The term DISEASE describes a collection of symptoms that are observed on the turf plant / sward and that occur as a result of changes to the normal functioning of the plants physiology, which is brought about by the direct action of any living organism on that turfgrass plant.
What this means in simple terms is, if a turfgrass plant expresses changes in the way in which it grows or functions and these changes occur as a direct result of the activity of another living organism, the collection of these changes or symptoms is termed a disease.
By using this definition, it is apparent that many organisms can cause disease in turfgrass. Fungi are perhaps the most well known of likely causes of disease and indeed are almost synonymous with the term. However, bacteria and viruses are also known to cause disease in turf but their effects and importance in cool-season turf are negligible. Certain nematodes (microscopic roundworms) can also directly injure turfgrass plants and cause a change in the plant growth and as such, nematodes too can be agents of disease. Not only can nematodes directly affect the growth of the turf but they can also facilitate entry of bacteria and fungi in to the colonised turf.
The organisms that are responsible for disease, e.g. fungi, are generally present in the turfgrass system all of the time. However, the symptoms of disease that they are able of causing will only be expressed during periods when the growth of the fungus exceeds that of the turf. Many of the fungi that cause disease on cool-season grasses are relatively weak pathogens. They rely on stresses to weaken the turf and render it more susceptible to attack whilst at the same time take advantage of favourable growing conditions to allow their populations to increase to epidemic proportions.