2009 Year in Review

Gray Mold

"Herbaceous plants and woody plants can be attacked," said James E. Shuster. "The disease is not host specific. The number of plants that gray mold will attack numbers in the hundreds maybe even the thousands."

The disease needs moisture as one of its criteria for infection. The wetter it is, the more gray mold shows up on plants. Not only are the numbers of infected areas increased; so are the number of plants attacked as well as the severity of the infections--quicker growth of the disease and death of tissue.

"Botrytis is sometimes confused with old age and thrip damage," he said. "To help separate browning on petals caused by old age and the disease, check the petals closely.

"Browning from old age should occur on the outer petals and along the edges or at the tip of the petals first. When inner petals or the middle of the petals develop brown lesions before the outer petals, gray mold is probably the reason."

On leaves that are relatively thin such as redbud, lesions are light brown to brown and irregular. On fleshier leaves such as rhododendrons, concentric rings with different shades of brown may be visible.

"Symptoms may vary depending on the plants attacked, plant parts attacked and growing conditions," Schuster explained. "Under moist humid conditions a grayish thin mass of webbing--spore structures--may be visible. Misty or rapidly rising humidity may trigger the release of spores. Poor air circulation adds to the survival and growth of the disease."

When temperatures are between 68 and 76 degrees and humidity is high, it takes about 20 hours for the disease to infect. Warm to hot, dry weather tends to reduce or stop the growth and spread of the disease.

"The disease often overwinters on infected dead plant material," he said. "Sanitation is important. Deadhead dying flowers and remove all infected plant tissue. Infected plant material should be buried to help control the spread.

"Dig a hole at least a foot deep. And then cover the infected Botrytis plant material with the dug-out soil. Since gray mold often attacks flower petals, fungicides are frequently not suggested. Sanitation is the main control. Sanitation is not perfect since the disease can blow in from a long distance."

by  Editor, theCity1.com
May 21, 2009