White cottony webbing on your lawn? Here’s what it is
That white cottony webbing across the blades is fungal mycelium. Two diseases produce it: Fusarium (also called Microdochium patch) and dollar spot. They can look similar on a damp morning, so the time of year is your first clue.
If you’re seeing it in autumn or winter, cold and wet conditions, it’s almost certainly Fusarium. If it’s appearing through the warmer months, late spring into summer, dollar spot is the more likely culprit. Both produce white mycelium at the edges of patches in the early morning, but the conditions that drive them are different and so is the treatment. Getting the identification right matters before you reach for anything.
With Fusarium, the mycelium disappears as the sun hits it in the morning but comes back overnight. The patches underneath tend to be tan to orange-brown, often with a darker border. Dollar spot patches are typically smaller and more defined, straw-coloured, and the mycelium has a slightly more thread-like appearance.
What causes Fusarium
Fusarium is a cold weather disease. The fungus is present in most lawns year-round, sitting in the soil and thatch doing nothing, but it gets going when a specific set of conditions line up.
The main driver is leaf wetness duration. Fusarium doesn’t need standing water, it just needs the leaf to stay damp for extended periods. That’s why you see it most on lawns where the dew sits for hours in the morning, where air movement is poor because of fences or hedges boxing the lawn in, where there’s shade keeping the surface cool and slow to dry, or where drainage is poor and the soil stays boggy after rain.
Temperature plays into it too. The disease is most active in the 0 to 15 degrees Celsius range, which is why it peaks in autumn and winter across most of New Zealand. Once night temperatures drop and soil cools, conditions are close to ideal on many lawns.
Then there’s fertility. High nitrogen going into cold damp conditions produces exactly the kind of soft, lush growth the disease thrives on. A grass plant pushed into rapid leaf growth heading into winter has thin, vulnerable cell walls and reduced natural resistance. This is why people using fast-release fertilisers late in the season often see disease shortly after. Fast-release nitrogen forces soft growth at a time when the lawn should be hardening off, and sets it up for trouble.
Thatch makes it worse. A thick thatch layer holds moisture against the crown of the plant and gives the fungus organic matter to work with. Lawns with heavy thatch are chronically susceptible year after year until the thatch is dealt with.
Compaction is a factor too. Compacted soil holds water at the surface, slows drainage, and keeps the canopy wet for longer after rain. It also puts the plant under stress, and a stressed plant is less able to fight off infection.
Most lawns that get badly hit have two or three of these going on at the same time. Fix one or two and the disease pressure drops considerably the following year.
Fix the conditions
Knock the dew off in the mornings. Dragging a hose or a brush across the lawn makes a real difference because you’re getting the leaf to dry faster. If you’re watering at night, move it to the morning so the lawn has the full day to dry off.
Avoid fast-release fertiliser heading into the cooler months. It forces soft growth at exactly the wrong time of year and is a common trigger for Fusarium in lawns that were otherwise managing fine.
Treatment
Get Fungus Pro on as soon as you can. It’s a contact fungicide, so it knocks the mycelium down where it touches it and protects the healthy grass around the patch straight away. With Fusarium, the sooner you get onto it the better.
Leave it on the surface for a few days after application. No irrigation. Once it’s washed off it’s done, contact chemistry only works while it’s present on the leaf.
Follow up with Azoxy, which can be applied in the same tank as Fungus Pro. Azoxy is systemic, so it moves into the plant and provides around 28 days of preventative protection. The two products work differently and cover different ground, which is why running them together makes sense.
One thing to avoid: throwing nitrogen at it. It won’t help the lawn outgrow the disease, and in cool soils the plant can’t use the nitrogen efficiently anyway. Iron+ is the right foliar through this period, lifting colour and hardening the plant without forcing soft growth.
If the disease is showing up year after year in the same spots, the underlying conditions haven’t been addressed. Fungicide manages the disease; fixing the drainage, thatch, airflow and fertility programme is what stops it coming back.
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