The simultaneous occurrence of a pathogen and a suitable host does not necessarily result in infection. Other conditions need to be favorable for pathogens to succeed. These conditions include but are not limited to temperature, humidity, precipitation (rain or snow), base fertility balance, pH, nitrogen management, irrigation management, height of cut, soil biological activity, and plant stress.
Like the human body, a plant has the ability to protect itself from diseases. Plants produce many different chemical compounds that are toxic to fungal pathogens. The ability of the plant to successfully resist disease depends on many different factors. When plants are stressed compounds called oxidants accumulate and cause the breakdown of lipids, proteins, and nucleic acids. This further stresses the turf plant. The plant responds by using a lot of energy to create more lipids and other cell components that were oxidized. However, photosynthesis occurring in turf during stressful periods is often producing less energy than is being consumed because of heat, drought, or often height of cut. If turf is mowed too closely, less leaf surface area is exposed to synthesize energy from the sun. As stress increases and energy production decreases, the plant cannot produce defensive chemicals and soon becomes defenseless against pathogens.
Other protection comes from the soil or, more accurately, from the soil organisms. As cliche as it may sound, there is truth to the axiom that a healthy soil promotes healthy plants. There are thousands of different organisms in the soil that exist for two common purposes—to survive and to reproduce. The most limiting factors to their existence are resources such as food, water and a proper atmosphere. If these organisms were given an unlimited supply of these resources, their populations would increase beyond the earth’s spatial capacity in a matter of weeks. Soil organisms must compete with each other for the limited amount of resources in their habitat. As resources dwindle, certain segments of the biomass may become dormant or die off, giving surviving organisms a distinct competitive advantage. If those surviving organisms are plant pathogens, then the microbial imbalance can be manifested as plant diseases. In some cases poor soil conditions can cause an organism to change from being beneficial to pathogenic. Fusarium, for example, is a saprophytic fungi that normally decays organic residues but it can become a pathogen when it encounters poorly nourished roots. Unfortunately, its reputation is only that of a pest and it is usually treated with a fungicide, which often depletes or destroys other groups of beneficial organisms. Saprophytic organisms are extremely valuable for their production of carbon dioxide, the most consumed nutrient in a plant’s diet.
A rich soil, as its name implies, contains a wealth of resources. Most of us imagine a rich soil to be one that contains an abundance of plant nutrients capable of producing excellent crops. The term, however, has a deeper meaning. Resources are in many forms and benefit many different parts of the soil system. Most of them are linked, either directly or indirectly, to one important soil component —organic matter. Although the fraction of a mineral soil that is organic matter is relatively small, the function of this component is responsible for the lion’s share of the system’s benefits. The proteins, carbohydrates, lignin, cellulose, fats, waxes, hemicellulose, humic substances, and other compounds that comprise the soil’s organic fraction are resources for billions of soil organisms. Paying attention to the needs of these organisms with applications of compost, biostimulants, or organic fertilizers and mowing higher with sharp blades will go a long way toward eliminating disease without using fungicides.