This is the time of year, around mid summer, when we can expect an increase of the moisture content in our weather. Arizona has it worse. They, and New Mexico, frequently call it their “summer monsoon”. It accounts for 35 to 45% of the state’s annual precipitation. In Mexico, it accounts for 60% of the annual rainfall in Sonora. Cloud development typical of summer monsoons. The summer monsoon is not well understood but we know that it moves inward from the Pacific Ocean because the land heats up faster than the Pacific Ocean. The monsoons mostly stop around the Phoenix area but we do see an increase in clouds building from the South and, more importantly, an increase in our relative humidity. So why is this important? Because it causes an alteration in some plants susceptibility to diseases. This all goes back to the plant disease triangle. The plant disease triangle was probably one of the most important concepts I was ever taught in plant pathology while in school. The disease triangle relates to a three-legged stool; each leg coinciding with an increased potential for disease development. These three factors are: the predisposition of the plant to disease, the presence of the disease organism, and an environment that is conducive to the development of disease. When the summer monsoons come into play, all three of these legs of the stool become activated. Let me explain. Plant disease triangle Predisposition of the plant. Healthy plants normally ward, or can fend off, diseases. Just like us, the healthier we are, the better able we are to withstand diseases. The plants more susceptible to diseases are those that are weakened, in poor health, old, malnourished or in some way compromised. This is why I encourage you to feed your plants on a regular basis. This helps to maintain their health. As summer temperatures increase, plants have a reduced capacity to withstand diseases. The summer monsoon comes at a time when plants have been hit hard with high temperatures and are under a lot of stress. Presence of the disease organism. We can make some assumptions. The assumption I like to make is that most common disease organisms are always present in our garden or landscape. Two factors that contribute to the possibility that a disease may become a problem are the amount of inoculum (that is the total amount of bad disease organism) present and the virulence (the strength) of that organism. If the disease is highly virulent, it will not require a lot of that organism to be present for the disease to occur. If there is a lot of inoculum present, the potential for disease is high. Environment. The third factor is the physical environment surrounding the plant and the disease organism. Fungal diseases, our most common plant diseases, require a wet environment to prosper. Bacterial and viral diseases, less so. When the humidity increases, the moisture in the environment increases, and the propensity for diseases to occur increase. If the other two factors in this three-legged stool are leaning towards a disease problem, higher humidity may send it over the top. The adage in plant disease control is to manage plants and their environment to minimize disease problems by considering all three legs of the stool. Eliminate one of the legs and the disease can be managed or controlled. These summer monsoons come at a time when plants are under a lot of stress due to high temperatures, high light intensities, poor soils, poor irrigation practices and a host of other things. The disease organism is present. Count on it. They are there and ready for any opportunity that presents itself. Now, the summer monsoons come, increasing humidity. Our biggest disease problems will come from fungal diseases that prefer higher humidity, higher temperatures and plants that are susceptible.