Rain

Normally, rain is considered to be beneficial for trees as opposed to a stress agent, however, too much of anything can be bad. Heavy rains can actually damage leaf tissue or cause minor defoliation. Prolonged periods of leaf wetness can increase susceptibility to certain diseases such as anthracnose or needle cast. Splashing rain is a common mode of pathogen transmission for diseases such as fire blight and sudden oak death. Saturated soils also tend to be conducive to root pathogens such as Pythium and Phytophthora. Shallow rooted trees are susceptible to wind-throw or root-lifting when the surrounding soil remains saturated for a prolonged period of time, especially when strong winds occur. Soils that remain saturated for long periods may also inhibit a tree’s ability to conduct many of the essential oxygen-requiring physiological processes that occur in roots, stressing the tree and making it susceptible to secondary stress agents. Excessive rain in the fall may delay the “hardening off” of succulent tissues before winter, leading to an increased risk of late season frost damage.