If red thread appears, application of nitrogen to the affected area will often be sufficient to control it. Nitrogen is best given as sulphate of ammonia at 15g per sq m (½oz per sq yd). Do not apply after August to avoid the production of soft growth which is prone to snow mould.
The threat from red thread can be reduced by taking action to improve the drainage and aeration of the turf. Scarifying the turf with a lawn rake or electric scarifier will remove thatch and moss and increase aeration. Poor drainage and compacted areas can be alleviated by forking or by the use of a solid-tine or hollow-tine aerator. Ensure that the soil is not deficient in nitrogen. Disposing of (not composting) grass cuttings will reduce the amount of fungus present to re-infect the lawn.
The only fungicide available to home gardeners for control of red thread is trifloxystrobin (Provanto 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.
Some lawn maintenance companies offer treatment with professional fungicides, but there is strict legislation controlling the application of professional products to domestic gardens. You will therefore need to satisfy yourself that the company is operating within the law.
Inclusion of a fungicide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Chemicals: using safely and effectively
Chemicals: storing and disposing safely