Rust of peach and prune—Tranzschelia discolor
Rust is a fungal disease that on peach causes lesions and spores on fruit, leaves, and twigs. The T. discolor fungus can occur on other stone fruits, but with the exception of prune the rust it causes is generally not an important disease on other stone fruits.
Identification
Small cankers on twigs are the first symptom to develop in spring. But unless twigs are closely inspected for these small lesions, the yellow spots on leaves are commonly the first obvious symptom observed.
Leaf lesions. Leaf lesions usually develop after cankers form in spring. New leaf spots may continue to appear through summer and into fall. The lesions appear as bright yellow, angular spots on the upper surface of leaves. On the lower leaf surface, the lesions develop reddish brown spore masses of the fungus. A high incidence of early leaf infections may cause midseason defoliation and numerous fruit infections at harvest. Early and severe defoliation may stimulate the production of new buds and leaves 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 about 1/10 inch (2.5 mm) in diameter with green halos on mature, yellow fruit. When fruit reddens, lesion halos become greenish yellow. The lesions are sunken and extend about 1/8 inch deep into the fruit.
Fruit lesions may be confused with damage from feeding of stink bugs. Rust is distinguished by the presence of yellow leaf lesions and spore masses within lesions on fruit, the underside of leaves, and twigs.
Twig cankers. Twig cankers develop after petal fall on infected, 1-year-old fruiting wood. They appear as elongate splits in the bark about 1/8 to 1/4 inch long. Lesions can most easily be seen using a 20X hand lens.
Life cycle
Rust is more commonly a problem during years when rainfall is more abundant or later than on average or sprinkler irrigation wets foliage. It also is more likely to occur when surface water or low-growing vegetation such as weeds are nearby because these situations increase humidity around the trees.
The rust fungus survives the winter in twig lesions. Spores produced in these lesions during wet weather in the spring infect leaves. The disease continues to spread to other foliage and fruit during periods of wet foliage or high humidity. Foliage infections become dormant during the summer when conditions are dry and warm.
Leaf lesions are the source of infectious spores that cause lesions on fruit and twigs. Fruit become most susceptible to rust infection when they are nearly full sized. Twigs are infected during damp weather in summer and fall. But no spores are produced in twig lesions until the following spring.
Damage
Rust can become a problem when leaves and wet conditions occur together. If rust becomes abundant on foliage, the leaves may drop prematurely. Leaf drop can lead to sunburn of exposed bark and reduced fruit yield during the current or next growing season. Sunken, discolored rust lesions in fruit make it unappetizing.
Solutions
Use drip irrigation instead of flood, furrow, or sprinkler irrigation. Drip irrigation does not increase humidity around trees as much as these other methods and low humidity reduces the likelihood of rust development. If sprinklers are used, aim them at a low angle or shield them to prevent the wetting of foliage.
To be effective, fungicide must be applied before rust symptoms appear on leaves. If rust becomes abundant during the growing season, foliage and fruit can be thoroughly sprayed with sulfur. If rust was abundant the previous growing season, sulfur application may be warranted the following spring before leaves flush. If late rains or high humidity occur after treatment, additional applications of this preventive fungicide may be warranted in late May or early June.
Adapted from Integrated Pest Management for Stone Fruits and Pest Management Guidelines: Peach, University of California Statewide Integrated Pest Management Program (UC IPM). |
