Question
I have a customer whose front yard is all dwf. mondo and it is in filtered sunlight. Recently her mondo grass has been browning up and you can pull the dead leaves easily away from the crown similar to damping off. We have had over 60 days this summer over 100 degrees and virtually no rain fall. I think it would be very hard to over water now. Is Pythium active in temperatures this hot? Do you think it could be just heat stress? The plants have been in the ground for over 2 years and very established. I would not think it was a disease but the symptons point to it. Could you recommend a treatment if it is disease?

Answer
It is certainly possible that the problem is environmental, but Pythium is a good possibility.  With environmental stress, the roots should remain light colored and not easily fall apart.

Certain species of Pythium are active in the high temperatures.  It will cause a crown rot on the grass and result in the symptoms you describe.  If the roots and crowns appear brown-black, then I would suspect Pythium may be to blame.  Sometimes, the crown will be somewhat slimy and it may develop a bad odor if bacteria invade and speed the decomposition.

Pythium is more of a problem on plants under environmental stress.  If the soil is heavy or poorly drained,the plants are more likely to be infected.  Timing of watering is also important.  If the home owner is watering late in the day, the plants will remain moist overnight and they are more likely to have a problem.  

Infected plants should be removed and destroyed.  Plants that are severely yellowed and have crown rot will not recover.  It may also be helpful to remove some of the associated soil.  Replanting with the same host is not recommended.  Plants that appear healthy or only show minimial symptoms can be treated with a fungicide.  Products that will be most effective include Aliette (fosetyl-Al) or Subdue (mefenoxam) and they should be applied by drenching.  

For a picture and a little more information, see the following fact sheet, page 3:
http://plantpathology.uark.edu/Number3-2009.pdf

Good luck!