THE EFFECT OF WEATHER ON DISEASES The Effect of Weather on Diseases

Three things must happen at exactly the same time if an infectious plant disease is to occur. One, a susceptible plant must be in a vulnerable state. Two, the parasite that causes the disease must be in an infective stage. Three, environmental conditions must be favorable for disease development. The environment of a plant consists of the air around it and the soil in which it grows. The environment of a parasite alternates between the body of its host and either air or soil, according to whether it attacks plants above or below ground. The aerial environment actually is the weather. It is made up of light, temperature, snow, rain, atmospheric humidity, dew, cloudiness, sunshine, wind, air currents, evaporation, and atmospheric pressure. Perhaps each element of weather affects the occurrence of disease in some way, but temperature and moisture apparently are the limiting factors for most diseases. The amount of heat and moisture available in the soil environment depends on weather (or on irrigation), together with the ability of individual soil types to absorb and retain heat and moisture. The surface of the soil affects the air in contact with it and a short distance above it. The state of the atmosphere near the ground, therefore, is different from that higher up. Depending on topography, the direction of exposure, the color, type, and moisture content 83