InteractiveTurf Pest Bulletin

June 13, 2002

August 28 Update: The frequency and severity of dollar spot reports have peaked for this year so far. Heavy dews and milder temperatures have caused this severe outbreak, resulting in many sites noticing shortened control windows of previous fungicide applications. If populations of dollar spot are becoming more tolerant to fungicides, we would notice it later in the season more because(a) peaks in conducive weather like this one and (b) earlier selection for less sensitive strains in the season. We are interested in screening dollar spot strains from the region so if you think you might have a problem, please give Dr. Kane or myself a call.

Year in and year out, dollar spot is the most commonly occurring disease of golf turf in the upper Midwest. Dollar spot fungi frequently attack the bentgrasses (Agrostis spp.) and Poa species (esp. Poa annua and Poa trivialis) that grow on our greens, tees, and fairways. In some years, such as summer 2000, dollar spot persists through most of the growing season and becomes a severe problem. Therefore, repeated fungicide applications and large monetary inputs can be required to control this disease.

Climate: This year dollar spot flared up considerably on May 29th, (when four reports came in), and reports have been steadily trickling in since then. We know dollar spot occurs during moderate daytime temperatures (65-85 F) and cool, clear nights that promote heavy dew formation. Because of this, we generally see two peaks of dollar spot activity in northern Illinois, one in late May or early June, and another in late summer - around Labor Day (see Figure 1). Hot and/or dry weather in July and August generally reduce dollar spot "pressure".

Cultural Management

Turf Health: Turf management practices that reduce the plant's ability to resist infections, or reduce the plant's ability to recover from infections greatly increase dollar spot severity. Of major importance is the maintenance of adequate fertility levels - especially Nitrogen (see Photo 2). Plants that are N deficient are more prone to dollar spot and recover more slowly from infections once the disease is stopped. On fairways, N rates are often kept low to keep leaf clippings to a minimum, since most courses collect and dispose of clippings. Also important is keeping turf properly irrigated, as studies have shown that turf suffering from moisture stress was more susceptible to dollar spot infection.

Figure 1

Photo 2

Photo 3

(click on an image to enlarge it.)

Host Resistance: Unfortunately, most turf managers have not had much of an opportunity to use some of the newer resistant cultivars, unless they are managing a new stand of turf. In the Chicago area, most fairways consist of mixed Poa annua, P. trivialis, and old south German or 'Seaside' bentgrasses - all of which are highly susceptible to dollar spot. Certain clones of Poa trivialis appear to be especially susceptible and often show dollar spot symptoms well before surrounding patches of bentgrass (see Photo 1). Research is underway at Rutgers and other northern universities to identify and develop dollar spot resistant bentgrasses (both creeping and colonial types). Hopefully, in the not too distant future, we will have dollar spot resistant bents available for green height and tee/fairway height turfs.

Others: The combination of high rates of N with applications of Primo results in suppression of dollar spot without increasing mowing frequency. However, PGR applications just prior to or during a dollar spot outbreak can prolong disease symptoms by slowing turf recovery, even if the disease is stopped by a fungicide or weather change. Removing early morning dew with poling or mowing practices reduced dollar spot incidence by as much as 53% on putting green turf (anyone pole fairways?). Lowering humidity in the turf canopy by increasing air circulation is also a recommended practice, and can be achieved by using fans or by pruning and removing trees and shrubs.

Application: Cultural practices alone do no provide adequate control of dollar spot, making regular fungicide applications the most effective and necessary tool for dollar spot management. Fungicide application parameters play a critical role in dollar spot control, and when evaluating a poor performing fungicide, the first consideration must be whether application rates and spray intervals were appropriate.

Most fungicide labels state very clearly: "when under high disease pressure, use the higher (curative) rate and the shortest interval to insure control." Unfortunately, on fairways the lowest label rate (or lower) and the longest interval are being used. Note that contact fungicides with chlorothalonil as the active ingredient (eg. Daconil) usually do not provide adequate control for more than 7 to 10 days. Assuming no resistance problems, dicarboximides (Chipco GT, Curalan) may last 14-21 days, and demethylation inhibitor (DMI) fungicides (Banner, Bayleton, Eagle) may last 21-28 days.

On fairways it is much more likely that the volume of spray solution per acre is lower than the manufacturer recommends. Multiple trips to refill the sprayer are usually required when treating fairways according to manufacturer's recommendations, leading to increased time, labor, and cost. However, lower application volumes results in poor coverage of leaf tissue, and can lead to lower levels of disease control than anticipated.

Fungicide Resistance: Lastly, development of dollar spot resistance to fungicides remains an ongoing concern, and shouldn't be discounted by superintendents who are having dollar spot control problems. If you are experiencing chronic dollar spot control problems and you have correctly applied the appropriate fungicide(s), then it becomes prudent to investigate whether or not fungicide resistance is developing on your course. In general, we find that a relatively simple laboratory test can provide evidence to a fungicide resistance problem.

Strains of dollar spot have been found in Illinois that are resistant to the benzimidazole, dicarboximide, and DMI fungicides. Benzimidazole (e.g. Cleary 3336) resistant dollar spot has developed at most golf courses in the Chicago area, resulting in a long lasting ineffectiveness of the fungicide. Dicarboximide resistance has been relatively scattered and unprevalent in Illinois. Similarly, obvious DMI resistance has developed at few courses (<5%) in Illinois, with cases resulting in a shorter interval of control than the 21-28 days normally achieved with the DMIs. Resistance to the DMIs is a gradual process, as opposed to benzimidazole resistance, and may take many DMI applications to develop and become apparent. Due to its multisite mode of action, resistance to chlorothalonil has never been found in Illinois after 30 years or more of use.