Symptoms on the Evergreen Host

Cedar-apple rust and cedar-hawthorn rust have similar symptoms on juniper.

In mid-spring, swellings or galls develop on juniper needles that were infected with spores during the previous year. These galls are brown to dull red in color and globe-shaped.

In general, the galls of cedar-apple rust are larger and more regular in shape than those of cedar-hawthorn rust.

As the galls mature, circular pits or depressions are found over the surface of the galls, giving them a golf ball like appearance.

Cedar-quince rust does not develop the rounded galls. This disease produces elongated swellings along the stems of the juniper host.

Symptoms on the Deciduous Host

Cedar-apple rust and cedar-hawthorn rust produce similar symptoms.

Bright yellow, orange, or sometimes red spots develop on the upper surface of the leaves in late spring. These spots gradually enlarge. On the undersurface of the leaves, small blister-like lesions develop.

In midsummer, these rust lesions on the lower side of the leaf develop tiny, cylindrical tubes which release spores into the air. These spores will infect the juniper host.

While defoliation is not common with these rust diseases, there may be some loss of infected leaves when the host tree is experiencing other stresses like dry summers.

These rusts also cause yellow to orange spots on the fruit, but the spots are usually much larger. Fruit infection causes an inferior fruit quality or premature fruit drop.

Cedar-quince rust symptoms develop on the fruit, young twigs, and petioles, rather than on the leaves.

The rust causes a swollen corky gall on the current year’s growth, usually no more than one inch in length. The swelling eventually develops the characteristic cylindrical fruiting bodies, which will produce spores to re-infect the juniper host.

Seriously affected twigs may become girdled and die.

Cedar-quince rust also infects the fruit of the host tree. Infected fruit will produce the tiny cylindrical tubes that produce spores to re-infect junipers. These tubes give the fruit a spiky appearance.

Disease Cycle

The cedar-rust diseases overwinter as galls on the juniper host. The rust organism spends one full year of its life cycle on junipers.

During the second spring, usually around the time crabapples are in flower, the galls become rain-soaked and swell, producing jelly-like tendrils (spore horns) that project out of the pitted areas of the galls. Cedar-quince’s flat galls along the stem will ooze spores in a gelatinous mass.

As the spores are released, they are carried by the wind to young, newly developing leaves of the deciduous hosts. Dispersal of spores can range up to two miles from a juniper, but most infections develop within several hundred feet.

About a month after crabapples have flowered, most leaves are no longer susceptible.

Ten to 14 days from initial infection, small yellow spots can be seen on upper surfaces of infected leaves (on stems and fruit in the case of cedar-quince rust).

Several weeks later, the fungus appears as spots with cylindrical spore horns on the underside of the leaf (fruit and stems with cedar-quince rust). In late summer, the rust spots release the spores and are carried to nearby junipers.

Management

Cultural Management

Because this disease requires two hosts, the separation of the hosts for a distance of one mile will help reduce infection. This is not always feasible, as host plants are common in urban and suburban landscapes, and many of the hosts are found growing in the wild as well.

Resistant Cultivars

To minimize disease problems, plant trees and shrubs that are resistant to rust diseases. There are many apples, crabapples, hawthorns, and junipers that exhibit resistance to these diseases.

Chemical Management

Protective fungicides can be applied to help minimize infection. For most fungicides, three applications will be needed. These applications protect the new leaves from spores that are dispersed from the juniper host in mid-spring. Spraying apple, crabapple, and hawthorn foliage after symptoms develop has no controlling effect. Treatment of the evergreen host has little value and is not recommended.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.