- Yellow patches that turn brown and grow a white cottony fuzz in wet weather are a lawn fungus, and small patches recover once the grass dries out and spring growth starts.
- Staying off the grass, raking the matted blades gently and knocking the dew off in the morning slow the spread, while heavy feeding and wet mowing speed it up.
- No spray reverses damage you can already see, so the fix is drier grass, fewer footsteps and fresh seed only on the patches that stay dead.
Why the Patch Looks Bigger Every Morning When the Weather Stays Mild and Wet
Keep your feet off the patches, rake the flattened, matted grass very gently so air and light reach the soil, brush the dew off the blades first thing in the morning, and leave the fertiliser in the shed. Small patches usually recover by themselves. Only the ones that are still brown once the grass starts growing in spring need seed.
You do not need a spray, and in many regions you cannot buy one. The RHS says no fungicides are currently available to amateur gardeners for lawn diseases. Where preventative products are sold, they go on before the patches appear. Applied to damage you can already see, they do nothing, and fertiliser on a diseased patch only weakens the grass trying to heal.
Every common mistake in a mild, wet winter feeds the fungus, and the sections below sort the jobs that help from the ones that make it worse.
What You Are Looking At
It starts as small patches of yellowish, dying grass that turn brown. The patches grow to 30cm (12in) or more across, and neighbours merge until a whole stretch of lawn looks scorched. In wet weather a white or pinkish cottony fuzz appears, most of all around the edge of each patch. Gardeners sometimes call it fusarium patch or snow mould, but you do not need the name to deal with it.
Check it is not something else. Red thread is a wet summer disease. Slime moulds do not attack or kill the grass. A fairy ring is a circle of dead grass or toadstools. Brown footprints after frost are damaged blades, not a fungus, and take many weeks to fade.
A second, greyer type forms under snow that lies for a long time. The growth is greyish white rather than pink, and tiny, brown, hard resting bodies, like small seeds, sit scattered over the grass.
The pink type is the one that worries people. Roots and blades can both die, which is why a patch that stays brown after the weather warms needs seed. The grey type usually harms only the blades, and the roots stay alive to push up new ones.
Why It Turns Up When the Weather Turns Mild and Wet
The fungus survives temperatures as low as -20°C (-4°F), but it grows fastest between 12 and 19°C (54 and 66°F). It can infect grass in summer, yet fast growth hides the damage. When the weather cools and the grass slows down, the leaves stop outgrowing the fungus and the patches show. A mild, wet spell in late autumn or winter hands the fungus the conditions it likes while the grass slows down.
Water on the leaf is the trigger. The fungus needs surface moisture to infect, so a lawn that stays wet after dew or rain is the lawn that gets it. Shade, overhanging branches, leaf litter and slow drainage all keep blades wet for longer. Alkaline soil favours the disease too.
Feeding is the mistake nobody connects with it. Grass pushed hard by high nitrogen is more prone to attack. Wet weather, overfed grass and grass left a bit too long are the usual set-up for yellow or brown patches in December. A late autumn dose of the wrong fertiliser leaves you with exactly that kind of grass.
Some grass is also more at risk. The disease is most troublesome on annual meadow grass, and also affects bents, fescues and perennial rye-grass. Annual meadow grass is the coarse, light green grass with shallow, fibrous roots that flowers and sets seed below the height of your mower blades, throughout the year. It colonises bare patches by seed, so every bare patch you leave open invites more of the grass that suffers most. Dense, healthy turf is harder for it to take hold in.
The last piece is you. The fungus spreads as spores or as infected grass debris carried on equipment or shoes. Walk through a patch and across the rest of the lawn, or mow while the grass is wet, and you plant the next patch yourself.
What to Do Now Without Making It Worse
**Keep off the lawn.** Footsteps move spores, and a frosty lawn has brittle blades that walking damages. If the route across the lawn is the problem, set stepping stones level with the soil so the mower still passes over them.
**Stop mowing wet grass.** Spores and infected grass debris travel on equipment, so a mower that crosses a patch carries the disease to clean grass. If a mild spell has kept the grass growing above 7°C (45°F), mow only when the surface is dry and raise the cutting height to about 5mm (¼in) higher than in summer. Wet grass tears instead of cutting cleanly, which is why mowing a wet lawn rarely ends well. Wipe the deck and your shoes before you move to healthy grass.
**Rake the matted patches gently.** Run a spring-tined rake lightly across the flattened grass to loosen it, so the soil dries faster. Keep the strokes light: frosty blades are brittle and break easily.
**Knock the dew off.** Draw a switch (a long, pliable rod) or a bamboo cane across the lawn in the morning. Heavy dew is a long drink for the fungus.
**Clear the leaves.** Rake fallen leaves off the lawn so they do not smother the grass, blocking light and moisture.
**Let in air.** Prune back overhanging trees and shrubs. Overhanging growth slows the drying of the grass underneath, and moving air shortens the time the blades stay wet.
**Skip the feed.** The grass is barely growing, and fresh nitrogen only pushes the high-nitrogen growth that is more prone to attack.
How to Fix the Patches That Do Not Recover
Wait. Small patches should recover, and you cannot tell how far a patch has gone until the weather warms and the soil dries. Seed sown now lands on the same wet surface the fungus needs, and the lawn has nearly stopped growing, so nothing fills the gap before spring. For a bare patch near the lawn edge you can cut turf from a spare corner of the garden. Otherwise, wait until spring to sow seed.
When spring growth starts and a patch is still brown, rake out the dead grass. For small patches, scratch up the soil with a rake, scatter the seed, press it down, and keep the soil moist until the grass sprouts. For bigger areas, work through the patch with a de-thatching rake, pulling out dead material and loosening the soil so the seed can take hold. A power rake does the same on a big lawn. Rake off the debris, sow, and water lightly at least once a day, so the seed germinates without washing away.
Roger Cook of This Old House set out the recovery steps for a homeowner with a severe outbreak: aerate the affected area, overseed with fresh grass seed, apply a balanced fertiliser, and top-dress with compost. That homeowner mowed a fully recovered lawn six months later. Leave the fertiliser until the patch has dried and the seed is down. Feed too early and you weaken a lawn that is trying to heal.
How to Stop It Coming Back
**Fix the drainage.** The aim is a lawn that dries quickly after dew and rain, which takes a combination of scarifying, spiking and hollow or solid tining. If water pools after winter rain, spike the lawn with a garden fork or a mechanical aerator, and once the surface is fairly dry, brush a mix of sharp sand and loam into the holes. Aerating a compacted lawn walks through the technique. Fill low hollows with topsoil so water has nowhere to stand.
If the same patch comes back year after year, dig a small hole in it and look below. Roger Cook did exactly that on a lawn that kept getting a fungus disease, and found a layer of stone dust holding the water up. “That stops the water from draining down any farther into the soil,” he explained. “The water comes in, floods. It leads to a fungus disease.” A buried layer like that never shows on the surface, and no amount of raking fixes it.
**Control thatch.** Dethatch two or three times a year and keep the thatch layer under 1.9cm (¾in). Bag the clippings, so they do not keep the ground damp.
**Go easy on nitrogen.** Avoid high doses of nitrogen fertiliser in late summer or autumn and use a proprietary autumn lawn feed instead. In cold northern climates where snow lies for weeks, do not fertilise within about six weeks of the first expected snowfall. Feeding your lawn in autumn covers what to buy and when to apply it.
**Think about the last cut.** Roger Cook’s advice for a lawn that goes under snow is: “When you cut that lawn the last time, cut it as short as possible. What it does is it moves the blade so there’ll be nothing there for that fungus disease to feed on.” That lawn lies dormant all winter, so the short cut does no harm. A lawn in a mild climate keeps growing through every warm spell, and the RHS rule for those lawns is to raise the cutting height by about 5mm (¼in) above the summer setting. Use the rule that matches the weather in your winter.
**Keep the surface clear.** Do not let snow pile up on the lawn when you shovel a path. Anything that traps moisture is bad news for the grass.
**Choose the grass.** Bentgrass is more susceptible, fine fescue more resistant, so pick the seed mix with that in mind.
**Know when to call someone.** Some lawn companies offer professional fungicide treatments. Strict legislation controls how those products go on to domestic gardens, so ask the company to show that it operates within the law before you sign anything. Call a drainage or turf specialist when the same patches return in the same place every winter after you have aerated and cut back the overhang. That pattern points to something under the surface.
