Managing dollar spot: what does the research say?

Dollar spot has become a growing concern for turf managers across the UK and Ireland.

Once considered a disease of warmer, humid climates, recent years have seen its incidence and severity increase in temperate regions.

Combined with the gradual loss of fungicide availability and mounting resistance, dollar spot has become one of the most pressing disease challenges in modern turf management.

Dollar spot (Clarireedia spp.)

This article distils insights from refereed research papers to outline practical, research-led strategies for managing dollar spot with reduced reliance on fungicides.

The goal: a sustainable, integrated approach tailored to professional turf conditions in the UK and Ireland.

Understanding the pathogen

The fungus historically identified as Sclerotinia homoeocarpa was reclassified in 2018 as part of the Clarireedia genus.

Several species are now recognised, including C. jacksonii, C. monteithiana, C. homeocarpa and C. bennetti, the latter two more commonly found on turf surfaces in the UK.

Dollar spot thrives between 21–27°C in humid conditions above 70% relative humidity.

It overwinters in the thatch layer and activates rapidly under favourable summer conditions, making it increasingly relevant in UK summers.

Fungicide resistance: a growing limitation

While fungicides such as Ascernity and Instrata Elite can still be effective, repeated use of single actives has led to widespread resistance for some active ingredients.

Resistance is now well-documented in the literature, especially from the US and parts of Europe.

Best practice includes rotating actives and using disease prediction models, such as the Smith-Kerns model, to time applications more precisely.

However, to future-proof disease control, turf managers are increasingly looking to reduce fungicide dependency through cultural and nutritional strategies.

Integrated management: what research recommends

Cultivar and Species Selection

A clear gradient of species susceptibility exists. Tall fescue is among the most tolerant, followed by perennial ryegrass, then red fescue and creeping bentgrass. Poa annua turf is the most vulnerable.

Modern creeping bent cultivars such as Riptide and the upcoming Piranha show improved resistance.

Fescue cultivars with high endophyte levels such as Rubicus have also demonstrated reduced disease incidence. No cultivar is immune, but selection and overseeding with more tolerant varieties can significantly reduce disease pressure.

Dollar spot (Clarireedia spp.)

Rolling: A Proven Physical Suppression Method

Perhaps the most consistent non-chemical strategy across research trials, regular rolling has been shown to reduce dollar spot severity significantly. In multi-year trials:

• Rolling five to ten times per week yielded the greatest disease suppression.
• Morning and afternoon rolling were equally effective, suggesting benefits are not solely due to dew removal.
• Benefits likely stem from microclimate modification and thatch layer compaction.

Rolling is especially valuable during peak risk periods and has shown potential benefits on greens, approaches and even fairways.

Moisture Management

Unlike many turf diseases, dollar spot thrives in conditions where the soil is dry but the canopy remains moist and humid. A fine balance must be struck:

• Dew removal is critical, through switching or rolling.
• Maintain consistent soil moisture at around 75% of field capacity.
• Light, frequent irrigation regimes are preferred over deep, infrequent watering.

Managing moisture in this way helps reduce plant stress and creates less favourable conditions for pathogen activation.

Thatch Reduction

Research confirms that Clarireedia survives in the thatch, not the soil. This makes effective thatch management critical:

• Scarification, hollow coring, and sand topdressing all contribute to lowering thatch volume.
• Less thatch should mean fewer infection reservoirs and less disease carryover.

Nutritional approaches: Getting nitrogen right

Nitrogen availability has a direct, documented impact on dollar spot severity. Low nitrogen encourages disease; appropriate feeding suppresses it.

Research suggests:

• Weekly applications of 3–5 kg N/ha during summer are a good starting point.
• Higher rates (e.g. 10 kg N/ha every two weeks) produce more significant reductions, though practicality and species tolerance must be considered.
• Urea performs as well as other mineral N sources, and often better than organic N sources.

Exploring Non-Fungicidal Applications

Biostimulants

Many biostimulants will have been tested in dollar spot trials, with few providing real significant benefits.

Biostimulants designed to stimulate systemic acquired resistance (SAR) or induced systemic resistance (ISR) offer potential, but results are mixed.

A key takeaway from recent studies is that cultivar response to SAR/ISR products varies widely. Some treatments, such as silicic acid (not be be confused with salicylic acid, show consistent benefits. Others can be ineffective or even exacerbate disease depending on the turf cultivar.

This highlights the importance of testing SAR/ISR products in a site-specific context, particularly on Agrostis species, which display high genetic variability.

Biological based products

Products containing beneficial microbes like Pseudomonas or Bacillus have shown occasional promise, but consistency in response remains a problem.

Frequent application, sometimes weekly is often necessary to see any benefit. Success in laboratory-based trials has not often translated into reliable field performance.

Iron Sulphate

A traditional tool with modern relevance, iron sulphate applied at high rates (up to 48 kg/ha biweekly) has demonstrated suppression of dollar spot in trials. The mechanism appears to involve direct toxicity to the fungus. Sulphur alone and chelated iron do not produce the same effect.

Applications of iron at this high rate are not often desirable however, due to the subsequent turf blackening.

Practical summary

Based on the literature and trials reviewed:

• Fungicides: Effective but use sparingly, rotate actives and follow disease models for precise applications.
• Cultivar selection: Prioritise disease-tolerant species and cultivars and overseed where possible.
• Rolling: Roll frequently, especially in summer to disrupt disease progression.
• Moisture: Remove dew daily and avoid drought stress.
• Thatch: Reduce organic matter via physical methods.
• Nitrogen: Maintain feeding rates of 3–5 kg N/ha/week or more.
• Biostimulants: Use selectively; silicic acid appears most reliable.
• Iron sulphate: Can be integrated as a supplementary control method.

Ongoing research

Ongoing trials in Italy, France and Australia are evaluating ITM programmes and a combination of new and existing products. Initial results are expected in late 2025 and will provide further evidence to refine IPM strategies for dollar spot in UK and Irish contexts.

Conclusion

Dollar spot management is getting a lot of focus across the industry now.  Continued trial work is necessary to allow turf professionals to make informed decisions to help protect their surfaces.

It is expected that an integrated approach combining better understanding, precise fungicide applications with supporting nutritional inputs and targeted cultural practices will help to manage the disease.