Symptoms of leaf rust on prune.

Adapted from the article “Prune Rust Management Plan” by Carolyn DeBuse, UC Farm Advisor, Solano/Yolo Co. in the April 2010 Sacramento Valley Prune News.

Leaf rust is caused by a fungal pathogen Tranzschelia discolor which over winters as spores on the buds and branches of the tree. Orchards that were infected heavily last year have a good chance of having the disease show up again this year. Wet or humid weather promotes the growth and spread of the disease. The disease starts on young leaves in spring and progresses through the summer until it causes defoliation of the infected leaves. If defoliation happens after harvest there is no effect on prune quality or the next year’s return bloom. But if defoliation happens before harvest, the fruit may have a reduced dry away and lower quality.

• Symptoms: The top of the infected leaf has yellow angular spots and the underside of the leaf has red-brown spore masses.
• Monitoring: Starting May 1^st, monitoring weekly. Select 40 trees randomly in each block or orchard. If the orchard is larger than 40 acres increase number of trees monitored. Look at the lower leaves in the canopy for the disease symptoms. The disease favors vigorous trees and younger replants in the orchard check those as well as rust “hot spots” last year.
• Treatment Threshold: If you find one leaf with disease symptoms treat the orchard for rust.
• Treatment: Treat with sulfur or a registered fungicide for rust on prunes. For preventative treatment, apply fungicide before a rain event in spring or summer in orchards with a history of rust.
• Follow Up: After treatment continue monitoring for new signs of the disease and treat again when found. Monitor and treat until July 15^th. Keep a record of where the rust was found in the orchard for increasing monitoring efficiency next year. Rust often shows up in the same area from year to year.

For more information see the UC IPM website.