Identifying and treating lawn diseases: snow mould, red thread and more
When patches appear in the lawn, a fungus is usually behind them. The pathogens are almost always already in the soil. Whether a disease breaks out, however, depends on whether the weather is right and the lawn is weakened. That is also the good news: if you remove the cause, you do not need fungicides in the home garden. Here you can read how to recognise the most common diseases and what helps in each case.
In short
- A lawn disease breaks out when three things come together: a pathogen, a weakened lawn and suitable weather.
- The most important one in Central Europe is snow mould. It appears in late autumn and winter, often without any snow at all.
- Red thread and rust usually indicate a nitrogen deficiency. Snow mould, on the other hand, tends to follow too much nitrogen in autumn.
- The best prevention: fertilise appropriately, keep the soil free-draining, water in the morning, remove leaves and mow with a sharp blade.
Why lawns get sick: the disease triangle
Plant diseases only develop when a pathogen, a susceptible plant and a suitable environment meet. If one of the three sides is missing, the disease does not break out. You can hardly influence the pathogens in the garden; they are practically everywhere. The other two sides, however, are in your hands: a well-fed lawn on free-draining soil that dries quickly after rain rarely gets sick.
The most common lawn diseases at a glance
| Disease | When | What it looks like | Favoured by |
|---|---|---|---|
| Snow mould | October to April, at 0 to 8 °C and wet conditions | round patches, first orange-brown, later grey-white to pink with a cotton-wool-like growth | nitrogen in late autumn, long grass in winter, thatch, waterlogging, leaves |
| Red thread | late summer and autumn, warm and humid | pink to reddish patches, fine red threads at the blade tips | nitrogen deficiency, long leaf wetness |
| Rust | late summer and autumn | yellow-orange pustules on the leaves, orange dust on your shoes | nitrogen deficiency, drought, slow growth |
| Dollar spot | summer, warm days and cool nights with dew | small, straw-coloured, sharply defined spots, often only coin-sized | nitrogen deficiency, drought stress, dew |
| Leaf spot | spring and autumn, cool and damp | spots on the leaves with a dark edge, later thinned areas | leaf wetness, too much nitrogen in spring, thatch |
| Powdery mildew | late summer, shady | white, floury coating that can be wiped off | shade, little air movement, lush growth |
Ring-shaped fungi and fairy rings are a separate topic. More on this in Mushrooms in the lawn.
Snow mould: the most important disease of the winter half-year
Snow mould (Microdochium nivale) does not need snow. It appears in cool, damp weather between about 0 and 8 °C, so in our climate often from October to April. Under a long-lasting snow cover, grey snow mould can also occur. After the snow melts you then see grey, matted patches.
How to prevent it:
- Short and dry into winter: long grass flops over, stays damp and offers the fungus ideal conditions. Keep mowing in autumn as long as the grass grows. Raising the cutting height slightly in autumn is fine, but very long grass in winter is not.
- No more nitrogen from October: soft, nitrogen-rich leaves are particularly susceptible. In autumn, fertilise with a potassium-rich product instead.
- Remove leaves: wet leaves create exactly the microclimate the fungus needs.
- Free-draining soil: aerate and top-dress with sand so that the surface dries after rain. More on this in Waterlogged lawn.
If it has appeared: loosen the patches with a rake in spring so that they dry out. As soon as it gets warmer, the lawn usually grows back by itself. Overseed badly damaged areas in spring.
Important
Do not walk on infected patches while they are wet, and clean your shoes and tools afterwards. Otherwise the spores are spread across the whole lawn.
Red thread and rust: the lawn is hungry
The two diseases look different, but the cause is usually the same: the lawn is growing too slowly because it lacks nitrogen.
- Red thread can be recognised by pink patches and fine, red, antler-like threads at the blade tips.
- Rust can be recognised by orange pustules on the leaves. After mowing, your shoes and mower are dusted orange.
What helps: a balanced nitrogen application in late summer so that the lawn grows again. Regular mowing removes the infected leaf tips. Collect the clippings with the grass box. Both diseases usually disappear within a few weeks as soon as the lawn is growing strongly again.
Dollar spot, leaf spot, mildew
- Dollar spot: these spots tend to appear on very closely mown, dense areas that are not adequately fertilised. What helps: sufficient nitrogen, brushing off dew in the morning or mowing early, watering deeply in dry spells.
- Leaf spot: it appears in cool, damp weather when the leaves stay wet for a long time. What helps: water in the morning instead of the evening, reduce thatch, do not apply too much nitrogen at once in spring.
- Powdery mildew: it is almost always a shade problem. What helps: thin out trees and hedges, cut a little higher in the shade and overseed with shade-tolerant varieties.
Prevention is everything
| Measure | Works against | Why |
|---|---|---|
| Fertilise appropriately | Red thread, rust, dollar spot, snow mould | enough nitrogen during the season, none in late autumn |
| Aerate and top-dress with sand | Snow mould, leaf spot | the surface dries faster, less waterlogging |
| Water in the morning | all leaf diseases | the leaves dry off during the day |
| Reduce thatch | Snow mould, leaf spot | less damp habitat for fungi |
| Remove leaves | Snow mould | no wet microclimate on the lawn |
| Mow with a sharp blade | all leaf diseases | clean cuts offer pathogens less of a point of attack |
| Clean tools and shoes | all | spores are not carried around |
Tip
A clean cut is an underrated form of prevention. The frayed blades left by a blunt mower blade give fungi a large point of attack. A sharp reel mower cuts the grass off cleanly. Why that is so is explained in Reel mower: what it does better than a rotary mower.
Note for reel mowers
In autumn, leaves and twigs often lie on the lawn. Then it is better to mow with a rotary mower so that the reel is not damaged. For disease prevention, what matters most is that the leaves are removed regularly.
What you should avoid
| Common tip | Why it does not help |
|---|---|
| Spraying fungicides | Not necessary in the home garden if the cause is removed. The pathogens remain in the soil, and without removing the cause the disease comes back. |
| Another heavy nitrogen feed in October | Soft, nitrogen-rich tissue is a feast for snow mould. |
| Watering in the evening | The leaves stay wet overnight, which encourages almost all fungal diseases. |
| Liming on suspicion | A high pH tends to encourage snow mould. Only lime after a soil analysis. |
Frequently asked questions about lawn diseases
How can I tell whether it is a disease or drought?
Drought damage covers larger areas and often follows sunny spots or slopes. Diseases usually form round, defined patches, and you can see coatings, pustules or threads on the leaves.
Does snow mould go away on its own?
Usually yes, as soon as it gets warmer and drier in spring. Overseed badly damaged areas.
Is rust dangerous for children or dogs?
No. The dust looks unpleasant but is harmless.
Do I have to cut out diseased patches?
As a rule, no. The grasses sprout again from the base once the cause has been removed.
Can I compost clippings from diseased areas?
With a light infection yes, if the compost rots down well. With a heavy infection, it is better to put them in the green waste bin.
Tools for a healthy lawn
- Lawnpecker: aerating with hollow spoons so that the lawn dries faster.
- REWUKIL: for dethatching and against thatch.
- Falkenstein: a clean scissor cut with clean cut edges.
- Sharpening and maintenance service: so that the reel stays sharp.
