Snow Mould or Fusarium Patch Disease in Turf
The following advice has been issued by the Royal Horticultural Society
Symptoms
This common lawn disease is caused by the fungus Monographella nivalis (syn. Fusarium nivale). Attacks can occur at any time of the year, but are most severe in October and during mild spells in the winter. As the name suggests, however, the disease sometimes follows a thaw of snow, particularly on areas of lawns which have been trodden on when snow covered. The disease first shows as small patches of yellowish, dying grass that later turn brown, increase in size and coalesce. In moist weather the patches become covered with a white or faintly pink cottonlike fungal growth, which mats the dying vegetation together. The disease is encouraged by lack of aeration and excessive use of nitrogenous fertilisers, especially when applied after August. Lawns containing a large proportion of the weed grass Poa annua (annual meadow grass) are liable to be severely affected but other grasses including Festuca rubra (red fescue) are also susceptible to infection.
Biology
The fungus is considered to be soilborne, but mainly survives on infected plant material. It is a lowtemperature organism and can survive temperatures as down to –20°C. When the conditions become favourable, the fungus begins to grow. This initial growth is slow and may go undetected for several weeks, but, as conditions become favourable, the infection spreads rapidly. The optimum conditions in Britain are during wet weather, when the temperature is between 12 and 19°C. The disease can therefore occur in summer, but fast lawn growth often masks its presence. As the weather cools down and grass growth slows, the disease becomes more apparent. Short periods of cold, moist weather can predispose turf to attack, as can sharp frosts which slow grass growth. Spores of the fungus and infected debris can be easily transferred on equipment or shoes to cause ne
w infections elsewhere. The fungus requires surface moisture to infect leaves and grass which is particularly lush due to high nitrogen levels is more susceptible. Poor drainage and long blades of grass also increase humidity and may encourage the disease.
Prevention
As snow mould develops where the turf is poorly aerated, the disease can be prevented by good cultural treatment. Scarifying the turf with a wire rake to pull out dead, matted material will help to improve the aeration of the surface. Aeration by
spiking the surface of the lawn is also advisable. Spiking is best carried out using a tool that gives at least 7.5cm (3in) penetration and preferably 10–15cm (4–6in). For small areas of compaction, an ordinary garden fork is suitable: use the fork backwards to pierce the surface vertically. Drive it in with the foot and then ease it gently backwards and forwards. This will slightly enlarge the hole, yet allow the fork to be lifted out without disturbing the surface of the lawn. Space each set of holes 10–15cm (4–6in) apart, working backwards to avoid treading on aerated turf. If the soil is heavy or waterlogged, hollowtine aerators give better results. Each hollow tine removes a core or plug of soil, expelling it on to the lawn surface at the next penetration. This relieves compaction by allowing the soil to expand. The holes act as air and water channels, and stimulate new root growth. After treatment, sweep up the cores, then top dress with a sandy mixture and brush this into the holes. This creates permanent cores of more freedraining material in the soil. It should only be necessary to hollow tine every third or fourth year. In spring and summer, spike localised areas of compaction with a garden fork or solidtine aerator. Repeat the treatment as necessary at fouror fiveweek intervals when the soil is moist, and preferably in cool, showery weather, then water afterwards. Spiked rollers may be used during the summer months to break up light surface compaction and improve moisture penetration.
In September carry out deeper and more general aeration, including hollow tining every three to four years as part of the autumn programme of renovation, scarifying and topdressing afterwards. Ensure that the soil is moist, since this enables the work to be done easily. In winter carry out localised spiking where necessary to disperse standing water.
In addition to aerating the lawn, it is advisable to brush off heavy dew early in the morning with a besom or bamboo cane, to keep the grass as dry as possible. The turf should be kept firm and no nitrogenous fertilisers should be given in late summer or early autumn. Regular mowing will also help to prevent this disease and, even in winter, the grass should be kept short and open.
Control
The only fungicide available to amateur gardeners for control of this disease is trifloxystrobin (Lawn Disease Control). The fungicide can be used all year round except during drought conditions or when the lawn is frozen. There is a risk of resistance occurring in the fungal population therefore the product should not be applied more than twice a year and is best used in conjunction with other control methods.
The Sports Turf Research Institute recommends using iron sulphate, a moss killer (see Horticultural leaflet on ‘Weedkillers for Amateur Gardeners’ number 1330 [114]), to reduce the severity of the disease. This compound not only toughens the grass plants, but acidifies the turf microclimate, which in turn discourages snow mould.
Chemical available to the gardener
FUNGICIDE PROPRIETARY PRODUCT
(active ingredient)
Trifloxystrobin Wettable powder: Lawn Disease Control (Bayer Garden)
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