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Archive date:  May 18, 2002

What Do May Showers Bring?


Don't you just love the cool weather we are having! Everything is bright green and working out in the yard is a pure joy! Unfortunately, plants and people are not the only things enjoying this cool spell. Many diseases thrive in cool, rainy, humid conditions.

Three elements are required for the spread of disease, a susceptible host plant, the disease causing organism, and a favorable environment. Boy howdy, are we in the midst of a favorable environment! This coming week should bring a few sunshiny days and hopefully some relief. For now keep an eye out for the following diseases.

Rusts – Apple trees are very susceptible to rust. It appears as small, circular, rust colored spots on the upper and under sides of leaves. Again, this disease enjoys cool, moist conditions. Other plants that may show symptoms of rust are snapdragons, roses, blackberries, and hollyhocks.

Black Spot – If you have roses, odds are you have black spot. The leaves will have black blobby spots on them and eventually the leaves will begin to turn yellow. This disease is common, especially during cool, humid conditions. It will quickly make a rose bush unsightly and spreads readily.

Powdery Mildew – This disease looks just like it sounds. It creates a thin layer of white film in patches on the upper sides of leaves. Watch for it during cool, damp weather in areas with little circulation. Powdery mildew loves crapemyrtles. In fact, one very good reason to *not* top crapemyrtles is that the resulting heavy, dense branching reduces circulation and increases the incidence of powdery mildew. The mildew will not kill the tree but it may greatly reduce flowering by infesting flower buds. Powdery mildew may also be found on dogwoods, grapes, roses, phlox, zinnias, and bee-balm.

Botrytis – This is most often the culprit when peonies have trouble blooming. Botrytis will attack practically any herbaceous, or fleshy, plant. It loves cool, moist areas with poor air circulation. Watch for it on geraniums, petunias, and any other blooming annuals, especially those with large flowers. Be very careful when removing infected leaves and buds. When the infested area appears grey and fuzzy it is primed to release spores that will poof like a cloud of dust when disturbed.

All of these diseases are fungal and may be treated in a similar manner. First, remove as much of the infested areas as possible. If in doubt about what to remove, contact your local nursery. Be certain to rake any debris from beneath the plants as well. After removing the damaged plant parts apply a fungicide to the plant. There are many fungicides available. If you are unsure which product you should use, take a sample of the problem plant to your local nursery for expert advise. When applying any chemical, please read and follow all label directions. Be absolutely certain that the ailment your plant has is a fungus and that whatever product you decide to use has been approved for that fungus and the infected plant.