Dollar spot was our most troublesome disease in 2014. Dollar spot is one of the most common diseases of turfgrasses worldwide. It was a chronic problem in tall fescue and Kentucky bluegrass lawns from late May to November on Delmarva this year.

There are three annual dollar spot epidemics in our region, but the departure of one epidemic and the beginning of another is blurred by imperfectly understood environment conditions. Basically, dollar spot comes and goes throughout the season. In 2014, we had a mild summer and timely rains. In a so called normal summer, high night temperatures (i.e., average ?77oF) normally shut down dollar spot activity, but we had remarkably few hot nights to slow disease activity for very long. In autumn, night temperatures averaging below 54oF normally bring dollar spot activity to a stop, but we usually don’t observe consistently lower night temperatures until about mid-October. In most years, we experience frosts beginning in late October that bring the disease to a halt. In most years, however, we usually have a period of Indian Summer after frost, which re-activates the disease. Indeed, we experienced heavy frosts in mid-November, 2014 followed by a few days in the 60’s and 70’s, which reactivated dollars spot on some of our lawns.

Dollar spot is identified by characteristic hour-glass-shaped leaf lesions (i.e., a bleached-white girdling of leaves bordered by a brown band where white and green tissue meet) in Kentucky bluegrass. In tall fescue, dollar spot may cause hour-glass-shaped lesions, but more often there is a bleached-white tip dieback and smaller oblong, white lesions with brown borders. Sometimes, whitish mycelium (i.e., the cottony body of fungi) can be seen on leaves. The overall effect of blighting is that it gives lawns a whitish or straw color, but the disease seldom kills lawn grasses. The real problem is that it is disfiguring for long periods of time and breaks-down density giving weeds a more competitive advantage.

Our fungicide program is designed to provide about 28 days dollar spot control in the average year. As previously noted, 2014 weather conditions were most favorable for dollar spot and break-down in control occurred in some of our lawns. Some homeowners observed that their lawn was badly affected, but the neighbor had little or no observable blighting. This begs the question of whether there is something in our program that has caused the problem. The answer is NO, because if it were so most of our lawns, instead of a very few, would be affected similarly.

We could blame the problem on too much irrigation, but research has shown that frequent irrigation has no effect on dollar spot. Some possible reasons for chronic dollar spot on some lawns may have something to do with mowing height and/or the varieties of grasses grown. Mowing substantially above 3.5” creates a tall canopy, thus providing a microclimate more conducive for infection since high humidly and lingering leaf surface wetness promote this disease. The most likely reason is plant genetics. There literally are hundreds of different varieties of tall fescue and Kentucky bluegrass. Some of these varieties are simply more susceptible to the disease. Highly susceptible varieties develop the disease much earlier and the blighting is more rapid and severe, when compared to a resistant variety. If you search the net you will find references to insufficient nitrogen being a major contributing factor. This is myth in that early scientist observed more dollar spot in non-fertilized turf , compared to that well fertilized. In fact, a timely application of nitrogen can reduce dollar spot, but only shortly during early phases of the epidemic. Our lawns are fertilized with near maximum amounts allowed by Delaware Nutrient Management law. So it is not how much nitrogen that is applied, but how close to the epidemic it is applied. Despite adequate nitrogen being applied, turf will still get blighted severely if weather conditions are conducive for dollar spot, especially if you have susceptible varieties.

In our next blog we will review other disease issues encountered this past year, but dollar spot was Number 1 in 2014.