Understanding Cedar-Apple Rust
Cedar-apple rust can be a destructive apple disease in this area. It also attacks red cedars but usually causes minimal damage. Understanding this disease and its life cycle will help in proper identification and control.
Cedar-apple rust is a general term that covers three separate fungi that attack apples, quince, and hawthorne. All three fungi spend part of their life cycle in cedars. The disease cycle is similar for each type of fungus. This article will cover only the fungus and disease affecting cedars and apple trees because of the large economic importance of apples.
On apple: Infections occur on the leaves, fruit, and sometimes on small limbs. Small pale yellow spots appear on leaf surfaces in April or May. These spots enlarge and darken turning orange. A black spot will appear in the center in later summer. This is the fungus spore maturing to leave the host leaf and return to the cedar. Severe infections will cause leaves to drop and prevent fruit from maturing properly or sun scalding of fruit because of inadequate foliage.
On cedar: This fungus produces reddish-orange-brown galls up to two inches in diameter. Each gall has small circular depressions (like golf balls). In each depression is a small, pimple-like structure that elongates to produce an orange spore-bearing horn. These horns swell during warm, rainy periods of April and May and release spores that the wind carries to infect apples.
This disease is complex and requires two host plants – apple and cedar. The fungus requires two years for a complete cycle. Warm, wet springs produce fantastic numbers of spores. These spores can infect apple leaves in as little as four hours under favorable conditions. Yellow spots will take two to three weeks before they are visible. In August spores from infected apple leaves are carried back to cedar trees.
There are three ways to control cedar-apple rust disease: 1) plant resistant varieties of apples (Red Delicious, Winesap, Redfree, Jonafree, Prima, Priscilla are a few.) A nursery or mail-order catalog will have these and additional varieties; 2) remove cedars adjacent to the orchard. This may only be practical for a large commercial orchard, but may not be feasible for a homeowner with neighbors who want to preserve their cedars; 3) spraying fungicides effective against this rust disease. Apply four sprays at 7 to 10 day intervals of a fungicide labeled for cedar-apple rust starting at pink bud stage. This fungicide can also be used in July, August on cedar trees, but usually protecting apples is of greater importance. Always read and follow pesticide label instructions.