CO-Horts Blog

Thursday, July 1, 2021

Ascochyta Leaf Blight: Facts and Myths

Tony Koski, Extension Turf Specialist
Dept. of Horticulture and Landscape Architecture

Ascochyta leaf blight on a lawn June 2021
The visible wheel tracks does NOT mean that mowing 
"tracked" the ALB fungus on to the lawn. Rather, the 
lawn was drought-/heat-stressed when it was mowed. 
The wilted grass leaves were killed by the weight of 
the mower - but the grass plant is still alive.
It's back, in full force, on Colorado's lawns. Cool, wet springs, like this year's, create perfect conditions for Ascochyta leaf blight (ALB) outbreaks. When the transition from spring to hot, dry summer conditions occurs as suddenly as how it happened this year, ALB becomes epidemic in lawns.

We've blogged about ALB a number of times, so there's no reason to repeat what you can read there (2013, 2015, 2016, 2017). Without re-hashing what was written in years past,  the most important takeaway: it's a problem of drought, heat stress, and lack of water (or a sign of an irrigation system problem); mowing the stressed turf often increases the injury. The good news is that the grass is NOT dead, but can be revived with sensible watering and by paying closer attention to future irrigation needs.

Some ALB facts:

  • While it is a disease, ALB is really a problem of water - not enough, or not frequently enough. 
  • ALB can affect bluegrass, fescue, and ryegrass lawns.
  • Fungicide applications will NOT make it recover more quickly - so don't apply them (or don't pay for your lawn care company to apply them!)
  • While an ALB outbreak looks terrible and the grass appears dead, this disease does NOT kill grass.
  • An infected lawn will recover, though it may take 1-3 weeks for full recovery. Recovery rate depends on severity, species of grass, and how you water (not swampy, but consistent moisture).
  • Light fertilization - if your lawns has not yet been fertilized this year - can help speed recovery.
  • The ALB fungus does infect grass leaves through the mowed off tip of the leaf blade - but that doesn't mean you should stop mowing the lawn when disease is active (mowing doesn't spread the fungus, nor does it significantly increase the amount or severity of the disease). 
Some popular myths about ALB:
  • Maybe you've read on the never-wrong internet that the ALB fungus is spread by mowers? Wrong! The wheel tracking often seen on ALB-affected lawns occurs because the lawn was mowed when it was drought- and/or heat-stressed. The weight of the mower crushes the wilted leaves under the wheels, leaving brown tracks a day or so later. Often there is no ALB disease IN the wheel tracks - but plenty of blighted turf between the wheel marks. 
  • You will also find claims that lawn mowing equipment should be disinfected between lawns - or after you mow you own lawn. Wrong again! The fungal spores that can result in ALB are everywhere already. Disinfecting your mower with bleach solutions or Lysol will not prevent future ALB on your lawn, nor will it prevent its occurrence on lawns of other clients (if you own a lawn care or mowing business).
  • You will also read on that you should throw grass clippings from lawns suffering an ALB outbreak into the trash, because leaving them on the lawn will make the problem worse - or will cause it to happen in the future. Again, there is no evidence to suggest this to be true. As before, the spores are EVERYWHERE already. Grass clippings can be safely composted or used as mulch in gardens (because the fungus causing the lawn disease doesn't affect vegetable or flower garden plants).
While people will always claim that "it can't be water", it ALWAYS is. When you can see where the sprinkler heads are in a lawn affected by ALB, that should be a sure indicator that it is a watering issue. Read the previous blogs listed above for suggestions on getting a lawn back to health after an ALB outbreak.


  1. Thanks again Tony, my lawn is almost back to 100%.

  2. Thanks! Your article just helped me diagnose my lawn issue!

  3. I read this and all the linked articles on this topic. Thank you for the valuable information. I understand that temperature and water conditions exacerbate the fungus. But, I am still unclear as to what introduces the fungus to a lawn that has been free of it for years. Does it come from new sod? Top dressing? Airborne?