Controlling diseases like anthracnose or insects like tussock moth is often accomplished by spraying the tree canopy with the appropriate fungicide or insecticide. Both of these pests can cause defoliation of foliar canopies. This is not healthy for trees for the same reason heavy pruning is not beneficial. Defoliation decreases photosynthesis; the process in which a tree convert carbon dioxide and water to sugar and oxygen to feed itself and provide us with fresh air. This process occurs in the leaf, which is why we take special care in trying to protect the foliage.