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Plant diseases

first_imgBy Cat HolmesUniversity of GeorgiaDespite last year’s drought, the most commonly diagnosed plantdisease in Georgia was caused by excess water, say University ofGeorgia experts.”Out of all the samples of diseased plants we got fromhomeowners, nurseries, landscapers and greenhouses, root rot wasthe problem more than 40 percent of the time,” said AlfredoMartinez, a UGA plant pathologist. “We saw this on a fullspectrum 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 stuntedgrowth, and they think the problem is lack of water,” he said.”So they water more.”Causes of root rotUnfortunately, the causes of root rot — the fungi pythium andrhizoctonia — 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 wateredtoo 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 ahole 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. Thewater should drain out in 24 hours or less. If it takes more thanthat, you need to add sand or some other amendment.Soil samplesTake soil samples to the county extension agent to determine whatamendments, if any, would be helpful and what sort of fertilizerwould help, Martinez said.”It is much easier to improve the characteristics of the soilthan to treat disease that has set in,” he said.One good watering each week is enough for most plants. “Avoidlight watering that gets the top layer of the soil wet butdoesn’t penetrate the 2 to 3 inches plants really need,” Martinezsaid. “Often people overwater simply out of habit or because thetop layer of soil is dry.”It’s important to check the soil from time to time to see howwell it’s draining and whether plants are getting moisture. To dothat, he said, dig about 6 inches down to see how much moisturethe soil contains.Dig around roots”Don’t dig in the root systems of plants,” he said. “Dig aroundthem. But make sure you get down below the root zone — about 6inches, in most cases. If it’s dry and powdery that far down, itneeds to be watered. Well-watered soil will stick together whenit’s pressed into a ball.”Another key to preventing root rot is to carefully check newplants 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, Martinezsaid, and rinse the roots.”Roots should be white or silvery,” he said. “If they’rebrownish, soft or sparse, then the plant is probably infectedwith a root rot-causing pathogen. Don’t introduce sick plants tothe growing site.”The treatmentIf root rot is diagnosed, fungicides are on the market that, ifwisely chosen, can reduce or alleviate the problem, Martinez said.However, he said, the best thing to do is to correct the realproblem: overly wet soil. After all, the root of the problem, hesaid, is in the roots.(Cat Holmes is a science writer with the University of GeorgiaCollege of Agricultural and Environmental Sciences.)last_img read more

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