Cedar-apple rust on Plants

Symptoms

This problem looks different on apple and on juniper. On apple, you see yellow to bright orange spots on the leaves. Spots can also appear on the fruit. On rosaceous hosts in general, the swellings and colorful spots on fruit, leaves, and twigs are nonlethal.

Groups of small, cup-shaped fruiting bodies hold orange fungal spores on the fruit, green stems, and leaves of those hosts. The orange color runs through the whole list, from the leaf spots on apple to the gelatinous masses on juniper. Both kinds of host can show swollen growth on twigs, with swellings on rosaceous hosts and galls on juniper.

On juniper, look for smooth, round galls on the twigs, up to golf-ball size. Dark green, tight bunches of shoot growth, known as witches' brooms, can appear on juniper and other conifer hosts. These brooms can exceed 3 feet (0.9 m) across. The spore-forming bodies on conifer hosts appear as orange, gelatinous masses. The galls and brooms belong to the conifer hosts, while the leaf and fruit spots belong to apple and its relatives.

Causes

The cause is a fungus, Gymnosporangium juniperi-virginiana. Cedar apple rust is caused by fungi of the genus Gymnosporangium. The fungus requires two hosts in different plant families to complete its life cycle. If either host plant is not present, the fungus dies.

The hosts listed for this problem are apple, crabapple, juniper, eastern red cedar and hawthorn. On conifer hosts, the fungus forms spore-forming bodies that appear as orange, gelatinous masses. On rosaceous hosts, it forms groups of small, cup-shaped fruiting bodies that contain orange fungal spores. Infected conifer twigs and limbs produce spores in the spring.

The damage to rosaceous hosts is nonlethal swelling and colorful spots on fruit, leaves, and twigs. Galls on juniper twigs can reach up to golf-ball size. In most landscape cases, damage to either host is not severe enough to warrant fungicide applications. If a broadleaf host such as crabapple or apple is the desired plant, the listed advice is to remove wild junipers.

Diagnosis

Check the host list first, because cedar apple rust is tied to apple, crabapple, juniper, eastern red cedar and hawthorn. A plant outside that list is not one of the hosts named for this problem.

On apple, the spots on the leaves are yellow to bright orange. Spots can also appear on apple fruit. On juniper, the signs are smooth, round galls on twigs, witches' brooms that can exceed 3 feet (0.9 m) across, and orange, gelatinous masses. The symptoms are split between rosaceous hosts such as apple and conifer hosts such as juniper.

What sets this problem apart is that the fungus requires two hosts in different plant families to complete its life cycle. If either host plant is not present, the fungus dies. The spots and swellings on rosaceous hosts are nonlethal. Cup-shaped fruiting bodies that contain orange fungal spores form in groups on fruit, green stems, and leaves.

Orange, gelatinous masses on juniper are spore-forming bodies. Inspect junipers during each dormant season for galls, and prune and destroy any gall tissue before the spore horns develop.

Treatments

The listed treatments are mostly cultural, with chemical options for nurseries. Cultural steps cover most of the list: inspection, pruning, spacing, removal and resistant cultivars.

Inspect junipers during each dormant season. Prune and destroy any gall tissue before the spore horns develop. Prune and dispose of infected conifer twigs and limbs in the fall, before spores are produced in the spring. Pruning of conifer twigs and limbs is timed for the fall, while inspection of junipers is timed for each dormant season.

Avoid planting the alternate hosts close together. If possible, remove susceptible non-crop plants from the vicinity. If a broadleaf host such as crabapple or apple is the desired plant, remove wild junipers. Plant resistant cultivars of crabapple and apple, which are available.

In most landscape cases, damage to either host is not severe enough to warrant fungicide applications. The chemical options are described for nursery use. In the nursery, a fungicide can be applied to crabapples in the spring and early summer. In the nursery, a fungicide is also applied to junipers in the summer.

If you use any fungicide, the product label is the legal instruction and must be followed for the crop and site being treated. This page gives no dose, rate, dilution or spray interval, because those belong on the label.

Prevention

Prevention comes down to the plants you grow and where you place them. Avoid planting the alternate hosts close together. The fungus requires two hosts in different plant families to complete its life cycle. If either host plant is not present, the fungus dies.

If possible, remove susceptible non-crop plants from the vicinity. If a broadleaf host such as crabapple or apple is the desired plant, remove wild junipers. Choose resistant cultivars of crabapple and apple, which are available.

Inspect junipers during each dormant season. Pruning gall tissue before the spore horns develop is part of that routine. Prune and dispose of infected conifer twigs and limbs in the fall, before spores are produced in the spring.

The hosts to keep in mind are apple, crabapple, juniper, eastern red cedar and hawthorn. In most landscape cases, damage to either host is not severe enough to warrant fungicide applications.

Plants most often affected

Apple, crabapple, juniper, eastern red cedar and hawthorn are the hosts for this problem. On apple, it shows as yellow to bright orange spots on the leaves, and on juniper it shows as galls on the twigs. The fungus requires two hosts in different plant families to complete its life cycle. If either host plant is not present, the fungus dies.

• Eastern Redcedar