What causes plant disease? Most people would answer this question by saying that plant pathogens, such as fungi or bacteria, cause plant disease. But this is only part of the answer. Plant pathologists talk about a combination of three crucial factors that must be present to have plant disease. These three factors are often referred to as the plant disease triangle.
In order to have plant disease, we first need a susceptible host plant. For example, to have Septoria leaf spot on tomato plants, we need a tomato plant that is able to get this disease. Secondly, a pathogen must be present--some organism that causes disease, such as the Septoria fungus that attacks tomato leaves. Thirdly, the host plant and pathogen must interact in a favorable environment. For example, the tomato plant and Septoria fungus must interact in warm, moist conditions in order for disease to occur. This third factor is critically important--even the most susceptible plants exposed to huge amounts of a pathogen will not develop disease unless environmental conditions are favorable.
Because all three factors are necessary for plant disease, we can prevent disease in our gardens by altering any one of the three factors. For example, the host plant can be changed by growing disease-resistant varieties or species that are relatively disease free. Using growing practices that maintain good plant vigor will also make plants less susceptible to disease. We can reduce the pathogen by removing debris and weeds where it may survive, rotating crops so that pathogens do not survive year to year on the same crop, controlling insects that carry pathogens to plants, or using fungicides to kill the pathogen. The environment can be managed so that it is less favorable for disease, such as by spacing, staking, and pruning plants to promote airflow and reduce humidity, removing weeds that impede airflow, avoiding overhead watering that increases leaf wetness, and watering in the morning rather than evening so leaves have time to dry out.
Effective management of plant disease relies on an understanding of the three factors that interact to cause plant disease--a susceptible host plant, a pathogen, and a favorable environment.