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blackberry rust
blackberry rust (Phragmidium violaceum)

Images courtesy of Ken French, Oregon Dept. of Agriculture
Phragmidium violaceum or blackberry leaf rust fungus, was discovered in April 2005 on Himalaya Blackberry (Rubus fruiticosus) in Oregon. This appears to be an accidental introduction and is the first official report in North America. P. violaceum is a naturally occurring plant disease and is native to wild blackberry in Africa, Middle East and Europe. This fungus has been used for decades as a biological control of wild blackberry in Chile, Australia and New Zealand.
P. violaceum, is a defoliating plant disease that mainly attacks the leaves of blackberry. The youngest, fully opened leaves at the cane tips are the most susceptible. It can also infest the buds, unripe fruit and green growing parts of the cane. In late spring and summer, uredineospores, "summer" spores, appear as yellow powdery pustules on the underside of the leaves. When high levels of uredineospores buildup, all the leaves of susceptible plants can defoliate. In late summer through fall, black teliospores are produced which remain attached to the under surface of leaves through the winter, or are deposited on the soil when the leaves fall. These "winter" spores can remain dormant in the soil, or germinate in spring to start a new cycle of infection.
In Oregon, P. violaceum has been found on wild Himalaya Blackberry (Rubus fruiticosus aggregate), wild Evergreen Blackberry (Rubus laciniatus) and commercially farmed "Everthornless" Thornless Evergreen Blackberry (Rubus laciniatus sp.). It has not been found on native Rubus or other cultivated varieties.
Before P. violaceum, Himalaya blackberry remained in leaf all year, effectively suppressing the growth of other plant species under a growth of blackberry canopy. One of the results of high rust levels is seasonal defoliation of blackberry. Defoliation, repeated annually, allows much more light to reach the soil below the bramble thicket, enabling seeds to germinate and have a better chance to grow up through the blackberry. This change could be a slow gradual process, unless desirable plants are established early as part of a restoration effort.
Another result of high rust levels has been the reduction in tip rooting (daughter caning), dramatically reducing the competitive advantage that blackberry has enjoyed. Although rust infections can be spectacular in appearance, blackberry is a very vigorous plant, which requires many rust attacks, over a number of years to impact the root system.

Australia's experience, shows that P. violaceum levels are higher and the impact on blackberry greater in areas of high rainfall. Distribution of P. violaceum occurs in 16 out of 18 Western Oregon counties. Based of surveys conducted in 2005 only the two driest western Oregon counties (Jackson, and Josephine) did not have reported or observed rust infections. (K. French 2005)

The accidental introduction of P. violaceum to the Pacific Northwest presents a unique opportunity to control blackberries locally. Where blackberry infestations are heavily impacted by the rust, use the opportunity to establish native or desirable vegetation to replace blighted blackberry patches. Blackberry resistance to P. violaceum is likely to occur. The control of rust resistant plants will slow the replacement of susceptible Himalaya blackberry by resistant biotypes.