Lawn Diseases: Red Thread & Fusarium

Noticed strange patches appearing on your lawn? Pink or red threads on the grass blades? Circular brown areas that seem to spread? These are classic signs of lawn disease, and while they look alarming, most are treatable with the right approach.

Red thread and fusarium patch are by far the most common lawn diseases in UK gardens. Both are fungal infections that thrive in our mild, damp climate. The good news is that understanding what causes them reveals the solution, and it is usually simpler than you might think.

Red Thread: The Most Common Lawn Disease

Red thread (Laetisaria fuciformis) is the disease you are most likely to encounter in a UK lawn. It appears as irregular patches of bleached or pinkish grass, typically 5-20cm across. Look closely and you will see the telltale sign: pink or red thread-like strands binding the grass blades together, particularly visible in the morning dew.

Red thread typically appears in late summer and autumn when conditions are warm and humid. It does not actually kill grass, it just causes the leaf blades to die back, giving that bleached, straw-like appearance. The grass will recover once conditions change.

What Causes Red Thread?

Here is the important part: red thread is almost always a symptom of nitrogen deficiency. The fungus attacks weak, underfed grass that lacks the vigour to fight off infection. Lawns that have not been fed regularly, or those on poor soil, are far more susceptible.

Other contributing factors include compacted soil, poor drainage, and heavy thatch buildup. All of these stress the grass and make it vulnerable. If your lawn has yellowing patches alongside the red thread, nitrogen deficiency is almost certainly the root cause.

Fusarium Patch: The Winter Threat

Fusarium patch (Microdochium nivale), also called snow mould, is most common from autumn through to spring. It creates circular patches that start small but can grow to 30cm or more, often with a distinctive darker ring around the edge. In damp conditions, you might see white or pink fungal growth at the margins.

Unlike red thread, fusarium can kill grass if left untreated. It particularly affects annual meadow grass (Poa annua), which is present in most UK lawns whether you planted it or not. The disease thrives in cool, wet conditions with poor air circulation.

What Causes Fusarium?

Fusarium often appears after prolonged wet weather, especially when combined with mild temperatures. Lawns with excessive thatch, poor drainage, or compacted soil are prime targets. Walking on wet grass can spread the fungal spores, making patches multiply.

Overfeeding with nitrogen in autumn can actually encourage fusarium, which is why timing your lawn feeding correctly matters so much. Autumn feeds should be lower in nitrogen and higher in potassium to harden grass for winter rather than promoting soft, disease-prone growth.

How to Treat Lawn Diseases

The frustrating truth is that there are no fungicides available to home gardeners for treating lawn diseases in the UK. Products that were once available have been withdrawn for environmental reasons. Professional greenkeepers have access to some treatments, but for domestic lawns, we need to focus on cultural controls.

For Red Thread

The solution for red thread is simple: feed your lawn. A nitrogen-rich fertiliser applied when you first spot symptoms will usually clear the problem within a few weeks. The grass will grow out of the infection as it gains strength from the nutrients.

Our Spring Lawn Treatment or Summer Lawn Treatment are ideal depending on when you spot the problem. Regular feeding throughout the growing season, following a proper spring lawn care routine, prevents red thread from taking hold in the first place.

For Fusarium Patch

Fusarium is trickier because it often strikes when feeding is not advisable. If you spot it in autumn or winter, avoid high-nitrogen feeds which will make things worse. Instead, focus on improving conditions:

Reduce thatch through scarifying in early autumn before disease season. Improve drainage by aerating compacted areas. Avoid walking on wet grass, and if possible, brush morning dew off affected areas to reduce moisture.

Once spring arrives and soil temperatures rise, a balanced feed will help grass recover and fill in damaged areas. Severe patches may need overseeding to restore full coverage.

Preventing Lawn Diseases

Prevention is far more effective than cure. A well-maintained lawn rarely suffers serious disease problems because healthy grass can resist infection naturally.

The key prevention strategies are regular feeding to maintain nitrogen levels, proper mowing height to reduce stress, good drainage through annual aeration, and thatch control through scarifying. Following a complete autumn lawn care routine sets your lawn up to resist disease through the vulnerable winter months.

Avoid overwatering and improve air circulation by trimming back overhanging plants. Remove fallen leaves promptly in autumn since they trap moisture and create perfect conditions for fungal growth. For more detailed prevention strategies, browse our lawn health and maintenance hub.

Other Common Lawn Diseases

While red thread and fusarium are the main culprits, you might occasionally encounter other issues.

Lawn rust appears as orange or yellow powder on grass blades, usually in late summer during dry spells. Like red thread, it indicates nitrogen deficiency and responds well to feeding.

Fairy rings create dark green circles or arcs in the lawn, sometimes with mushrooms appearing. These are caused by fungi breaking down organic matter in the soil and are difficult to treat. The best approach is to mask them with extra feeding to even out the colour.

Dollar spot creates small, bleached patches about the size of a coin. It is less common in the UK but appears in dry conditions on underfed lawns. Again, proper nutrition is the solution.