To defoliate a plant or a tree is to make its leaves fall off — and in the sense you are learning, it means doing that with a chemical, a defoliant, not picking the leaves by hand. Farmers may spray cotton or other crops so the leaves drop and machines can pick the fruit more easily. Armies have also used defoliants in war, to strip trees so people and paths cannot hide. You will sometimes hear the same verb for insects or disease that leave a tree bare, but the core idea here is: someone puts a defoliant on the plant. It is a formal, technical word. Everyday English is more likely to say they sprayed the fields, or the trees lost their leaves. Autumn leaf-fall on its own is not this: the tree is not being defoliated; the season is just doing what seasons do.

使落葉；噴落葉劑