Latin name: Phragmidium violaceum
Reason for Concern:
1) A problem on cultivated Evergreen and wild Himalayan blackberries.
2) Severely infected leaves may curl and dehydrate.
3) Severely infected flowers and fruit fail to ripen.
4) Continuous defoliation and stem infections may cause dieback of canes.
5) Evergreen blackberry is the only commercial cultivar that has shown a susceptibility to this.
6) First observed in 2004 and recorded in 2005, the rust has been found in all west coast states.
Wine colored spots appear on the top of infected leaves. Directly under these spots, on the underside, will be circular patches of cream to yellow spore masses surrounded by a violet tinge. Upper leaf symptoms may resemble Septoria leaf spot while lower leaf symptoms may resemble cane and leaf rust. Black spores mixed in with the yellow spores appear in the advanced stages. Older leaves close to the canes are the first infected and can eventually die. Defoliation of entire canes has been seen in severe cases. Blossoms and unripe fruit can also carry spores. All green portions of primocanes and floricanes can be infected. Cane infections on evergreen blackberries are rare even under severe conditions.
The fungus overwinters as black pustules principally on leaves that remain attached to canes and/or those trapped in cane bundles, then germinates in spring and infect primocane and floricane leaves leaving purple upper leaf spots, yellow lower leaf spores, and infected flower buds, leaves and stems. New fungi are produced from these infections and continue the epidemic, building up disease in rapid, successive cycles. Black pustules develop in late summer/early fall on infected leaves. In fall, old canes can be a significant source of spore infection of primocanes in the Pacific Northwest. More than 6 hours of leaf wetness daily with mean temperatures between 49 and 64°F favor disease development. Spores are easily spread by wind; young leaves at ¾ expansions are most susceptible to infection.