Turfgrass diseases are difficult to understand because the biological organisms causing the problems are rarely observed. Fortunately, grasses maintained using proper cultural practices (water, mowing, fertility) are not as likely to become diseased or be as severely damaged as grasses not receiving proper care.

Observing spots and patches of yellow or brown turfgrass does not automatically mean the turfgrass has a disease. While turfgrass injuries or disorders may look like diseases, they are not diseases and should be treated differently. Because diseases are difficult to diagnose, it is often faster to rule out involvement of other factors than to verify the presence of disease. Diseases are the exception and not the rule for most turfgrass plantings.

An injury to turfgrass is a destructive physical occurrence such as pesticide damage, mowing the grass too short or a fuel leak.
A turfgrass disorder is an interaction between the plant and its environment that is usually associated with imbalances of physical or chemical requirements for turfgrass growth. Examples would be nutritional deficiencies, cold stress, drought or excessive rainfall.
A disease is an interaction between the plant and a pathogen that disrupts the normal growth and appearance of the plant. It consists of three components: host, pathogen and the conducive environment in which the host and pathogen interact. The environment is the key factor for disease development since the turfgrass host and turfgrass pathogens are naturally always present. Individual turf diseases are active for only a few months each year, usually due to weather patterns and the environmental effects. However, any stress (environmental or man-made) placed on turf will weaken the turf, and make it more susceptible to disease development.

Turfgrass diseases are most often identified as very symmetrical brown areas in the turf. Because diseases are only active when all environmental conditions are met AND it is dark the disease spreads like the waves that appear when a rock is dropped in a pond. Insects on the other hand tend to damage the turf in a more irregular pattern. Absolute identification of many diseases can only be conclusively done in a lab.

Prevention is the key to disease control. Water early in the morning hours to keep the grass dry as long as possible in the dark. Balanced fertilization, especially reducing the amount of ammonical nitrogen (primarily Urea) during the last half of the summer will help reduce the chances of your lawn being attacked by diseases. Our experience is that most every yard we see with serious disease problems has poor watering practices and high nitrogen fertilization program.
Except for one disease caused by a virus, fungi cause turfgrass diseases in Florida. Fungi are unable to produce their own energy and must rely on living or dead hosts for energy and growth. Most fungi living in the turfgrass environment are totally harmless. And in fact are beneficial in that they help encourage decomposition of thatch in the lawn. A very limited number of fungi, at some point in their life cycle, cause plant diseases by infecting living plants.

It is important to know that when a fungal pathogen is not actively attacking the plant, it has not disappeared from the turfgrass area. It is simply surviving in the environment in a state of dormancy (like a bear in hibernation) or as a saprophyte (non-pathogenic phase) living off dead organic matter in thatch and soil layers. As soon as all the environmental conditions are met, including finding a susceptible target, the disease becomes active.

The one viral disease, centipede grass mosaic or St. Augustine grass decline (SAD) is not observed very frequently in Florida. The St. Augustine grass cultivars commonly used are resistant to the virus. In centipede grass, the virus alone does not cause decline and death, but it does add another stress factor to the turfgrass.

For more information go to: http://edis.ifas.ufl.edu/LH040
Portions of this Information Sheet taken from Turfgrass Disease Management, M. L. Elliott and G. W. Simone. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.. First published: July 1991. Revised: April 2001.