Gymnosporangium rusts

Vulnerable plants
Common broadleaf hosts of cedar-apple rust (Gymnosporangium juniperi-virginianae) include apples and crabapples, and the usual conifer hosts are eastern red cedar, Rocky Mountain juniper, as well as a few Chinese, common and prostrate junipers. Hawthorn rust (G. globosum) commonly infects the same hosts listed for cedar-apple rust, as well as hawthorn, pear, quince and serviceberry. On the other hand, quince rust (G. clavipes) not only can infect the broadleaf hosts listed, but over 450 additional rosaceous plants.

What Gymnosporangium rusts look like
Gymnosporangium rust fungi typically live alternately on two very different host plants to complete their life cycle. They spend part of their life cycle on broadleaf hosts such as apple and crabapple, and part of it on conifer species such as juniper. On the broadleaf host (primarily members of the rose family) cedar-apple and hawthorn rusts cause yellow-orange spots that develop into protruding fruiting structures (aecia) on the underside of infected leaves. Heavily infected leaves fall off prematurely. Quince rust causes discoloration, distortion and death of infected fruit, leaves and branches of infected broadleaf hosts. Cylindrical, yellow to pink aecia develop on affected plant parts. If the infection girdles branches, dieback beyond that point results.

On the conifer host, spherical to spindle-shaped galls develop on leaves and branches. These can be from one-eighth to two inches in diameter. When the galls are producing spores, the gelatinous fruiting structures (telial growths) can be red to orange to yellow colored. When the galls are dormant or are no longer producing spores, they are green to brown in color.

How infection occurs
The Gymnosporangium rusts typically require both a broadleaf and a conifer host in relatively close proximity to complete their life cycle. Gymnosporangium rust spores blow from galls or infected branches on the conifer host during rainy periods in April and May. If conditions are cool and wet when these spores land on the leaves, fruit or green shoots of susceptible broadleaf hosts, infection occurs. By July and August, the rust fungus releases spores from leaves, fruit and shoots of the broadleaf hosts. These spores blow to needle-leaves and green shoots of susceptible junipers. Typically the rust fungus will grow in the juniper for about a year after the infection before galls or other symptoms of infection are visible.

Management strategies
In many cases no interventions are carried out. Infections are seldom seriously damaging to ornamental woody plants.

Resistant varieties - Where Gymnosporangium rusts are a persistent problem, replace affected plants with resistant varieties or species. These are available for both broadleaf and conifer hosts.

Sanitation - Prune out galls or swollen branches while the fungus is still dormant. This is most beneficial if there are only a few juniper hosts in the surrounding area of the rosaceous hosts.

Modifying the growing environment - Avoid overhead irrigation of broadleaf hosts in April and May, and conifer hosts in July, August and September. Separation of the alternate hosts by more than one-quarter mile reduces infection of the broadleaf host.

Chemical control - The cost of fungicide sprays may not be economical except for specimen or nursery trees in areas where rust is a recurrent problem. Apply a fungicide to the broadleaf host at 7 -10 day intervals in the spring when juniper galls are producing telial growths. Initial applications are made just as buds open, and are repeated at labeled intervals to maintain protection of developing leaves, shoots and fruit. Fungicide applications to the evergreen host are not usually made. Fungicides may be beneficial for high value junipers. Spray the junipers in mid to late summer when cool, moist weather persists as spores are being released from the broadleaf host.

Dan Gillman
UMass Extension Plant Pathologist
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Diseases index:
Apple Scab
Cedar-Apple Rust
Crown Gall
Dogwood Anthracnose
Fire Blight
Gynosporangium rusts
Juniper Tip Blight
Powdery Mildew
Seedling Root and Crown Rots of Woody Plants
Spore Shooting Fungi
Verticillium Wilt of Woody Landscape Plants
Volutella Blight and Stem Canker on Pachysandra

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