Rust

Infections of young twigs and leaves are the most common symptoms of rust, but in California, fruit infections may be a major component of the disease as well.

Twig cankers

Twig cankers are the first symptoms of the disease in spring. Cankers develop after petal fall on 1-year-old fruiting wood. They appear as blisters and longitudinal splits in the bark about 0.12 to 0.25 inches long. They can most easily be seen using a 20X hand lens.

Leaf lesions

Leaf lesions usually develop after cankers form in spring and may continue to develop through summer and into fall. The lesions appear as bright yellow, angular spots on the upper surface of leaves. The lower surface of the leaves contains brown spore masses. A high incidence of early leaf infections may cause midseason defoliation and numerous fruit infections at harvest. Early and severe defoliation also may reduce yields and stimulate the production of new leaves and buds late in the growing season.

Fruit lesions

Fruit lesions may develop during the growing season after leaf symptoms. They first develop as small, brownish spots (0.1 inch) with green halos on mature, yellow fruit. When fruit reddens, lesion halos become greenish yellow. The lesions are sunken and extend several millimeters into the fruit.

Transchelia discolor survives in twig cankers or on other host parts, and airborne spores depend on wetness for infection. Fruit symptoms may resemble damage caused by stink bugs; confirm rust by the presence of rust spores within the fruit lesion or by leaf or twig symptoms.

In orchards where rust develops it is managed with a fungicide treatment in spring. If the problem was severe the previous year, several fungicide treatments may be necessary in spring as soon as the trees leaf out. Because damp conditions favor rust development, angle sprinklers to avoid wetting the foliage. Drip irrigation is the least favorable to development of this disease because it doesn't increase the humidity in the orchard as much as flood or furrow irrigation.

Take fruit samples every other week after color break (see PREHARVEST FRUIT SAMPLES) to detect any developing problems in the orchard and a fruit damage sample at harvest to assess the effectiveness of the current year's IPM program and to determine the needs of next year's program, see FRUIT EVALUATION AT HARVEST. Record results (PDF) for harvest sample.

Sulfur treatments are acceptable for use in an organically certified crop.

To be effective, treatments must be applied before rust symptoms appear on leaves. Examine one-year-old fruiting wood for small blisters or longitudinal splits. If twig cankers are found and rain is forecasted, make a treatment. If wet weather persists, additional applications may be necessary in late May or early June. Disease severity in the preceding year is an important factor in determining potential of disease during current year.

Treatment with sulfur is both cost-effective and efficacious. The sterol inhibitors are also efficacious but more expensive than the sulfurs.

• Fruit sampling for pest damage in harvest (PDF)