What is leaf disease?

If your lawn is being watered and fertilized but still seems yellowish, it could be caused by one of several common leaf diseases. There are many organisms in your lawn that are beneficial; however, there are also a few harmful organisms. The harmful ones are a parasitic form of plant life called fungi. Fungi live in soil, thatch and dead leaves all year round. These fungi feed on the grass by drawing nutrients from the plant and destroying plant cells.

The disease spreads in the form of spores throughout your lawn by wind, air, water and humans. The disease may reappear from year to year if desirable conditions are met.

What causes leaf disease in lawns?

Three factors must be present for a leaf disease to occur. They are a plant, which in this case would be the grass plant, a pathogen (the fungi itself) and an environment favourable for the disease. If any one of these factors is missing, disease symptoms will not be present. Even with all three factors present significant disease will not occur unless favourable conditions continue over a long period of time.

Some of the most common causes of summer leaf disease include the following:

  • Extended periods of warm humid weather

  • Watering grass at night or significant evening rainfall. Watering your grass in the morning allows excess moisture to evaporate during the heat of the day. Watering your grass at night allows the grass blades to stay wet and this gives an environment for the disease spores to spread

  • Cutting grass when the lawn is wet - This again encourages an environment where disease spores spread

  • Mowing with a dull mower blade - When a mower blade is sharp it makes a clean cut, which heals quickly. When a mower blade is dull it rips the grass blade making a rough and jagged cut. This takes longer for the grass plant to heal and the jagged cut leaves more openings for the disease spores to spread.

  • Cutting grass too short - This puts the lawn under stress and more susceptible to disease

  • Nitrogen deficiency - Grass plants lacking proper nutrients are more vulnerable to leaf diseases.


LEAF SPOT (Helminthosproium) - Leaf Spot is identified by small circles of yellow with a dark border. These lesions will appear all over the leaf blade. This disease can be a problem when the weather is warm and humid. Leaf spot often occurs in lawns that are mowed too short.

DOLLAR SPOT (Sclerotinia Homeocarpa) - Dollar spot is a disease that can occur in lawns during warm humid weather. The fungus enters the grass plants when they are wet. Large tan coloured wounds appear on the blades. It appears as an hourglass shape in the middle of the blade. These infected grass blades form rounded damaged patches the size of a silver dollar. Over time the patches will start to overlap each other causing large dead patches.

LEAF BLIGHT - Leaf blight occurs on lawns during hot humid weather. The disease enters the grass plants after evening mowing followed by excessive night watering. In heavily infested patches the fungus damages the lawn in circular patches that often form large bleached areas. Closer inspection of the plant reveals individual leaves dying from the tips down with a brownish purple line running horizontal across the plant.

If you are not certain which leaf disease you may have, please call your local Weed Man Professional to have a free healthy lawn analysis to diagnose the disease and inform you of the correct treatment options.

How can I treat leaf disease in my lawn?

A lawn suffering from leaf disease is like us having a cold. There is no treatment that will cure it and most are only treating the symptoms. It may reduce the symptoms but if conditions continue that are favourable for the disease it will just come back again and again. The best treatment available is to prevent the disease from developing by changing the environment. As described before, for a disease to occur it needs a plant, a pathogen and an environment suitable for the disease to grow. The only one of those factors that we can change is the conditions favourable for the disease to grow.

Ways to change the environment to prevent leaf disease include:

  • Avoid evening watering

  • Avoid evening mowing

  • Maintain a cutting height of 6 to 8 cm throughout the season

  • Make sure your lawn mower blade is kept sharp

  • A healthy well maintained lawn is the best defense, so it is important to keep the lawn fertilized regularly [link]

  • Although out of your control, improved weather conditions will greatly assist in the lawn recovering from leaf disease

If you have any further questions about leaf disease in your lawn or have any other questions or concerns, please feel free to contact your local Weed Man Professional.