Preventing plant diseases before they start

January 30, 2013 

I had the flu recently. I figured I would get over it in a week or so. Boy, was I wrong. I continued to be weak and lacked stamina for a while. I reasoned that this flu must be worse than the flu I had last time. That had to be the reason that I recovered so slowly. Certainly, the reason could not be that I am just getting older.

Diseases leave lasting effects on people and on plants. My bout with the flu means that I will plan for a flu shot next year. Prevention is also best with plant diseases as well. Preventing a disease is better for the plant and easier on the gardener as well.

Like people have physicians, there are doctors for plants as well. Plant doctors who work with diseases are called plant pathologists.

Jean Williams-Woodward is a University of Georgia pathologist who diagnoses diseases and gives information on preventing or managing diseases in ornamental plants -- shrubs, trees, flowers and bulbs. Williams-Woodward offers some advice on preventing diseases before they start in the landscape.

Some plant types are susceptible to certain diseases. Annual flowers often get root rot or leaf-spot diseases. Powdery mildew is common on crape myrtles, phlox, roses, dogwoods and other plants. However, some varieties of certain plants are resistant to some diseases. Plant breeders actually breed and select varieties of plants that are disease-resistant. If we plant these resistant varieties, our plants will be less likely to get these diseases. Resistance does not mean the plants will never get the disease, but it does mean the plant is less likely to get sick.

When planting crape myrtles, select varieties that are resistant to powdery mildew. Select them based on their variety name. Powdery mildew resistant crape myrtle varieties have names of Indian descent such as Natchez, Tonto and Muskogee. The variety name also will tell you important information such as plant size at maturity, flower color and fall leaf color.

Some pansy varieties are resistant to leaf-spot diseases. These include Bingo Red & Yellow, Crown Blue, Crown Golden, Crystal Bowl Supreme Yellow, Crystal Bowl True Blue, Dynamite Red & Yellow, Majestic Giants Yellow and Viola Sorbet Blackberry Cream. If you grow the larger Patiola pansies, then look for these leaf-spot-resistant varieties: Purple Passion Mix, Pure Yellow, Pure Lemon and Pure Orange.

Downy mildew attacks snapdragons, pansies and especially impatiens. The disease can be so bad on impatiens that all you are left with is bare stems. Remove infected plants. If you have downy mildew in a location, it will probably reoccur the next year. Plant another type of flower that is less likely to get downy mildew.

Root rots are very troublesome on annual flowers. The plants may wilt or suddenly die. The lower stem and roots may be black, brown or water-soaked in appearance. Once a planting gets root rots, all you can do is to replace infected plants. Remove the entire plant if you can, roots and all. Prevent root rots (and other diseases) with several practices.

Avoid plants from the “almost dead” rack. There is a reason they look so bad, and you may actually bring a disease into your garden. Black root rot is a common root rot disease that leaves chains of black spores in the soil that affect other plants. Take a plant sample to the local extension office to help diagnose the problem. If you have black root rot, avoid planting vincas, pansies, violas, snapdragons, impatiens, petunias, calibrachoas, verbenas and begonias. Plants that are less susceptible to black root rot include salvias, geraniums, marigolds, zinnias, dusty millers, coleuses and celosias.

Build raised beds and plant into these. Most diseases require excess moisture. Raising the beds 6 to 8 inches higher than the surrounding soil improves drainage, helps beds warm more quickly in the spring and reduces diseases.

Add fertilizer and lime based on a soil sample. Some practices, like adding too much lime, can increase diseases such as black root rot. Follow good sanitation by never planting sick plants or using soil from suspected areas. Clean your tools as well, removing soil that can carry diseases.

Willie Chance works with the University of Georgia Center for Urban Agriculture and helps to train the turf and landscape industry.

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