Turfgrass Diseases: Dollar Spot (Causal fungus: Clarireedia jacksonii)

It is perhaps the most common disease of high-maintenance, irrigated turf, and one of the most expensive diseases to control on golf courses due to the frequency of fungicide applications required for acceptable control.

Symptoms and Signs

On golf course putting greens mowed at or below 1/8 inch, dollar spot usually appears as white or tan spots of dead turf about the size of a silver dollar (hence the name dollar spot). On golf course fairways and tees, spots can be a bit larger, typically 1 to 3 inches in diameter.

Figure 1. Dollar spot symptoms on a golf course putting green (left), and on fairway turf (right). Photos: Peter Landschoot, Penn State

On athletic fields and home lawns mowed at 2 to 3 inches, spots may reach 2 to 5 inches in diameter. When conditions favor disease development, spots coalesce, often producing large areas of dead turf.

On individual leaves, initial symptoms appear as yellow-green blotches, which progress to a tan-colored lesion with reddish-brown borders. Occasionally, white mycelia (thread-like webbing produced by the causal fungus) can be observed on diseased leaves in early morning following rainy periods and on dew-covered grass.

Disease Development

In Pennsylvania, dollar spot can occur anytime during the summer and typically reaches peak disease activity in late summer under high humidity and air temperatures ranging from 60° to 85°F. Symptoms also may appear in early to mid-fall when nights are cool and days are warm. The most severe cases of dollar spot take place on turf receiving closely spaced summer irrigation. The disease may also occur on non-irrigated turf when humidity is high from prolonged muggy summer weather. Dollar spot is more severe on turf that does not receive enough nitrogen, or when grass foliage grows slowly.

Disease Cycle

The causal fungus, C. jacksonii, spreads via bits of mycelium on infected plant parts (primarily grass clippings). Mycelia may be carried by water or on turf equipment and shoes. This fungus does not produce spores. C. jacksonii survives cold periods as thick-walled crusts of mycelium called pseudosclerotia in dormant infected plant tissues.

Cultural Management

Late spring nitrogen fertilizer applications can help to minimize dollar spot severity, since foliar turfgrass growth will be stimulated during early summer when dollar spot infections begin. On athletic fields and lawns, fertilizer that releases nitrogen slowly over an extended period is usually more advantageous than quick-release products at this time of year. On golf courses, quick-release nitrogen fertilizers can be applied in liquid form frequently at low rates instead of using a slow-release fertilizer.

Managing soil and turf canopy moisture is also an important cultural practice for managing dollar spot. Irrigation cycles designed to apply more water less frequently, thereby wetting the entire rootzone of susceptible turf, can help reduce dollar spot severity. Research at Penn State and other universities has demonstrated significant reductions of dollar spot through removal of dew and guttation fluid from turf foliage by dragging fairways with weighted hoses or by mowing in early morning.

Lightweight rolling of putting greens is another practice that has shown some benefit in reducing dollar spot. Research at Michigan State University demonstrated that rolling creeping bentgrass greens five days per week with a lightweight roller following morning mowing generally reduced dollar spot. Rolling greens twice in the morning was more consistent in reducing dollar spot than a single pass with a roller.

Figure 5. Lightweight rolling of putting greens can reduce dollar spot severity. Photo: Peter Landschoot, Penn State

Some cultivars of creeping bentgrass that have shown improved tolerance to dollar spot in trials at Penn State include Proclamation, Declaration, 007, Barracuda, Luminary, Pin-Up, V8, Memorial, and Benchmark DSR. Differences among other turfgrass cultivars with respect to dollar spot susceptibility have been reported in data from the National Turfgrass Evaluation Program (ntep.org).

Chemical Control

On irrigated turf with persistent dollar spot problems, fungicides are very effective at controlling the causal pathogen. Fungicides are usually applied when symptoms of dollar spot are visible; however, early-season applications just before dollar spot symptoms first appear can reduce the severity of outbreaks later in the season. Both contact and penetrant fungicides are effective in controlling this disease. Because resistance to penetrant fungicides has occurred with C. jacksonii, alternating penetrant fungicides with different modes of action, mixing contact fungicides with penetrant fungicides, or using only contact fungicides in a control program is suggested. Perhaps the best way to lessen the chance of resistance developing is to employ a broad-based cultural management program that reduces the number of fungicide application over the course of the growing season.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide's ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. Chemical control of turfgrass diseases 2020 [2024]. University of Kentucky, Rutgers University, and University of Wisconsin.

Giordano, P.R., T.A. Nikolai, R. Hammerschmidt, and J.M. Vargas. 2012. Timing and frequency effects of lightweight rolling on dollar spot disease in creeping bentgrass putting greens. Crop Science 52(3):1371.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, Minnesota.