Grass turns orange due to the presence of a rust fungus. Rust fungus can thin and destroy grass. Maintaining the health of the lawn, collecting and disposing of grass clippings, and using a fungicide can help treat and prevent rust fungus.
Grass that shows vigorous growth does not have time to develop rust fungus. Grass should be watered and fertilized on a consistent basis. Watering is best done in the morning. Fertilizer should be applied at the beginning of the growing season and two to three more times in the spring, summer and fall. Local nurseries can advise on the correct amount of watering and fertilization for the particular grass variety and climate.
Grass clippings can harbor rust fungus spores. Prompt removal is necessary to prevent spreading the disease. If a lawn has problems with rust fungus, plant a mixture of grasses that are more resistant to the fungus to avoid problems in the future.
Though it is not the preferred method, the application of fungicides containing chlorothalonil or mancozeb can kill rust fungus. These fungicides can be purchased at gardening supply stores. They contain toxic chemicals and should be used with caution. One application is usually not enough to kill rust fungus.