Almost all grass diseases are caused by fungus, and St. Augustine grass is no exception. St. Augustine grass particularly is susceptible to brownpatch, gray leaf spot, Helminthosporium, Pythium, rust and downy mildew. Battling lawn diseases can be extremely frustrating after all the time and energy you have put into making your lawn the beautiful green haven it is, but you do not have to sacrifice your yard to harmful fungus. There are both preventative and reactionary steps you can take.
Water your lawn with 1/2 inch of water at least twice a week, either before the sun comes up or after the sun goes down.
Mow your lawn at least once a week, but don't cut it shorter than 2 inches.
Clean your mower regularly in case it has come in contact with diseased grass, and if you use a lawn care service talk to your lawn technician about cleaning his mower blades before cutting your grass.
Improve drainage in your yard if your grass is covered in standing water after it rains.
Lawn thatch, or dead grass between the new growth and the soil, can be a breeding ground for diseases. Aerate your lawn at least twice a year and keep thatch to 1/2 inch.
Identify the disease that is affecting your lawn. You can use the chart listed as a reference below, or ask a lawn expert.
Purchase fungicide that treats the disease that is affecting your lawn.
Mix the fungicide according to the instructions and fill the sprayer with the fungicide.
Spray the affected area with fungicide and wait 24 hours before watering your lawn.
Repeat after a week if the fungicide does not seem to be working.