Friend or Foe?:A New Rust of Blackberries

Upper and lower leaf of infected Himalayan blackberry. Photo (ODA)

In the spring of 2005 a very mixed blessing arrived in the Pacific Northwest. The Oregon Department of Agriculture discovered a fungal rust causing significant damage to the scourge, Himalayan blackberry, Rubus discolor. The rust was identified as Phragmidium violaceum, which was not known to occur in North America.

The blackberry rust is native to Europe, North Africa and the Middle East. Various strains of the blackberry rust exist, and each strain can cause different amounts of damage to the weedy Himalayan blackberry. The rust has been used as a biological control agent against weedy blackberry species in Australia, New Zealand, and Chile, and in some cases is very effective at killing blackberry plants.

Blackberry rust has been considered as a potential biological control agent for invasive blackberry species in the Pacific Northwest. However, there is concern about its ability to infect our desired Rubus species, such as commercial evergreen blackberry and related crops. Since its discovery in the Pacific Northwest, the Oregon Department of Agriculture and commercial growers have found the rust infecting weed and crop plants in 14 counties in Oregon and one site in Washington State. It has not been found in the Puget Sound counties, but keep an eye out for it! The disease can spread rapidly in favorable conditions and we will likely have it soon, if it’s not here already.

Identification and Lifecycle: The rust undergoes sexual reproduction in spring via three different spore types. Beginning as early as November, the rust can continue to produce multiple generations throughout the next growing season as “golden summer spores.” Golden summer spores rely on wind to disperse to other plants. New growth is most susceptible to rust infection. The rust over winters on infected leaves as black, sticky teliospores.

Infected leaves have small purple-red spots on the upper surface; while the underside will have yellow-to-creamy-white pustules. You may also see the black teliospores on the leaf’s underside. Other green tissues can be infected, such as unripe berries, flower buds, and stems. As the infection progresses, leaves drop, and the entire cane can become defoliated in severe cases. Other native rusts occur on blackberry but are not as damaging.

Infected evergreen blackberry fruiting buds and flowers. (Photo T. Peerbolt)

Bushes infected with rust still produce edible fruit. Jenny Glass at WSU’s Plant Diagnostic Clinic, does offer the good advice to wash berries off before eating them; it never hurts to do so anyway, given all the other critters that visit blackberry bushes.

Management: Currently federal, state, university researchers and commercial berry growers are testing the susceptibility of different Rubus species, especially the commercial varieties. Blackberry growers are likely to adopt fungicide applications to slow the disease’s progression through susceptible fields. If you have a favorite blackberry bush, regularly prune off infected tissue and dispose of it. Do not let infected leaves drop or stay on the canes.

In other areas of the world, the rust can reduce the foliage by as much as 50% in a five-year period. The inability to hold onto leaves and the vulnerability of new growth can severely inhibit blackberries’ usual talent for prolifically producing daughter plants and new suckers. Some strains of the rust appear to be very effective biological control agents against weedy blackberries.