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Root rot and lawn fungi in spring

Symptoms, causes, treatment, and prevention

Root rot, and fungal or cryptogamic diseases in general, affect almost all plants: vegetable and aromatic plants, fruit trees, ornamental plants (both flowering plants, shrubs, and trees). Lawn grass is no exception, becoming diseased mainly due to fungi, much less so due to viruses and bacteria.

Fungi survive in diseased grass, in the soil, or in the thatch that accumulates on the lawn. When the time is right, they activate, invade the tissues of grass seedlings, and feed on them, damaging them. They spread spores which, carried by water, wind, grass cutting equipment, shoes, etc., reach other seedlings, triggering further infections.

Fungal diseases of the lawn do not develop due to the pathogen alone: weakened grass and favorable environmental conditions (for the fungus) are also necessary. If the lawn is not healthy, it is more susceptible to fungal diseases. The environment and mistakes in turf management facilitate the onset of cryptogamic diseases: just think of mild/warm temperatures (or frost in winter), persistent humidity, clayey or compact soil (which facilitates water stagnation), cutting the grass too low or too high, infected clippings left uncollected, and the presence of thick thatch.

In this article, we will discuss cryptogamic diseases, including root rot, that attack lawns in spring. The period from March to June is critical because the risk of infection increases as temperatures and humidity rise, reaching its peak in the middle of summer.

The main fungi that affect lawns in spring

After briefly describing the life cycle of fungi that infect lawns and defining the factors that favor them, we will now outline the main spring fungal diseases that affect turf. For each one, you will find the pathogen, symptoms, conditions, and period in which it develops.

Red thread

Fungus responsible: Laetisaria fuciformis.
When: April to May (also occurs in autumn).
How to recognize it: roughly circular discolored patches appear on the lawn (which is why you might confuse it with dollar spot); on the leaves (i.e., the blades of grass), you will see pink-red filaments (this is the fungus itself). The damage is mainly cosmetic: it does not affect the roots or the grass collar.
Favorable conditions: temperature 15-24°C, nitrogen deficiency, potassium deficiency, poor and compact soil.

Dollar spot

Fungus responsible: Clarireedia homeocarpa (also known as Sclerotinia homeocarpa).
When: May to September.
How to recognize it: on the lawn, look for round discolored patches measuring 1-3 cm that cluster together, even spreading into large damaged areas. It does not affect the roots or the collar, but the leaves.
Favorable conditions: temperature 15-30°C, high humidity, prolonged wetting of the grass, nitrogen deficiency, water deficiency.

Leaf blights or leaf spots, helminthosporiosis and other names

Fungus responsible: various fungi, particularly of the genera Helminthosporium and Drechslera.
When: spring (also occurs in autumn).
How to recognize them: symptoms vary depending on the pathogen. Generally, the lawn darkens and thins out; brown spots form on the grass leaves, then the infection spreads to the lower part of the plants, causing root rot and irreversible wilting. You may confuse this with damage caused by drought.
Favorable conditions: temperature 12-21°C, high humidity, poor ventilation, low light, nitrogen excess/deficiency, potassium deficiency, compact soil, presence of thatch, cutting too low.

Pythium, also known as root rot

Fungus responsible: fungi of the genus Pythium.
When: May to September.
How to recognize it: you will see round patches of 3-15 cm on the lawn, sometimes covered with a whitish mass (this is the fungus); the grass seedlings are dark and slimy, then dry out. The patches can merge into very large areas. It is a destructive and rapidly spreading fungal disease of lawns that causes root rot.
Favorable conditions: temperature 25-35°C, high humidity, waterlogging, poor ventilation, nitrogen excess, calcium deficiency, poorly drained soil, already diseased lawn.

Brown patch

Fungus responsible: Rhizoctonia solani.
When: May to October.
How to recognize it: round brown patches measuring 10-30 cm to over a meter appear on the lawn, which can group together into larger patches; the leaves darken and then dry out. It causes root rot. A typical sign is the gray halo visible around the patches on very humid, sunless days (revealing the advancing fungus).
Favorable conditions: temperature 22-30°C, high humidity, poor ventilation, nitrogen excess, excessive irrigation, poorly drained or compact soil, presence of thatch, cutting too low.

Rust

Fungus responsible: fungi of the genus Puccinia.
When: usually between September and October, but can also occur in spring.
How to recognize it: the lawn turns orange; yellow spots appear on the grass leaves, which become yellow-orange pustules (these are the spores of the fungus). The damage is mainly cosmetic: it does not affect the roots, but usually the stem. It is not a serious fungal disease of the lawn, but if neglected or prolonged, it can weaken it.
Favorable conditions: temperature 20-30°C, high humidity, prolonged wetting of the grass, poor light, nitrogen deficiency, water deficiency, tall grass, uncollected clippings.

How to treat fungal diseases and root rot in lawns

Even limiting ourselves to the spring season, we have seen that there are numerous fungal diseases affecting lawns, some of which—such as pythium and brown patch—cause root rot. So what can you do to protect your lawn? The first step is to pay attention to any abnormalities that may appear, such as spots of varying sizes or regularity, color changes, or dry patches. Compare the symptoms you observe with reference photographs—online or in specialized books and magazines—and with the current season. Then assess the temperature and humidity, and the general condition of the lawn (whether it is suffering from incorrect fertilization or mowing).

An empirical diagnosis of this type may be sufficient to identify the disease and intervene with the appropriate treatment, i.e., a fungicide specific for turf and for the cryptogamic disease identified. In the case of red thread and rust, however, it may be sufficient to apply a fast-acting nitrogen fertilizer.

In order to intervene in time, it is essential to recognize the plant disease as soon as possible. Unfortunately, in the case of the dreaded pythium, due to the speed with which it spreads and the extent of the damage it causes, treatment is not very effective and the best defense is prevention (which we will discuss shortly).

Once the diagnosis is clear, proceed with treatment:

Do not mow or walk on the lawn, so as not to spread the fungus to healthy areas.
Suspend all activity on the infected area: watering, fertilizing, and so on.
Apply the turf fungicide, for example with a backpack sprayer, following the product instructions (dosage, timing, etc.). It should usually be applied to dry grass: if it is wet, you can dry it with a blower.
Do not mow, water, or walk on the grass for the necessary time.

Once the lawn has overcome the fungal disease:

In case of permanent damage, regenerate the areas damaged by lawn fungi, for example by overseeding or localized reseeding.
Treat the entire lawn (even the part not affected by the fungus) with products that stimulate the grass’ defenses.
Correct any lawn management practices that may have facilitated the development of the disease: watering, mowing, and nutrition, particularly with regard to nitrogen and potassium (see the section on prevention below for some basic guidelines).
Reduce shaded areas, particularly to prevent the onset of leaf diseases and rust.
Improve soil drainage in the garden, for example by digging drainage trenches in areas prone to waterlogging.
Make monitoring a rule to catch any lawn infections at the first sign.

If the lawn has suffered irreversible damage—that is, not so much cosmetic as functional—or the affected area is very large, it needs to be regenerated. With overseeding, you distribute seeds on the existing lawn to thicken it by growing new seedlings. With reseeding, on the other hand, you rebuild it from scratch: after removing what remains of the grass in the affected area, add new soil, fertilize it, and work it with a spade or (if the area is large) with a rotary tiller or two-wheeled tractor, then sow the seeds.

How to prevent fungal diseases and root rot in lawns

Careful lawn management is the best cure for fungal infections. In particular, as we have already mentioned, in the case of pythium, prevention is practically the only effective weapon. Preventing cryptogamic diseases basically consists of creating an environment that is as healthy as possible for the grass and at the same time unfavorable for the fungus:

Water correctly: in the morning (so that the grass has time to dry and moisture does not remain on the seedlings), abundantly and infrequently (with breaks of up to 3-5 days, minimum 2).
Fertilize in a balanced way, avoiding excesses and deficiencies: a healthy lawn is not so much lush as it is robust and resistant, so don’t overdo it with nitrogen and make sure there is enough potassium.
Mow the grass when it is dry: here we explain why it is better not to mow the grass when it is wet.
Adjust the cutting height to the type of grass (generally around 6-7 cm) and the season (in summer and winter, raise the cut by 20-30%): cutting too low makes the lawn more susceptible to fungal diseases, among other things.
Remove the clippings: infected grass is a reservoir for spreading new infections.
Aerate the lawn with a scarifier to remove thatch—where fungi thrive—and to loosen the soil.
Help the grass fight disease with treatments that strengthen its defenses.

When it comes to cutting height, if the grass is very tall, cut it several times, no more than 1/3 of its height at a time. Choose the machine that suits your needs from riding mowers, lawn mowers, brushcutters, and robot mowers, and adjust the cutting height according to the circumstances. By the way, here is an article that will help you choose the most suitable machine for mowing your lawn based on the size of your garden.

Before sowing the lawn, prevention against fungal diseases begins with the choice of resistant grass varieties and good soil preparation. The latter, thanks to the addition of organic matter, makes it soft and provides a reserve of nutrients, or thanks to sand or other soil improvers, facilitates the drainage of excess water. A very clayey or compacted substrate tends to create water stagnation on the lawn, thus providing an ideal habitat for fungi.