// The Problem
Dollar spot, caused by Sclerotinia homoeocarpa, is a widespread and very destructive turfgrass disease that can be observed throughout the year in Canada. Dollar spot is known to attack most turfgrass species including annual bluegrass, bentgrasses, fescues, Kentucky bluegrass, perennial ryegrass, bermuda grasses, zoysia grasses, centipede grass and St. Augustine grass. Recently, university researchers and scientists have proposed new Latin names for the dollar spot pathogen through molecular techniques. The name Sclerotinia homoeocarpa will likely be split up into four separate species including Clarireedia bennettii, Clarireedia homoeocarpa, Clarireedia jacksonii, and Clarireedia monteithiana of which, C. jacksonii is likely the predominant species in Canada.
// What to Look For
Dollar spot is favoured by air temperatures ranging from 15-32 ºC (optimum 21-26 ºC) with extended periods of high humidity (>85% at night). The disease tends to be most severe under the following conditions: warm days, cool nights, infrequent rain but long dew periods, daily ground fogs that extend leaf wetness periods, and low nitrogen fertility. When the fungus is active and leaf surfaces remain wet, a fine, white, cobwebby mycelium covers the infection centres or diseased patches during early morning hours.
Symptoms of dollar spot can vary based on the turfgrass species and height of cut. Under close mowing heights, as with intensively maintained bentgrass or annual bluegrass, the disease appears as small circular straw-coloured spots of blighted turf about the size of a silver dollar. On coarser textured turf maintained under higher mowing practices, such as Kentucky bluegrass or perennial ryegrass, the blighted areas are considerably larger, straw-coloured patches 3-6 inches in diameter.
Affected patches frequently coalesce and involve large areas of turf. Grass blades generally die back from the tip with distinct hourglass shaped lesions that are straw-coloured or bleached white. Hourglass bands may not appear on warm-season grasses.
// Bayer Solutions
Implementing proper cultural practices is crucial to reducing disease severity. Management tactics include:
• Maintaining adequate nitrogen when dollar spot is active
• Making light frequent nitrogen applications, avoiding drought stress
• Removing dew by mowing, poling, or rolling
• Aerifying to reduce compaction and thatch
• Removing trees or adding fans to increase air circulation
Fungicides are a key part of an integrated dollar spot management program. Since dollar spot is a foliar disease, select spray nozzles and spray volumes that provide good coverage to maximize fungicide activity. Early spring applications of fungicides after the 2nd true mowing can control early dollar spot infections. Preventive applications of DMI fungicides, like Mirage Stressgard, on fairy ring and summer patch (average soil temperatures of 13 ºC at a 5-10 cm depth) also provide early dollar spot control. Routine fungicide applications are commonly needed when air temperatures are 21-32 ºC; extended periods of temperatures above 32 ºC may significantly slow dollar spot development. The use of dollar spot prediction models, such as the Smith-Kerns model, can be a valuable resource in determining the proper spray intervals. Integrating cultural and chemical control strategies provides the most effective control of this fungal disease.
Interface Stressgard is an ideal choice for dollar spot control in all weather conditions. Interface is especially helpful in hot weather, since it is not a DMI and can be used without risk of negative plant growth effects, as well as for fall dollar spot applications when Bipolaris leaf spot and other cooler-weather diseases are active and the impact of DMI resistance is highest.
Resistance to certain classes of fungicides can be significant for dollar spot. Resistance to the benzimidazole fungicides is common, and DMI resistance can also be problematic. Preventive applications and rotating fungicide classes, including the use of multi-site fungicides, are important for reducing the risk and impact of resistance. All Bayer solutions for dollar spot will control benzimidazole-resistant populations. Interface is effective against DMI-resistant populations. If DMI resistance is present, increasing the rates or shortening the application interval of Mirage Stressgard, or using a combination fungicide like Trilogy Stressgard, will help keep DMI fungicides effective in these situations.