Cedar-Apple Rust
Elizabeth Wahle, horticulture specialist

If your apple leaves have developed orange spots with black specks in the center, you most likely have cedar-apple rust, a fungal disease.

Cedar-apple rust initially develops on the upper leaf surfaces shortly after bloom. Lesions are pale yellow; but as they expand, they turn orange, which distinguishes them from other types of leaf spots. Later, small black dots (fungal fruiting structures called pycnia) appear in the orange rust spots.

By late summer, tiny fungal tubes (aecia) emerge from the lower leaf surfaces. Extensive leaf infection may cause early defoliation, which in turn weakens the tree. Fruit may also be affected. Fruit infection by cedar-apple rust is most common near the blossom end and can severely mar the fruit. Yellowish orange lesions, which are similar to, but much larger than, those on leaves develop on the infected fruit. The lesions have a dark green border and are up to 1/16 inch deep; the tissue beneath the lesions is unaffected.

The disease cycle of cedar-apple rust is relatively complex. Cedar-apple rust involves two hosts, three types of fungal fruiting structures, and requires two years to complete its disease cycle. The fungus overwinters in reddish-brown galls on cedar trees. When wet in the spring, orange gelatinous tendrils, or “horns,” containing resting spores (teliospores) emerge. The resting spores germinate to form reproductive spores (basidiospores) which are carried by wind to apple trees.

Fruit are most susceptible for two to three weeks starting at bloom; leaves are most susceptible when four to eight days old. Once release of reproductive spores from cedar trees has ceased (generally by second- or third-cover spray), there is no further infection of apple tissues.

Unlike apple scab, cedar-apple rust lesions on apple leaves will not produce spores that re-infect apple leaves and fruit. Instead, fungal mating occurs in rust lesions (pycnia), which result in the formation of tiny fungal tubes on the lower leaf surfaces (aecia). The fungal tubes release spores (aeciospores) which are carried by wind to cedar trees where they infect and complete the disease cycle. Galls start to develop on cedar shortly after infection but do not exude gelatinous tendrils containing resting spores (teliospores) until the second spring after infection.

Some apple cultivars are resistant to cedar-apple rust. Those considered very susceptible include cultivars ‘Golden Delicious,’ ‘Goldrush,’ ‘Jonathan,’ and ‘Rome Beauty.’

Removing cedars within a 2-mile radius of an orchard will disrupt the disease cycle, but this method is usually not practical in most settings. The spores that infect apples are produced and released from cedar galls starting at about the pink stage of flower bud development through first- (7 to 10 days after petal fall) or second-cover spray (7 to 10 days later); this is the most critical time for control with fungicides.

In the home setting, fungicides such as captan (common component of Tree Fruit Spray) directed at scab will not control rust. Spectracide Immunox Multi-Purpose Fungicide (active ingredient myclobutanil) is labeled for control of rust on apple.

pycnia

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