True or False: Diseases affecting only the foliage are serious only if defoliation occurs over multiple years.

The statement that diseases affecting only the foliage are serious only if defoliation occurs over multiple years is true because the long-term impacts of repeated defoliation can severely weaken a tree's health. While a single or occasional bout of defoliation might not significantly affect a mature tree's overall vitality, consistent defoliation over multiple years can lead to cumulative stress. This stress depletes the tree's energy reserves, making it more vulnerable to other pathogens, pests, and environmental stressors. Furthermore, foliage is crucial for photosynthesis, and its loss can hinder the tree's growth and ability to generate food for survival. Thus, those diseases that only affect the leaves can become a serious threat if they result in prolonged defoliation.