Dollar Spot Disease
By Dr Kate Entwistle
Dollar spot is a fungal disease that appears to be increasing in occurrence across the UK. Historically in the UK, dollar spot has been recorded as a disease of fine-leaved fescues but, over the last five years, we have seen a steady rise in the number of outbreaks of this disease developing on Poa annua swards. In Europe and across the USA, dollar spot can be a devastating disease on Agrostis sp. turf.
The fungus responsible for this disease has always been called Sclerotinia homoeocarpa even though we have known for many years that it is not actually a Sclerotinia fungus. The reason for this is quite complex and the difficulty in naming the fungus is evident from the fact that to date, there is no definitive name for it. It is suggested by some researchers that the fungus is a Rutstroemia species but until we have a confirmed re-naming of the fungus, we can continue to call it by the name that is generally accepted.
Regardless of its name, this fungus causes rapid outbreaks of disease on turfgrasses under a range of mowing heights but on close mown turf, the symptoms can be dramatic. The characteristic small (2 cm diameter) bleached spots of infected turf can develop so extensively that it can be difficult to stand on an affected area without putting your foot on the diseased plants.
Dollar spot can develop either during late spring/early summer or late autumn but more severe outbreaks appear to develop in the autumn. Drought-stressed turf is particularly susceptible to infection but free water, high relative humidity or heavy dew are necessary for disease development. High daytime temperatures and low nighttime temperatures, when combined with the presence of heavy dew, are ideal conditions for this disease. Turfgrass grown under low nitrogen nutrition is more susceptible to dollar spot but rootzone pH does not appear to influence susceptibility.
The disease shows as 2 to 4 cm diameter bleached spots which do not increase much in size but which do coalesce to form large areas of bleached turf. In all grasses apart from Poa, individual affected leaves show a characteristic 'hourglass' lesion which can be seen as a bleached, slightly narrowed portion of the leaf bordered at each end with a narrow, dark band. In Poa, the bleached lesion is present but the dark borders are not.
Management of this disease is based around cultural controls. Thorough deep, but infrequent irrigation is recommended as a way of providing sufficient water to the plant whilst reducing the overall period of leaf wetness. Removal of dew is essential to minimise the chance of infection. Adequate nitrogen nutrition is critical both to minimise the onset of the disease and also to aid recovery of an affected sward.
Several fungicides that are currently available for use on managed amenity turf have shown efficacy against this turf disease and, where necessary, can be used as part of an integrated programme to manage dollar spot. Always ensure that the disease is correctly identified prior to the application of any plant protection product.
Dr Kate Entwistle MBPR
The Turf Disease Centre
Waverley Cottage, Sherfield Road, Bramley, Hampshire RG26 5AG ENGLAND
Telephone: 01256 880246
Fax: 01256 880178
Mobile: 07879 468641