Q. I have some dead spots in my backyard that I attributed to the dry weather. After two days of spotty rain, I saw this white mold over all of the dead spots but not on healthy grass. Is it something to be concerned about, or will it go away when it cools off and we get more rain?

A. The writer included a photo of the problem that was very helpful in diagnosing this problem. Dollar spot is most severe on Kentucky bluegrass (Poa pratensis). The causal fungus, Sclerotinia homeocarpa, is believed to overwinter in dormant turf that was infected with dollar spot during the previous growing season. Development is favored by air temperatures in the 80-degree range and high humidity. It usually becomes active from late spring through early summer and can remain active all summer, right into fall.

Dollar spot spreads on infected clippings and can be carried by lawn equipment, foot traffic or wind. The disease starts out as tan-colored patches that are 2 to 4 inches in diameter. As it progresses, those spots run together to create large patches of dead grass. White mycelium or fungal strands are sometimes visible early in the morning when the grass is wet with dew; it looks as if the lawn is covered with cobwebs. Infected blades of grass may have tan spots that have an hourglass shape with dark brown margins. Dollar spot is most severe on lawns suffering from nitrogen deficiency and those that are watered lightly every day or every other day. However, it can also develop on non-irrigated lawns when we have long periods of warm, humid weather.

Cultural controls include maintaining adequate fertility levels, preferably by using fertilizers with a high percentage of slow-release nitrogen. Although fertilizing non-irrigated lawns in hot, dry weather is a waste of money, those that are irrigated should receive a moderate application of nitrogen in midsummer. One-half pound of slow-release nitrogen around July 15 should carry the lawn through until the late summer application in early to mid-September. It is important to water deeply and infrequently, applying an inch of water a week in one or two long, soaking sessions. Avoid watering at night, which keeps the grass wet longer than watering first thing in the morning. If dollar spot is a recurring problem on your lawn, overseed or replace susceptible Kentucky bluegrass with resistant cultivars. They include 'A-34,' 'Banff,' 'Eagleton,' 'NuStar,' 'Preakness,' 'Princeton 104' and 'Unique.'

Some fungicides that are readily available to home gardeners and labeled to control dollar spot include Cleary's 3336 (thiophanate methyl), Bayleton (triadimefon), and those containing mancozeb as the active ingredient. Mancozeb is preferred because it works on contact and fungi are less likely to build up resistance to contact-type fungicides. Systemic fungicides such as Cleary's 3336 and Bayleton can lose effectiveness over time as the target fungi develop resistance.