Annuals, Perennials, and Bulbs: Rust

Problem
Yellow or orange spots appear on the upper surfaces of the leaves. Yellowish orange, rust, or chocolate-colored pustules of spores develop on the undersides of the leaves. Infected leaves usually dry up and die. The plant may be stunted.

Analysis
This plant disease is caused by any of a number of related fungi. Most rust fungi spend the winter as spores on living plant tissue and, in some cases, in plant debris. Some rust fungi also infect various weeds and woody trees and shrubs during part of their life cycle. Flower infection usually starts in the early spring as soon as conditions are favorable for plant growth. The spores are spread to healthy plants by splashing water and wind. Some rust fungi cannot infect the flower host unless the foliage is wet for 6 to 8 hours. Rust is favored by moist weather, cool nights, and warm days.

Solution
Several different fungicides are used to control rust. Look up your specific plant in the index to determine which fungicide to use. Spray plants thoroughly at first sign of disease, covering both the upper and lower surfaces of the leaves. Some plants are so susceptible to rust that you may need to spray at weekly intervals throughout the summer. Fungicides will only protect uninfected tissues; they will not cure diseased leaves. Water in the morning rather than the late afternoon or evening. This will allow wet foliage to dry out more quickly. Remove and destroy all infected plants in the fall to prevent them from infecting new plantings. Plant rust-resistant varieties, if available. (For a list of resistant flower varieties, see Chrysanthemums Resistant to Chrysanthemum Rust.)