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Wet Weather Fungus Revival

September 26, 2001

For sure, it's been a crazy growing season. Some areas hardly dried out, while others hardly had rain. I guess it has been said that there is no "normal" growing season. For those who have been keeping an umbrella handy, this article points out a couple of common landscape diseases you may be experiencing.

Gray Mold (Botrytis). We had several reports of gray mold (Botrytis cinerea) rearing its ugly spores again this fall on various herbaceous plants such as mums, yarrow, and petunias. This fungus and its many strains can infect and damage flower parts, leaves, buds, shoots, seedlings, and fruits. The pathogen can attack a wide range of herbaceous, woody, and vegetable plants; and the disease progresses rapidly under cool, wet conditions.

Symptoms may vary depending on the plants attacked, plant parts attacked, and growing conditions. Under moist, humid conditions, a tan to gray fuzzy mold (fungal hyphae and spores) develops. Poor air circulation adds to the survival and growth of the disease. With temperatures between 68 and 76 and humidity high, it takes about 20 hours to infect. Warm to hot, dry weather tends to reduce or stop the growth and spread of the disease. Botrytis is sometimes confused with old age or natural dieback of petals. But check the petals closely: Browning from old age should occur on the outer petals first and along the edges or at the tip of the petals. When inner petals or the middle of the petals develops brown lesions first, gray mold is probably the reason.

The pathogen often overwinters on infected dead plant material, so sanitation is important in reducing potential for future infections. However, spores can blow in from far away. Dying flowers and all infected plant tissue should be collected and burned, buried, or otherwise removed from the growing area. For future plantings, space the plants to provide maximum air circulation. Fungicide options are provided in the 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard and Garden Pest Guide. However, you might think twice about using a fungicide for Botrytis on annuals because we are at the end of the growing season.

Dollar Spot. With a return to cool nights and dew-laden mornings, you may see the revival of several turf diseases, including dollar spot. If you have received excessive rainfall, your soil may be short on nitrogen (from leaching or denitrification) and thus more susceptible. Moderate fertilizing of your turf now is an effective way to stop this disease. For com-prehensive and easy-to-read guides, visit the University of Illinois Turfgrass Program Web site (www. turf.uiuc.edu) and click "Extension" and "Fertilization" or visit University of Illinois Extension's "Lawn Talk" (www.urbanext. uiuc.edu/lawntalk).

The dollar spot fungus can infect creeping bent-grass, Kentucky bluegrass, annual bluegrass, and fine-leaf fescues, even Bermuda grass and zoysia grasses. The disease appears as roundish, brown spots in the lawn. Initially, spots are silver-dollar size (thus the name) and later may enlarge to 4 to 8 inches. Merged spots could affect a larger area. The affected area turns straw colored and appears sunken in the lawn.

A quick and rather good diagnostic guide involves the appearance of the leaf lesions (dead areas). Look for these on plants at the edge of the sunken areas. The lesions girdle the blade, may be up to 1 inch long, and are usually bleached white to light tan, with a dark brown, reddish brown, or purplish border. When dew is present on the blades of grass, a white cobweb-like growth of mycelia may be seen on infected plants. The disease appears in warm (60F to 85F), wet, and humid weather, especially in lawns low in nitrogen. Control measures include maintaining balanced fertility, avoiding late-afternoon or evening watering, providing good air circulation by pruning surrounding plants, providing adequate surface drainage, and mowing at the maximal height. Fungicides can be used on a preventive basis but are generally used only on golf courses or high-visibility areas. Refer to the 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard and Garden Pest Guide for fungicide options. Also refer to Report on Plant Disease no. 407 for details on the disease, pathogen, and management options.

Author: Bruce Paulsrud Jim Schuster Nancy Pataky


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