(also called Rhizoctonia Blight)
Pathogen: Rhizoctonia solani Turf grasses Affected: All warm-season turf grasses, especially St. Augustine grass and zoysia grass.
Occurrence: This disease is most likely to be observed from November through May when temperatures are below 80 F. It is normally not observed in the summer. Infection is triggered by rainfall, excessive irrigation or extended periods of high humidity resulting in the leaf canopy being continuously wet for 48 hours or more.
Symptoms/Signs: The fungus infects the leaf area closest to the soil, disrupting transport of water and nutrients to the upper portions of the foliage and eventually killing the leaf. A soft, dark rot will occur at the base of the leaf. The entire leaf will pull out easily from the leaf sheath due to this basal leaf rot. Eventually, entire plants will easily pull off the stolons. Roots are not affected by this pathogen. Base of leaf is rotted due to Brown Patch.This disease usually begins as small patches (1 foot) that turn yellow and then reddish-brown, brown or straw-colored as the leaves start to die. Patches can expand to several feet in diameter. It is not uncommon to see rings of yellow/brown turf with apparently healthy turf in the center. Turf at the outer margin of a patch may appear dark and wilted. This is where the fungus is most active. Brown Patch symptoms on St. Augustine grass. Brown Patch symptoms on zoysia grass. Note that the outer edge is darker color indicating the fungus is active at this point.This disease is often confused with phenoxy herbicide damage on St. Augustine grass. Herbicide damage may cause the same overall symptoms of yellow or brown patches. The leaf may still pull out of the leaf sheath, but the base of the leaf is not dark and rotted. Instead, the leaf base is dry with a tan discoloration, and there is no distinct rotten smell.
Avoid excess nitrogen during potential disease development periods. Do not use readily available forms of nitrogen such as soluble liquids or quick-release nitrogen sources just prior to or during these periods. Instead, use slow-release nitrogen sources. Apply a balanced fertilizer containing equivalent amounts of potassium, preferably a slow-release potassium form.
Irrigate only when necessary and do so only in the early morning hours (between 2 and 8 AM) when dew is already present. Since mowers can spread this disease, mow diseased areas last, and wash turf clippings off the mower, discarding in a compost, before proceeding to the next site.