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Rust Diseases Three different rust diseases can occur on apple. However, it is not necessary to distinguish the subtle differences between these rusts for the purposes of scouting, since the disease cycles and control programs are similar for each.
SYMPTOMS: Small pale yellow spots appear on the upper surface of infected leaves shortly after bloom. These spots gradually enlarge up to 1/4" in diameter, depending upon the apple variety and the number of spots per leaf, and become bright yellow-orange in color. By early or mid-summer, yellow spots will also be present on the lower leaf surface; from these come small, orange-yellow tubular fruiting bodies which eventually project downwards. As the tubes mature, they split towards the base into narrow strips and curl back on themselves to form cup-like structures. Within these structures a mass of light brown spores can normally be seen with a hand lens (10x). Heavily infected leaves may fall off the tree.
Fruit spots usually appear near the blossom end, and may be of two types: 1) spots that are similar in color to the leaf spots, but larger (up to 3/4" in diameter), which may be outlined by a dark green border; 2) spots that are dark green and sunken. Fruits with Type 2 infection usually become puckered at the blossom end while the fruit is still an inch or less in diameter.
DISEASE CYCLE: Spores produced in the cup-like structures on the lower surface of infected apple leaves become wind-dispersed, and infect nearby cedar or juniper trees during the summer and fall. The rust fungus then grows and survives two succeeding winters in galls which are formed on infected cedars. When apple buds are in the pink to early bloom stage, the fungus produces spores upon the cedar galls, and these spores are blown to apple tissue, where they infect and complete the disease cycle two years after it began. The infection period for apple usually ends about 30 days after bloom. Unlike apple scab or fire blight there is no apple-to-apple (secondary cycle) spread of disease.
SCOUTING: Apple varieties vary greatly in their susceptibility to rust. Be especially alert for leaf infections and Type 1 fruit infections on Prima, Sir Prize, Lodi, Jonathan, Rome, Wealthy, and York Imperial. Be especially alert, also, for Type 2 fruit infections on Cortland, Golden Delicious, Gold Rush, McIntosh, Red Delicious, Stayman, and Winesap.
For each tree record the number of infected leaves per 100 observed (20 per limb x 5 limbs). Also, examine 100 fruit per tree and record the number with at least one rust spot. Make separate notations for Type 1 and Type 2 fruit infections, where possible.
Cedar Rust Gall
If you can see cedar trees near the orchard, look for the presence of the orange cedar galls from pink bud to 30 days after bloom. Note your findings so that future rust outbreaks can be anticipated or so that infected cedars can, if possible be eradicated.
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