Tony Dickerson problem solves!
Q What’s to blame for the brown patches on my lawn? Gordon Waugh, Edinburgh
A There are a number of turf diseases leading to brown patches on lawns and it’s not that easy to diagnose them by eye. One of the most common is red thread. This causes brown patches in turf, typically up to 25cm (10in) in diameter. They have a reddish tinge at first but later become bleached in appearance, which matches your description of the damaged grass blades appearing almost translucent. Early on, the red fungal threads are usually visible with a hand lens.
This disease frequently occurs in wet summers, especially where lawns are deficient in nitrogen. This can be put right earlier in the year with sulphate of ammonia applied at 15g per sq m (½oz per sq yd), but don’t apply it after August – instead opt for an autumn lawn fertiliser for now. Red thread rarely kills the grass completely and the patches should recover with appropriate remedial action, including improving drainage and aeration of the turf by forking or hollowtining, which you can do now.
More common from autumn onwards is fusarium patch, which starts off as patches of yellowish, dying grass that later turn brown. During wet weather you may notice white or pinkish, cottony fungal growth. It’s one of the most damaging diseases of turf grasses and can be difficult to control.
The disease is most noticeable after thaws of snow, hence the common name of ‘snow mould’. Patches may reach 30cm (12in) or more in diameter, often merging together, affecting large areas.
To prevent fusarium patch, avoid high doses of nitrogen fertiliser in late summer and autumn, but apply autumn lawn feed. Moss killers based on iron sulphate do help toughen up the grass, increasing the acidity of the soil, which discourages the disease. Use a combination of scarifying, spiking and hollowtining to improve aeration and drainage and cut back overhanging branches to improve airflow over the lawn.
If you don’t know whether you need to apply more or less nitrogen next spring, and it can be confusing, treat half the lawn and monitor the results. In addition, you can apply Bayer Garden Lawn Disease Control up to twice a year. It’ll be effective against both diseases.