All cedar-rust diseases spend a time of their life cycle on Eastern red cedar (Juniperus virginiana – along with other junipers) and another part of their life cycle on apple, hawthorn, and other members of the rose family. Both hosts are necessary for the fungus to complete its life cycle. Three of the most common rusts occurring in the Midwest are caused by Gymnosporangium juniperi-virginianae (cedar-apple rust), G. globosum (cedar-hawthorn rust), and G. clavipes (cedar-quince rust).
THE DISEASE CYCLE
The rust organism requires a full two years for the fungus to complete its life cycle and spends one full year of its life cycle on the juniper. During the second spring, coinciding with the blooming of the crabapples, the galls swell and produce jelly-like tendrils called, ‘spore horns’. As the spore horns begin to dry, the spores are released and carried by the wind to the newly developing leaves of the susceptible plants. Distribution of spores can range up to 5 miles from a juniper, but most infections occur within several hundred feet. The spores are finished in about a month and most leaves are no longer susceptible after that time. About 2 weeks from initial infection, small, yellow dots can be seen on upper surfaces of infected leaves. Several weeks later, the fungus appears as orange or brown spots with hair-like appendages on the underside of the leaf. In late summer, the rust spots release the spores and are carried to nearby junipers, thus restarting the cycle. Repeated infections of cedar-apple rust can be unsightly and seriously weaken and destroy the ornamental value and health of susceptible plants.
CEDAR APPLE RUST
Symptoms on Juniper
During mid-spring, swellings or galls mature on juniper needles that were infected with spores the previous year. The galls are brown to dull red in color, globular in shape, and may vary from pea-sized to an inch or more around. As they age, circular pits or depressions are all over the surface of the galls, like the dimples on a golf ball. After April showers and damp weather, yellow gelatinous tendrils or spore horns form in these dimpled areas. The tendrils elongate rapidly and release spores during dry, windy weather that follows the Spring rains. Spores produced on the juniper host blow to the apple, crabapple, and hawthorn hosts as their new growth emerges.
Eventually the galls dry out but remain attached to the tree for several years, resulting in some small twig and tip die back.
Symptoms on the Crabapple and Apple
Leaves: Bright yellow/orange spots develop on the top of the leaves in late spring. These spots gradually expand, becoming evident on the bottom of the leaves as small bumps. In midsummer, these rust lesions develop hair-like, cylindrical tubes (hyphae), which release spores into the air, which are carried to the juniper host. Infected leaves of apples and crabapples will most likely drop with defoliation more severe in drought. Galls that form on the juniper host do not show up until July the next year.
Twigs: The rust appears as a swollen bumpy gall on the current year’s growth, usually no more than 1 inch in length. The swelling eventually develops the characteristic cylindrical fruiting bodies. Extremely affected branches are stunted and may die.
Fruit: The rust produces yellow to orange spots comparable to those on the leaves, but the spots are usually much larger. Fruit infection causes lower quality fruits or premature fruit drop.
Cedar-hawthorn rust is very comparable to cedar-apple rust, in both appearance and occurrence, but it infects a broader range of plants within the rose family. The severity of the disease is usually minor on crabapples and apples (Malus sp.), pears (Pyrus), and mountain ash (Sorbus) but can be quite serious on many hawthorns (Crataegus sp.).
Symptoms on Juniper
Cedar-hawthorn rust galls are smaller than cedar-apple rust galls, less proportioned, and more chocolate-brown in color. Galls remain on the twigs of branches of junipers for several years, where they continue to produce spores, compared to the one season spore production of cedar-apple rust. Symptoms on Hawthorn.
Symptoms on Hawthorn
Leaves: Big yellow spots appear on the tops of the leaves turning yellow orange to gray-brown as they age. When rust is severe, all the foliage may turn bright yellow and drop in summer. The orange leaf spots are smaller on apple and crabapple.
Fruits and Twigs: Deformation of fruits and young twigs is particularly severe on hawthorns, but this damage is usually caused by the cedar-quince rust fungi and not cedar hawthorn rust fungi.
Symptoms on Junipers
Spindle-shaped swelling occurs on twigs and branches of junipers. Young branches are usually girdled, then die. In damp weather, older galls are covered with masses of gelatinous, orange to brown spore horns. Galls can produce spore horns for 4 to 6 years, sometimes longer.
Symptoms on Quince
Cedar-quince rust affects quince (Chenomeles), mountain ash (Sorbus), hawthorn (Crataegus), serviceberry (Amelanchier) and many other plants in the rose family. Though normally not as widespread as cedar-apple rust, it causes the greatest amount of damage to the fruits, twigs, and thorns of susceptible plants. During lengthy periods of wet weather, when temperatures range between 50F and 75F, severe infection can occur just 4 hours after leaf contact.
Leaves: Nothing obvious although veins or petioles will be swollen.
Twigs and thorns: Elongated, puffy cankers appear on twigs and thorns. In damp weather, you’ll see orange to brown spores.
This disease requires two hosts, the separation of the hosts for a distance of 1 mile will help reduce infection. Idyllically, to minimize disease host availability, plant trees and shrubs that are resistant to rust diseases. Just check the label or search the variety on-line to check resistance.
Prophylactic fungicides can be used to help minimize infection. Many are organic! A minimum of three applications are necessary. These applications protect the new leaves from spores that are airborne from the juniper host in mid-spring. Spraying apple, crabapple, and hawthorn foliage after the symptoms develop has NO controlling effect.
Junipers: Begin spraying susceptible plants in early July and continue at labeled intervals through August. Remove galls and cankers to reduce infection of alternate hosts.
Apples and Crabapples: Begin spraying when new growth appears and flower buds show color but are not yet open (balloon stage). Repeat three to four times at 12 day intervals.
Hawthorns: Spray as new growth appears and flower buds begin to open. Repeat 3 to 4 times, always read the labeled directions! Visible stem cankers need to be pruned out.