for most plants. Do, however, avoid light watering that gets the top layer of the soil wet but doesn’t penetrate the 3 to 4 inches plants really need. 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 is draining and whether plants are getting enough or too much moisture. To do this, dig about 6 inches down to see how much moisture the soil contains. Don’t dig into the root systems of plants, but rather 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.

Plant Healthy Plants

Another key to preventing root rot is to carefully check new plants before introducing them to the garden. Contaminated soil is another way that pathogens or diseases can be introduced. Take one or two plants out of a flat of bedding plants and take a close look at the roots. Roots should be white or silvery. 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. Always purchase fresh soil for your garden containers. We’re often tempted to reuse last season’s soil, but that’s like asking for trouble. Many soil pathogens overwinter in old soil and can easily contaminate your new plantings.

The Treatment

If root rot is diagnosed, fungicides are on the market that, if wisely chosen, can reduce or alleviate the problem. However, the best thing to do is to correct the real problem: avoid wet soil. After all, the root of the problem is in the roots.

Information about Extension Solutions for Homes and Gardens can be found on the University of Georgia Cooperative Extension website at www.caes.uga.edu/extension/cherokee or by contacting the Cherokee County Extension Office at 100 North St., Suite G21 in Canton at (770) 479-0418. The Georgia Extension Master Gardener Program is a volunteer training program offered through county offices of the University of Georgia Cooperative Extension.

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