Issue 7, June 12, 2017
Gymnosporangium Rusts
I covered Gymnosporangium rusts on Eastern Red Cedar in Issue 2 of this newsletter. That article focused on infections of evergreen hosts. We are now seeing symptoms on many rosaceous plants, which serve as alternate hosts. Apples, crabapples, hawthorns and quince are some of the more commonly affected plants. Symptoms vary depending on the host plant and the rust species involved.
Three common cedar rust diseases occur in Illinois:
- Cedar-apple rust (Gymnosporangium juniperi-virginianae)
- Cedar-hawthorn rust (Gymnosporangium globosum)
- Cedar-quince rust (Gymnosporangium clavipes)
Cedar-apple rust is the most common of the above three. Infections can occur on leaves, fruits and twigs of apples and crabapples. For ornamental trees, leaf symptoms are more of a problem than affected stems and fruits. Leaf symptoms first appear in May and June as pale yellow spots on the upper leaf surface. The spots eventually enlarge and turn orange in color. They will also begin to appear on the underside of the leaf eventually forming tube-like structures (aceia). Infections may result in yellowing leaves and defoliation.
Cedar-apple rust on apple.
Cedar-hawthorn rust can infect several species within the rose family and can cause similar foliar symptoms to cedar-apple rust. The pathogen is considered minor on apple, crabapple, serviceberry and pears. However, the pathogen can cause severe disease on certain hawthorn species (Crataegus spp.). The downy hawthorn (C. mollis) is considered very susceptible to this disease, while infections seem to be less severe on hawthorn species with glossy leaf surfaces (C. crusgalli, C.viridis 'Winter King'). This rust disease most often affects leaves, causing yellow spots that enlarge eventually and develop a gray-brown color. Severely infected hawthorn leaves often turn bright yellow before dropping prematurely. This pathogen can also damage fruits and twigs. However, this type of injury is more likely the result of cedar-quince rust.
Cedar-hawthorn rust on Downy Hawthorn (Crataegus mollis).
Cedar-quince rust will also infect various members of the rose family. Serviceberry, chokeberry, quince, hawthorn, and apple are some of the more noteworthy hosts. Leaf symptoms for this disease are limited to infections of petioles and veins. This pathogen causes noticeable damage to stems, thorns and fruits of susceptible species. Stems and thorns may become enlarged and deformed. Fungal aecia cover infected fruit, giving them an orange, fringed appearance.
Cedar-quince rust on hawthorn. Stem and fruit infections.
Cedar-quince rust on hawthorn. Old branch swelling from previous season’s infection.
Control
Both juniper and hawthorn are necessary for these diseases to occur. Removing any unwanted junipers from landscape, or at least the infected juniper branches may help reduce the occurrence of the disease. However, it may not be a practical management tool to separate the hawthorns from their alternate host (junipers), since spores may still blow in from neighboring properties. If cedar rust diseases are a yearly problem, consider protecting high-value plants with fungicides. It is too late to protect this year's growth. Mark your calendars with a reminder to apply fungicides early next spring to protect new growth. (Travis Cleveland)
Author:
Travis Cleveland