Answer: That crusty gray growth on your fruit trees is called lichen. It?s made up of both a fungus and an algae living together, and grows on the trunks and branches of trees and shrubs, as well as on rocks. Since lichens can make their own food, they don?t directly hurt the trees and shrubs that they grow on. But, heavy growth can reduce the amount of light available to the tree, and you may not like the appearance of lichen on the branches. For trees and shrubs that lose their leaves in winter, a simple control method is available. A dormant spray of lime sulfur applied n late winter or early spring will effectively control lichens on deciduous plants. But be careful to keep this spray off evergreen plants like rhododendrons?it can damage their leaves.
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