Dollar Spot

Dollar spot, a significant disease affecting various turfgrass species globally, poses challenges in control due to fungicide resistance, particularly on golf courses. The disease impacts susceptible cultivars of Agrostis, Cynodon, Festuca, Lolium, Paspalum, Poa, Stenotaphrum, and Zoysia spp., leading to both aesthetic and economic consequences. In the past, uncertainty surrounded the taxonomy of the causal agent, but recent phylogenetic analysis unveiled six distinct fungi species responsible for dollar spot. These fungi now belong to the newly described genus Clarireedia, resolving a major controversy and highlighting the biological diversity within this genus.

 

Credit: Syngenta Turf

 

Causal agents

  • Clarireedia jacksonii – All cool-season grasses
  • Clarireedia monteithiana – All warm-season grasses
  • Clarireedia homoeocarpa – Original type species from the United Kingdom
  • Clarireedia bennettii – Cool-season turfgrass in the Netherlands, United Kingdom, and USA
  • Clarireedia paspali– Paspalum species

 

Primary Hosts

All turfgrass species are affected by Clarireedia spp. This is one of the most common diseases of turfgrass worldwide.

 

Disease Cycle

The dollar spot fungi have a complex disease cycle that involves survival in infected plants, debris, and thatch during unfavorable conditions. They can also be disseminated through infected leaf debris carried by various means such as equipment, people, animals, water, or wind. The pathogen can even be present in commercial seed lots. 

When environmental conditions become favorable, such as warm and humid days, cool nights, and heavy dew, the mycelium within previously infected tissues or from stromata begins to colonize the foliage. The mycelium grows prolifically and can extend into the surrounding air. When it comes into contact with a moist leaf surface, it can penetrate the leaf and initiate an infection. The pathogen can also enter leaves through stomata, cut leaf tips, or other mechanical wounds.

There is evidence that a toxin produced by the fungus may be associated with the necrosis of the apical meristems and roots of Agrostis stolonifera when grown in sand culture, although its exact role in the infection process is still not fully understood.

While spores have not been observed in North America, genetic variation within populations of the pathogen suggests that genetic recombination can occur. However, the formation of apothecia, the reproductive structures of the fungi, is rarely observed in nature.

Overall, the disease cycle of dollar spot involves the survival of the pathogen in various forms, its dissemination through infected debris, and its ability to colonize and infect turfgrass foliage under favorable environmental conditions.

 

Epidemiology

The epidemiology of dollar spot involves several key factors that contribute to its development and spread. The disease activity typically occurs from late spring through late autumn or early winter. Warm, humid weather combined with cool nights that produce heavy dews create favorable conditions for disease development. Temperatures ranging from 15-30°C (59-86°F) are conducive to dollar spot, although the optimal temperature for mycelial infection varies among different biotypes and geographical locations.

Moisture levels in the aerial canopy are crucial for infection, even though the disease is more severe in dry soils. Several epidemiological models have been developed to aid in the timing of fungicide applications for dollar spot control. These models use temperature and moisture thresholds to predict disease activity. However, early models like the Mills-Rothwell and Hall models had limitations in accurately predicting dollar spot development.

Recent advancements have led to the development of more sophisticated models, such as the growing degree day (GDD) model and the logistic regression model. The GDD model considers air temperature and calculates the accumulated degree days to predict the initial onset of dollar spot. The logistic regression model uses 5-day average air temperature and relative humidity to estimate the probability of new infections. This model-based approach has shown promise in providing effective control with fewer fungicide applications.

Factors such as nitrogen fertility levels influence the susceptibility of turfgrass to dollar spot, with low nitrogen fertility making the turf more susceptible. Soil pH and phosphorus levels do not significantly impact disease severity, while the effects of potassium application vary. Disease detection methods, including ELISA kits and real-time PCR, have been used to identify and diagnose the pathogen in infected plant tissue.

 

Symptoms

Dollar spot symptoms are commonly observed on closely mowed grasses such as golf greens and fairways. They appear as small, circular, sunken spots of blighted turf that usually do not exceed 5 cm (2 in) in diameter. In conditions favorable for disease development, these spots may merge and form larger, irregular patches. On taller turfs like residential lawns, the symptoms are typically irregularly shaped, bleached patches ranging from 2-15 cm (1-6 in) or more in diameter. These patches can coalesce and result in extensive areas of blighted turf.

Leaves affected by the dollar spot pathogen develop lesions that initially appear chlorotic, then become water-soaked, and eventually turn white or straw-colored. Individual leaf blades may have single lesions, multiple small lesions, or be entirely blighted. Dollar spot lesions are characterized by tan to reddish-brown margins and often expand to cover the entire leaf, except on grasses with coarse leaves. The lesions may exhibit an hourglass shape, although they can be oblong or oval with brown borders on certain warm- and cool-season grass species. Dieback from leaf tips is also common.

Credit: John Kaminski/Penn State University

Figure 10.7.1: Dollar spot symptoms vary based on height of cut. Under higher mowing heights, you can generally see (a) tan patches of affected turf and (b) typical hourglass lesions that span across the margin of leaves. On shorter mown turfgrass, the spots are typically (c) silver dollar sized infection centers that, in severe situations, can (d) kill the turf forming small pits.

 

Signs

When dew is present on grass blades and the disease is active, a white, cottony, or cobwebby growth of fungal mycelium may be visible on the affected turf. This aerial mycelium disappears as the leaves dry. It is important to note that this fungal growth can be mistaken for similar growth produced by other fungi like Pythium and Rhizoctonia.

Clarireedia signs

Credit: John Kaminski/Penn State University

Figure 10.7.2: Signs of the Clarireedia species include profuse development of mycelium in the field (a-c). Under microscopic observation, the septate mycelium has variable widths and often is associated with “Y-angle” branching (d).

 

Management

Dollar spot, a common turfgrass disease, can significantly impact the health and aesthetics of turf. To effectively manage dollar spot, a combination of cultural and chemical control strategies is recommended. Cultural practices focus on creating unfavorable conditions for disease development, such as proper irrigation, promoting airflow, and maintaining turf health. Chemical control involves the targeted use of fungicides to suppress the disease. By implementing these integrated management approaches, turfgrass managers can effectively mitigate the impact of dollar spot and maintain healthy and visually appealing turf.

 

Cultural

To effectively manage dollar spot, cultural practices should be implemented. These include proper irrigation techniques, where turf is watered thoroughly but infrequently, avoiding moisture stress while preventing prolonged leaf wetness. Irrigation should be avoided in the late afternoon or evening when dew is likely to form. Dew and guttation fluids should be removed from leaf surfaces through early morning mowing, rolling, or poling.

Good air circulation over the turf can be promoted by pruning or removing trees, shrubs, and other barriers. Mowing should be done at the recommended frequency and cutting height for the specific turfgrass species. Aeration can help reduce compaction and control thatch, while lightweight rolling several times a week has shown to decrease dollar spot pressure.

Sand topdressing has also been found to reduce disease severity. Maintaining adequate to high nitrogen fertility during active dollar spot periods is important, as light, frequent nitrogen applications help minimize disease severity and support recovery once the infection is controlled. Ferrous sulfate applications have shown to suppress dollar spot, especially when used in conjunction with a fungicide program. Some organic fertilizers and composted products have also been reported to reduce disease severity, likely due to their effects on nitrogen availability rather than enhanced microbial activity.

Hypovirulent isolates of certain fungal species and strains of beneficial bacteria have demonstrated suppression of dollar spot in the field, but the level of suppression is generally not considered acceptable in highly maintained turfgrass systems such as those found on golf course turf.

 

Chemical

Numerous fungicides are available for the control of dollar spot. However, it is important to consider local variations in pathogen sensitivity to different active ingredients, which can affect the effectiveness of specific products. Examples of effective fungicides include all active ingredients in the DMI and SHDI classes, thiophanate-methyl (Cleary 3336), iprodione (Chipco 26GT), fluazinam (Secure), isofetamid (Kabuto), and others. Select QoIs including fluoxastrobin (Fame), mandestrobin (Pinpoint), and pyraclostrobin (Insignia) can effectively suppress dollar spot; others are not effective against this disease. When curative applications are required, it is always wise to tank mix an effective fungicide with chlorothalonil (Daconil, Echo) to reduce the potential for the development of resistant strains of the pathogen.

Fungicide resistance has been a persistent problem, particularly in areas with frequent fungicide applications like golf courses. Resistance has been identified in the following chemical classes:

  • DMIs
  • dicarboximides
  • benzamidazoles
  • SDHIs

To minimize the development of fungicide-resistant strains:

  • follow label directions
  • avoid repeated use of chemically similar fungicides
  • consider tank mixing or alternating products with different modes of action
  • implement sound cultural practices that reduce stress and enhance plant vigor
  • use disease predictive models that include temperature and humidity values to time fungicide applications more accurately