Q: My lawn looked beautiful for several weeks this spring. Recently I’ve noticed brown streaks in the lawn that look like wheel tracks from my mowing company. Did they bring a disease to my lawn?

By Tony Koski
CSU Master Gardener in Larimer County and CSU Extension Turfgrass Specialist

Answer: It sounds like you’ve got Ascochyta leaf blight. The good news is you’re not alone, nor do you need to panic. Ascochyta is caused by drought stress or an inefficient irrigation system.

What is Ascochyta? Technically, it’s a fungus. But before you go reaching for a fungicide, first take a look at the cultural conditions in your lawn. No, the fungus was not brought to your lawn by the mowing company…or moved in your lawn by your mower. The straw-colored wheel track patterns occur when drought-stressed turf is mowed – essentially bruising the turf leaf blades. Unfortunately, this bruising kills the leaf blade – often all the way to the ground. Fortunately, the actual grass plants aren’t dead. With a little time and patience (and regular water) the lawn will recover in a few weeks.

Ascochyta is a fungus that lives on the leaf blade. The fungus enters the leaf blade through the cut end (when you mow). It causes the blade to turn a straw-color and wither to a point. Why the fungus happens or how it works is not well understood by researchers. It does seem to coincide with periods of cool weather followed by hot, dry weather — in other words, your normal, typical Colorado spring.

Stress in the lawn (from poor or lack-of irrigation coverage, mowing equipment or heavy foot traffic) encourages the fungi. While your lawn looks dead, the crowns and roots of your grass plants are still alive. We see this most commonly on bluegrass lawns, but any turf variety (tall fescue, fine fescue, perennial ryegrass) can get Ascochyta.

Don’t go to the store and reach for the “lawn-disease control” products. They will not cure the problem or hasten the recovery. Instead, focus efforts on your irrigation system. Look for broken or tilted heads and fix them. Adjust the spray/arc of your sprinkler stream to get more uniform coverage between heads. And water appropriately — your goal is to water as deeply but infrequently as possible. Under current conditions, for a bluegrass lawn, this will be about 1.5 inches of water per week. But make sure you have an idea of how much water you’re putting on by collecting water in cups during an irrigation cycle.

Mow as normal, but time mowing for a day or two after irrigation and do it during cool parts of the day — early morning or late evening. Keep your mower height at 2-3 inches tall and keep the blades nice and sharp. Leave the grass clippings on the lawn (to recycle your fertilizer) or if you collect them, it’s OK to use them in compost or in your vegetable bed. Remember, this is a lawn disease and will not spread to veggies or flowers.

While patience may not be your virtue, keep in mind that this disease usually disappears by mid-summer. And if your entire lawn has this, with regular irrigation and hopefully some natural precipitation, the lawn will recover to its former glory.

For more information on Ascochyta leaf blight, refer to CSU Extension Fact Sheet #2.901 at http://www.ext.colostate.edu/pubs/garden/02901.html.

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