Rust

You'll notice yellow pustules on the undersides of leaves in late spring or early summer, with light green discoloration on the upper leaf surface above each pustule. Later in summer, golden or orange powdery pustules appear as the fungus produces spores. By late summer and autumn, tiny raised dark brown or black crusts form and overwinter on fallen leaves. On some conifer hosts like Douglas-fir, you may see reddish-brown sunken cankers on new growth that can girdle stems and cause dieback.

Rust fungi often require two different plant species to complete their life cycle, which gives you a management advantage: identify and manage the alternate host. Early defoliation from rust reduces return bloom and weakens plants, so rake and destroy fallen leaves in autumn to eliminate spores. For susceptible trees and shrubs in high-value locations, removing nearby alternate hosts or separating them by 0.5 to 2 miles works if practical. Planting resistant cultivars is your best long-term strategy.