Made a house call yesterday to look at a lawn that was not doing as well as the owner would have liked and was able to take these photos for today’s disease. This is gray leaf spot, a common problem on younger St. Augustinegrass lawns that are receiving a bit too much nitrogen during the summer months and are being subjected to other stresses, in this case soil compaction, insufficient irrigation and take-all root rot. The fungus that causes gray leaf spot is now called Pyricularia (or Magnaporthe) oryzae. It attacks a wide variety of turfgrasses, but it is found most commonly on St. Augustinegrass in the Southeast. This disease occurs from mid-summer to early fall and develops most rapidly when the weather is warm (82-90F) and humid with sufficient leaf wetness to allow the fungal spores to germinate and grow. Under optimal temperatures, this takes about 9 hours, but takes longer when temperatures are suboptimal. The disease is managed by reducing stresses to the turf, such as soil aeration to reduce compaction, use of low rates of slow release fertilizers and improved irrigation to promote a deeper root system that can tolerate drought a bit better. Fungicides can also be used as part of the management program, but because the pathogen is capable of developing resistance to some of these fungicides, it is best to alternate or combine fungicides with different modes of action.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA