“Brown Patch is the most damaging turf grass disease” Brown patch is really a summer lawn disease that’s caused by a fungus called Rhizoctonia. The disease begins to show growth when temperatures reach 65°, but the most active growth of brown patch lawn disease occurs at temperatures of 80-85° when humidity levels are very high. The fungi survive the winter in plant debris (thatch) and enter the leaf tissue through wounds caused by mowing and through the pores (or stomata) when daytime temperatures get into the 70s.