Rusts are a type of plant and tree diseases caused by fungi from the pucciniales order. These only infect living plants and trees and the troubles begin when the spores land on the tree or plant surface and germinate. They infect the leaves, tender shoots, buds and stems causing them to wilt and wither with a characteristic red, rust-colored powdery residue on them. Rusts are considered some of the most invasive and harmful fungi for plants and trees. The spores are transported by wind, rain or by insect carriers. Affected trees will suffer leaf loss, spotting, reduced fruit yields, and deformities.
Symptoms: Characteristic reddish rust-colored flecks or bumps on leaves and tender shoots.
More information: University of Maryland Extension