With all the unusual late spring rains we have been having, diseases are an increasing concern for growers. Each specific disease has its own requirements of optimum temperature and wetness requirement for infection. Most fungal and bacterial plant diseases are driven by these two factors. The higher the temperature with leaf wetness, the shorter time needed for infection. Once this threshold for infection occurs, if the plant is in a susceptible stage. Infection will occur, if the innocuous or disease organism is present.
It is important to know what plant diseases are are in your area, in case treatment may be needed. All disease treatments are preventative, if you wait to see the disease it is to late to control it. That is why knowing what diseases are present in your area, and to monitor the environmental when the host plant becomes susceptible.
There has been much research in this area developing models that predict infection periods by researchers and extension people. Data and models are available to predict these infection periods. All you need to utilize them is you specific micro climate information in the orchard or crop you are interested in.
There are inexpensive data logging equipment that can record temperatures and leaf wetness very economically. These mini weather stations can provide you with important information to predict disease. In many areas there are whole systems of weather station that are tied together on the Internet that allows real time access to weather data. This can be a useful tool of the weather conditions in your area that disease conditions exist.