CO-Horts Blog

Monday, May 29, 2017

Ascochyta Leaf Blight on Lawns – 2017 Version

Mowing drought-stressed lawns can make
Ascochyta worse. Notice no disease at top
of photo - shadier and no drought stress here!
Tony Koski
Extension Turf Specialist

The appearance of Ascochyta Leaf Blight has become an annual springtime event on lawns in Colorado. We have written about Ascochyta before: see our 2013, 2015 and 2016 blogs on this lawn disease. For more detailed information on Ascochyta in lawns, read those blogs because I’m not going to repeat everything that we've already written about it.

What I do want to talk about is WHY the Ascochyta disease appeared so suddenly over the last week. We’ve been inundated with calls, emails and texts with a common theme: “My lawn was perfectly green, and now you should see it! What happened?”. The explanation of why – that the disease is incited by drought stress –  doesn’t make sense to people because it appeared after the very wet (2 plus inches of water) snowy/rainy storm that came on May 16-18. That abundance of moisture (here’s what people DON’T remember) was preceded by two WEEKS of above-normal temperatures and very dry weather (red flag weather, if you recall?). And while everyone who has seen this happen to their lawns claims they were watering, I saw a lot of drought-stressed turf in the week before the storm. All it takes to turn Ascochyta on is a day or two of drought stress and heat during the spring – which this graph clearly shows was probable in home lawns this May. Those who REALLY were watering their lawns (or have shady lawns, or north exposure lawns) are not seeing Ascochyta. 

Much of first half of May was warmer (red line) than average (green line).And there was almost no precipitation May 1-16. This is a perfect "recipe"for Ascochyta on lawns that aren't irrigated enough during warm springs.The disease cycle began BEFORE the wet storm, with the symptoms appearing immediately after the storm. The moisture came a couple of days too late!

Here are some pics of what we’ve seen the past week.

In these 2 photos, there is less/no Ascochyta in parts of the lawn that are shaded or receiving a little more water from gutter downspouts. Ascochyta is turned on by heat and drought stress in the spring

Some quick bullets on the disease (go here for more details):

  • As bad as it looks, it isn’t dead!
  • Affected turf will take 1-3 weeks to recover, depending on severity and turf species.
  • It is NOT spread by mowing equipment – so there is no need to sanitize your mower.
  • Fungicides are NOT EFFECTIVE for preventing or curing the disease – so don’t apply them!
  • Adjust watering (and fix sprinkler coverage issues) to maintain consistent soil moisture, but not so that it is swampy (which will slow down recovery).
  • This appears to be a disease that occurs when cool weather turns into hot weather (hence a spring disease), so it will pretty much disappear as we become consistently warm in the summer.
Ascochyta doesn't kill the grass plant. You will see new growth
under the dead leaves almost immediately following an outbreak
of this disease. Recovery can take 1-3 weeks, depending on
severity and grass species.

While you can see mowing patterns with Ascochyta, mowing and the mower
doesn't spread the fungus. The mowing is an additional stress when turf is
heat and drought-stressed - so the Ascochyta and mower stress combine to
cause the browning of the turf.


  1. This is exactly what I think happened to my lawn. So should I cancel the treatment my lawn care company says that my lawn needs? They are coming tomorrow if you have time to let me know.

  2. If it is a treatment (a fungicide application) for the Ascochyta leaf blight, I would advise against it. If they are coming to apply fertilizer and to do some weed control, then that would be OK. Be certain of what they want to do before you cancel. You (and/or your lawn care professional) can contact me directly by email: or by phone: 970-222-1450

    1. Thank you! And thank you for working on a holiday! This will be very helpful. If my company has a question I will have them call you.

  3. Wonderful relevant article! Great photos and explanations. Might as well plan on posting it on a yearly basis.

  4. The chart was the best roller coaster wake-up too.
    Good use of visuals for those (me) who benefit from them.

  5. Janet FitzgeraldMay 31, 2017 at 10:35 AM

    Thanks,Tony. I used the information twice this morning while staffing the MG Help Desk in Colorado Springs.

  6. Your "quick bullets" offer much data in a concise but informative manner. You 'know your stuff' and do a good job sharing your knowledge with others. Thanks for taking the time to address this turf issue. Jan Stoven

  7. This and the other articles describe how to get rid of the symptoms well but is there a way to treat the root cause and get rid of the fungus so it doesn't come back?

  8. Hi Kyle,
    Yes, regular irrigation will prevent Ascochyta. This disease only appears on drought stressed turf.

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