Rust diseases are common fungal infections that affect a wide range of floricultural crops, including aster, rose and viola in the nursery industry. Rusts have the potential to negatively impact floriculture production because these pathogens often cannot be detected on infected, but symptomless propagation material entering the U.S. or moving state-to-state. Rust fungi are obligate parasites, dependent upon a live host for growth and development, and seldom kill plants. However, rust infection reduces plant health and vigor, flower production, and aesthetic value.

Each type of rust has its own distinctive symptoms and its own specific plant hosts. The disease often first appears as chlorosis on the upper surfaces of leaves. All rust fungi produce powdery masses of spores in pustules, typically on leaf undersides that are yellow, orange, purple, black or brown. Some rust fungi produce pustules on upper leaf surfaces as well. Spores are easily spread on air or with splashing water. Lesions may coalesce resulting in large areas of necrosis; leaf distortion and defoliation often follow.

Let’s focus on rose rust, caused by Phragimidium mucronatum.

The facts about rose rust

Life cycle and symptoms:

This disease first appears in the spring as bright orange pustules on leaf undersides, leaf stalks and branches. In the summer, small raised orange spots appear on the undersides of leaves with small yellow specks on the upper surface beginning on the lower leaves and move upward. The spots may go unnoticed until the plant begins to exhibit a generally unhealthy appearance and a loss of the lower leaves.

Photos: Jody Feltzer,
Maryland National
Capital Park and Planning
Commission; Cesar Calderon
Pathology Collection,
USDA APHIS ITP,
BUGWOOD.ORG
Damage:

Leaves may become dry and twisted before falling off. As the leaves dies, the spots darken and produce the black teliospores that survive the winter to produce new infections in the spring. This disease may be mild or severe depending on the season and how early the infection began. Prune and burn infected leaves, petals, and canes as soon as they are detected. Cultivars vary in their susceptibility to rust diseases.

Control:

A combination of cultural and chemical control is often required to control rust diseases. There are many effective rust products and these should be applied on a protective basis according to label instructions. Be sure to rotate applications between chemical classes to prevent fungicide resistance from developing.

Management tips:
  • Purchase only disease-free plants or cuttings. Carefully inspect all incoming plant shipments for rust symptoms.
  • Keep new plants isolated from established plants for up to three weeks to allow rust diseases to develop, if present.
  • Scout regularly for rust diseases.
  • Frequently remove all rust-infected leaves and badly infected plants and destroy by burning, rapid composting, or burying.
  • At the end of the growing season, carefully clean up and destroy all crop debris. Sterilize benches and propagation rooms with an appropriate greenhouse disinfectant.
  • Keep the humidity within the greenhouse at less than 80 percent. Increasing air movement by adding fans will prevent moisture from condensing on the foliage.
  • Practice only surface watering and avoid splashing water onto foliage. If overhead irrigation is necessary, water in the early morning when plants will dry quickly.
  • Space plants to allow for good air circulation.
  • In the fall, carefully clean-up all leaf litter and destroy it. Severely infected roses can be dramatically pruned.

Source: UMass Extension Plant Diagnostic Lab, University of Massachusetts