Biology and Control of Dollar Spot Disease: 2022 Update - by Dr.. Tom Hsiang

Biology and Control of Dollar Spot Disease: 2022 Update by Dr. Tom Hsiang, University of Guelph

Dollar spot is the most prevalent disease of turfgrass in North America, particularly on intensively managed golf course putting greens and closely mown fairways. In Canada, dollar spot can be found in the Maritimes, the Prairies and the West Coast, but it is most severe in the Great Lakes Region. Fortunately for golf course superintendents, it usually can generally be recognized and managed without too much difficulty. A good understanding of the biology of dollar spot is key to controlling this disease.

The pathogen

For many years, the fungus which causes dollar spot was known as Sclerotinia homoeocarpa, but for over 30 years, we already knew that this was the incorrect name since this fungus does not have all the characteristics that would place it into the fungal genus Sclerotinia. In 2018, some researchers from Rutgers University in New Jersey, USA, wrote a research article that took this one fungus and split it into at least four species (Salgado-Salazar et al. 2018). They based this on some differences in gene sequences between a handful of genes.

The pathogen that attacks creeping bentgrass, annual bluegrass, Kentucky bluegrass and other cool season turfgrasses across North America was renamed Clarireedia jacksonii. This fungus is also occasionally found on warm season turfgrass hosts. There are other new species of Clarireedia, but these generally occur in other geographic locations and often on other turf species. For example, Clarireedia monteithiana occurs on warm-season grasses and also has a global distribution, but it occasionally is found on cool season turfgrass hosts. Interestingly, we found it on tufted bulrush, a seaside in Nova Scotia (Hsiang et al. 2014). At the time we found it in 2011, the various species were still known as Sclerotinia homoeocarpa, so that's what we called it then. But after the Rutgers paper was published, we found that it was actually a Clarireedia monteithiana. Two other Clarireedia species are found in Europe only at this point.

Conditions favouring the disease

Dollar spot in Canada mainly occurs during summer and early fall. As with every turfgrass disease, environmental conditions affect the occurrence and the development of the disease. The dollar spot fungus will start growing at 15oC, with optimal growth between 21 oC and 27 oC, but does not grow well above 35oC. As well as this optimal temperature range, the fungus requires relative humidity greater than 85% and long periods of leaf wetness (generally > 8 hours of continuous wetness under optimal fungal growth conditions) which can be provided by heavy dew at night or evening watering. Dollar spot is more prevalent on turf which has been grown under a low nitrogen fertility regime or cut at low mowing heights or stressed by factors such as heat or drought.

There have been many attempts to develop models based on environmental conditions that can help with the prediction of dollar disease outbreaks. Some of these are based on plant phenology such as when the flowering of certain plants occur, and some are based on running averages of relative humidity and air temperatures. You should try out implementations of different models and see what works well for your area and how they help you with fungicide application schedules.

Symptoms

Dollar spot patches start out on low cut annual bluegrass and creeping bentgrass with a few adjacent leaves turning brown (Fig01). After a couple of days these diseased areas take on a bleached appearance, ending with patches that have an average diameter of 3 cm (Fig02). The spots rarely expand, but they can coalesce to form larger dead areas (Fig03), and cover a broad area if allowed to continue unchecked (Fig04).

On longer grass leaf blades, individual lesions can be seen. They are hourglass in shape, with dark margins and a straw coloured centre

across the leaf blade (Fig05). On shorter turf, the lesions may be cut, resulting in a tip blight symptom (Fig06). The symptoms on longer grass appear as larger (Fig07) sometimes irregularly shaped patches (Fig08).

During periods of warm days and nights coupled with high humidity and heavy dew, dense, aerial mycelium can be found on the affected turf blades in the early morning (Fig09). When this occurs, this disease is sometimes misdiagnosed as Pythium blight (Fig10). Spider webs can also look similar (Fig11).

Figure 1: Small dollar spot patches on creeping bentgrass start out as a few brown or yellow leaf blades.

Figure 2: The dollar spt patches enlarge turning from yellow to a bleached straw colour and reach a maximum size of 3 cm across Figure 3: Dollar spot patches can coalesce to form larger dead areas.

Figure 4: This higher end golf course stopped dollar spot control at the end of summer, but the disease continued to progress with warmer early fall weather and humid nights.

Figure 5: On longer grass leaf blades, individual lesions can be seen as hourglass shaped with dark margins and a straw coloured centre across the leaf blade.

Figure 6: On short turf, the lesions can be cut by mower, and the symptoms resemble that of tip blights.

Figure 7: All the cool season turfgrasses are susceptible to dollar spot, and the Kentucky bluegrass on the left has larger patches than the creeping bentgrass on the right.

Figure 8: On taller cut grasses, the patches are larger and less regular in shape. On Kentucky bluegrass here, hourglass lesions are clearly visible. Figure 9: During periods of warm days and nights coupled with high humidity and heavy dew, dense, aerial mycelium can be found on the affected turf blades in the early morning.

Figure 10: Pythium blight also can have dense aerial mycelium, but is a much more aggressive disease. Typical fungicides which are used for dollar spot do not control Pythium blight and vice versa. (Photo courtesy of Kevin Goss).

Figure 11: Spider webs can also look similar to dollar spot webs.

Life Cycle

The dollar spot fungus is thought to overwinter as sclerotial flakes in the thatch near the edges of dollar spot lesions. These are very hard to see, since they are almost indistinguishable from soil. The fungus also likely survives as dormant mycelium in the tissues of infected turf plants. In North America, this fungus is not known to produce any spores, but research has found that sexually produced spores may also play a role in dispersal of this fungus (Plant Pathology 48:83-94).

When daytime temperatures warm above 15°C in the spring, fungal strands called mycelium start to grow out from the wet thatch onto wet leaf surfaces to cause the initial infections. The fungus can spread from diseased leaves to infect healthy leaves, and diseased tissue can be transported by grass clippings on golf shoes, golf carts and maintenance equipment to areas of healthy turf. This disease attacks the foliage and the crown of the plant, while roots and rhizomes are not infected. Thus in most cases, grass plants are not killed by the attack and can recover with proper maintenance, but repeated defoliation by this fungus will cause plants to weaken and die.

Cultural controls

Cultural controls can be very effective for managing this disease. As with most fungi, limiting the amount and duration of leaf wetness can greatly reduce the occurrence of this disease. Drag a hose or pole the turf in the early morning to reduce the duration of leaf wetness. Other ways of promoting drying of dew are as follows: encouraging air movement by pruning out the lower crowns of trees or thinning stands of trees surrounding greens and along fairways; reducing shade so that early morning sunlight hits the greens; mowing turf in the early morning to displace the dew; or avoiding evening watering during times very conducive to disease spread.

Dollar spot can also usually be minimized by growing turf with adequate nitrogen nutrition. Turf which is low in nitrogen is more susceptible to infection by the dollar spot fungus and is

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much slower to recover from dollar spot injury. It is important however, to maintain a balanced nitrogen status in the turfgrass plant. Overfertilization with nitrogen can encourage other diseases.

Resistant cultivars

Cultivars of creeping bentgrass vary in their susceptibility to dollar spot. L-93, A-1, Providence and Pennlinks have shown superior tolerance to dollar spot (Hurley, 1999). Cultivars which have been shown to be the most susceptible to dollar spot are Crenshaw, 18th Green and Century (Hurley, 1999). Putter, Emerald, Forbes 89-12 and SR1020 were also reported to be the most susceptible bentgrass cultivars based on trial conducted at the University of Guelph (Hsiang and Charbonneau, 1994). By choosing some of the newer cultivars that are less susceptible to dollar spot, fungicide treatments to manage dollar spot, especially on fairways, can be reduced.

Chemical control

Fungicides containing one or more of the following active ingredients are registered for control of dollar spot: boscalid, fluazinam, fluoxastrobin, fluxapyroxad, isofetamid, mineral oil, myclobutanil, penthipyrad, propiconazole, pyraclostrobin, tebuconazole, thiophanatemethyl, thiram, and triticonazole. Consult provincial publications and product labels for registered uses and recommended rates.

Biological control

Several commercially available organic fertilizers have shown suppression of the dollar spot fungus (Liu et al., 1995; Nelson and Craft, 1991), although the exact mechanism of suppression is not clearly understood. There has also been research on the use of strains of the dollar spot fungus which do not cause disease, but can inhibit the growth of strains which do cause disease (Zhou & Boland, 1998).

Fungicide resistance

Because dollar spot disease is so prevalent geographically and throughout the growing season, repeat applications of fungicides are made to control the disease. This has lead to

a long history of resistance development to fungicides.

After using demethylation inhibiting (DMI) fungicides on turf for over 10 years, American researchers have found fungicide resistance in dollar spot (Golembiewski et al., 1995) where normal applications failed to control the disease. Propiconazole was the first DMI fungicide registered on turf in Canada. A baseline sensitivity study conducted before the use of the fungicide in 1994 indicated that a population near the U.S. border had already been exposed to a DMI fungicide. However, the sensitivity of this particular population to the DMI fungicide has declined since 1994 (Hsiang et al., 1997). In 2003, 10 years after the start of DMI fungicide use, the study was continued again using many of the same turf locations from the 1994 study. The results revealed that locations which had multiple DMI fungicide applications (Banner or Myclobutanil) per year had populations of this fungus which were less sensitive to DMI fungicides, but not at the level of full fungicide resistance (Hsiang et al. 2007).

This study was continued 10 years later (Van Den Nieuwelaar & Hsiang 2014), and found even more resistance to DMI fungicides by the populations of the dollar spot organism sampled (14 locations in southern Ontario). However, an interesting finding was that some locations did not have the level of resistance which would have been predicted by their recorded use of DMI fungicides. These few courses had started anti-resistance strategies or decreased their use of DMI fungicides. To ensure that dollar spot does not become resistant to DMI fungicides, it is important to either tank mix DMI fungicides with a broad spectrum fungicide or to alternate DMI fungicides with fungicides from other families.

Literature cited

Hurley, R. 1999. Fungicide needs vary among top new bentgrass cultivars. Golf course Management 67(2):54-57. Hsiang, T. and P. Charbonneau. 1994. Managing dollar spot disease. GreenMaster 28(5):10-11. Hsiang, T., A. Liao and D. Benedetto. 2007. Sensitivity of Sclerotinia homoeocarpa to demethylationinhibiting fungicides 10 years after first use. Plant Pathology 56:500-507 Hsiang, T. and G. S. Mahuku. 1999. Genetic variation within and between local populations of Sclerotinia homoeocarpa. Plant Pathology 48:83-94. Hsiang T, Shi F, Darbyson A. 2014. First report of Sclerotinia homoeocarpa from the sedge Trichophorum cespitosum in eastern Canada which causes dollar spot disease on Lolium perenne and Poa pratensis but not on Agrostis stolonifera. Plant Disease 98:161. Hsiang. T., L. Yang, and W. Barton. 1997. Baseline sensitivity and cross-resistance to demethylation inhibiting fungicides in Ontario isolates of Sclerotinia homoeocarpa. Eur. J. Plant Pathol. 103:409-416. Liu, L.X., T. Hsiang, K. Carey and J.L. Eggens. 1995. Microbial populations and suppression of dollar spot disease in creeping bentgrass with inorganic and organic amendments. Plant Dis. 79:144-147. Nelson, E.B. and C.M. Craft. 1991. Suppression of dollar spot with top-dresssings amended with composts and organic fertilizers. Biol. Cult. Tests 6:93. Sadalgo-Salazar C, Beirn LA, Ismaiel A, Boehm MJ, Carbone I, Putman AI, Tredway LP, Clarke BB, and Crouch JA. 2018. Clarireedia: a new fungal genus comprising four pathogenic species responsible for dollar spot disease of turfgrass. Fungal Biology 122:761-773. Van Den Nieuwelaar A, Hsiang T. 2014. Changes in the sensitivity of the dollar spot fungus, Sclerotinia homoeocarpa, to the demethylation inhibitor fungicide, Propiconiazole, 20 years after first use. Eur. J. Turfgrass Science 45(2/15):43-44. Zhou, T. and Boland, G.J. 1998. Suppression of dollar spot by hypovirulent isolates of Sclerotinia homoeocarpa. Phytopathology 88:788-794.