Dollar Spot
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
Nematodes
NEMATODES are a large and diverse group of non-
segmented, microscopic roundworms that can inhabit both the rootzone and the turfgrass plants themselves. There are three main types of nematode but it is only the plant parasitic nematodes that have the ability to cause damage or disease to the turf.
All plant parasitic nematodes possess some form of stylet or hollow feeding organ in their mouth that is similar to the stylet used by greenfly to pierce herbaceous plants. The nematodes push this stylet in to the plant cells, inject chemicals to assist their feeding and remove nutrients and water from the turfgrass plant. Depending on their type, nematodes feed either on or in the roots, stem or leaf of the plant but it is generally those that feed on or in the roots that can cause the most damage to close-mown turf. For many years it was believed that plant parasitic nematodes only caused damage to warm-season turfgrasses but over the past 6 years, I have seen a steady increase in the number of amenity sites across the UK and Ireland that have been affected by nematodes at some time during the year. Damage appears to be most prevalent on stadia pitches, golf courses and racecourses though all amenity grasses have the potential to
become affected. The symptoms that develop on the
sward will vary depending on the nematode population present but generally, damage is first seen as chlorosis of the plant and reduced turf vigour. The turf generally doesn't respond well to application of either nutrition or irrigation and the damage often progresses to a yellowed sward in discrete- or diffuse-edged patches. Affected areas will tend to show up at the same time each year as rootzone temperatures increases and activity of the nematodes increases. Symptoms can often resemble those of fungal disease but fungicide applications fail to produce any recovery. In areas affected by nematode damage, foliar application of nutrients can offer some recovery from the symptoms, allowing the plants to bypass the roots and take in nutrients through the leaf. If the plants can continue to develop new roots to compensate for those damaged by the feeding nematodes, the symptoms will not become as sever and recovery will be quicker.
Nematode populations will vary
through the year following almost a cyclical pattern and if high populations coincide with severe physical or nutritional stress of the turf, extensive damage can occur. It will be possible to
find plant parasitic nematodes in most rootzones and on areas where the turf appears quite healthy. As with fungal disease, it is not merely the presence of the nematode that is the problem but the population size and the general health of the turf.
Although we have recently identified a completely new species of nematode that is causing damage to bentgrass turf, it is believed that all of the nematode damage seen in the UK and Ireland is being caused by native species that have developed to damaging population levels as a result of the sward composition and/or the inherent stress that comes with producing quality playing surfaces. If you think that you may have a
problem with nematodes affecting your turf, look for shallow root growth or a deformity in the root development (swellings, excessive branching, stunted or stubby growth). You will not be able to see the nematodes without the use of a microscope but if you send a sample through, I can check it for you.
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 The Turf Disease Centre Tel: 01256 880246
disease OF THE MONTH
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