Does your lawn look burned out? Does your lawn look like the photos in this post? If so, you likely have heat stress and/or Ascochyta leaf disease in your lawn.

Heat stress on lawns (also called heat shock) and Ascochyta leaf disease are triggered by intense sun, heat and dry conditions. They cause rapid yellowing of the leaf blade. Much like your skin turning red from a sunburn. Just as skin recovers with time and care, your lawn will too. 

When does Heat Stress and Ascochyta happen? 

According to Dr. Krueser, a turf researcher, “During hot weather, cool-season turf species can actually receive too much sunlight.” Heat Stress and Ascochyta lawn disease typically occurs during June, July and August (but has been observed in May and September) when temperatures are above 85 degrees F, or winds are greater than 15 mph or humidity is below 40% and the UV levels are high. These would be similar conditions to when our skin burns extra easily. 

Why does Heat Stress and Ascochyta happen?

According to Dr. Krueser, “Energy not converted to sugar leads to oxidative stress which bleaches leaves and kills cells.” Heat stress can appear to be random but it is typically not. Genetics and care practices often dictate whether or not someone gets a sunburn. So too in a lawn. The main factors are; the grass type, watering practices, mowing height, shade, soil type, and traffic on the lawn.

Why doesn’t my neighbor’s lawn show this? 

Typically, watering and mowing practices are the two major factors that dictate why one lawn gets it and one does not. One of the most common questions bioLawn gets is why the neighbor who doesn’t water isn’t getting the bleached areas. This occurs because regular watering discourages root growth to deeper parts of the soil. Under high temperatures an unwatered lawn is able to access water deeper in the soil, whereas a watered lawn cannot. 

Other reasons why some areas of a lawn get it and others don’t include:

• People playing on the yard.
• Sandy or poor soils drying out faster
• Windows and pavement heating up areas of soil
• Trees in your landscape fight for the same moisture and always win the moisture battle over grass.

Why do I see tire tracks in my lawn from my mower or your machine? 

These tire tracks occur because driving on the lawn can place extra stress on a lawn. It is very difficult to see an infection or heat stress as it is occurring. Think about how difficult it can be sometimes to tell if someone is getting burned in the moment. The same challenge exists when evaluating if a lawn is getting a sunburn. These tire tracks will recover, just like the other bleached spots. 

What can you do to limit damage? 

Watering – When temperatures are in the 90s, your lawn needs more water. It is not enough to continue watering with the pre-programmed times. Generally speaking you should double the amount of water. We recommend only watering between 5am – 7am. Watering in the afternoon and evening can exacerbate the burn.  We also recommend signing up for our weekly watering newsletter to get current recommendations on watering rates for the current weather. 

Mowing – We recommend mowing heights above 4 inches during the summer. Longer grass shades the soil and reduces water loss. We recommend not mowing when it is over 85 degrees. Mowing cuts a wound into the grass which increases water loss. On hot days, water loss can happen rapidly after mowing, which can exacerbate Heat Stress.

What do I do if I already have damage? 

The good news is your lawn will recover, just like a skin sunburn. It just takes time. The organic based fertilizers we use in our programs provide consistent release of fertilizer which will help the lawn grow. As a homeowner, you can speed the recovery process by watering more. This can be done by increasing the frequency that you irrigate, such as daily. Or by increasing the volume of water, such as doubling your watering time. We also recommend raising your mower height and mowing less frequently. If these practices are followed we typically see a lawn recover in 3-4 weeks.

The below photo shows heat stress that occured on a lawn around May 20, 2025 and then the status of the lawn on June 1, 2025. This is a good example of how much recovery can take place in 10 days.

Do you need help diagnosing heat stress, Ascochyta leaf disease, or another problem going on with your lawn? Reach out to our team and we can help!