Blackberry Rust
(Phragmidium violaceum)
Diseases
Symptoms
Blackberry Rust, Phragmidium violaceum, is a defoliating disease found on ‘Evergreen’ blackberries, which can result in significant crop loss. Other cultivars of blackberry can be infected, although none of these are as susceptible. Foliar symptoms include circular purplish leaf spots (1/8 inch) with yellow to tan centers on the upper leaf surface accompanied by cream to yellowish-orange pustules (aecia and uredinia) on the lower leaf surface. Upper leaf symptoms may resemble Septoria leaf spot while lower leaf symptoms may resemble cane and leaf rust. Yellowish-orange pustules may also form on the flower buds, fruit, and canes of infected plants. Severely infected leaves turn brown, shrivel and fall from the canes. Infected flowers and fruit may fail to ripen. Severe infections may result in complete defoliation of entire canes. In late summer/ early fall, black pustules (telia) will develop among the yellow spores (uredinia) on infected leaves. Cane infections are rare even under severe conditions.
Life History
This rust undergoes sexual reproduction in spring via three different spore types. It overwinters as black, sticky pustules (telia) on infected leaves attached to canes or trapped in cane bundles, which germinate in the spring and infect leaves of primocanes and floricanes. These primary infections produce purple upper leaf spots (aecia), which sporulate and also infect leaves. Yellow leaf pustules (uredinia) will begin to develop on the underside of leaves directly below the purple leaf spots. When these pustules sporulate, they can infect flower buds, leaves, and stems of primocanes and floricanes. New uredinia (yellow spores) are produced from these infections and continue the epidemic, building up disease in rapid, successive cycles. Telia (black pustules) develop among the uredinia in late summer/early fall on infected leaves. Old floricanes can be a significant source of spores for fall infection of primocanes in the Pacific Northwest.
Prolonged leaf wetness (>6 hours) with mean daily temperatures between 49° and 64°F favor disease development. Yellow summer spores, found only on the lower surface of blackberry leaves, rely on wind to disperse to other plants. New growth is most susceptible to infection.
Monitoring
‘Evergreen’ blackberries should be scouted regularly for signs of infection. Conduct a visual search within 3-5 areas of your field (depending on field size) for symptoms associated with blackberry rust. Evaluate 10-20 plants spaced 3-5 hills apart at each site. Record severity on a scale of 0-3 for each site scouted.
During the dormant season, look for black overwintering spores (telia) on leaves attached to canes or trapped in cane bundles. Early growing season infections can be indentified by circular purple leaf spots (aecia). As the season progresses, yellow pustules (uredinia) will develop on the lower leaf surface. Minor infections will show on leaves closer to the cane, while more severe infections will move out toward the tip.
Thresholds and Management
No threshold or tolerance level available.
Cultural tactics and dormant season lime sulfur will help reduce initial inoculum sources while chemical tactics will focus on protection of young, healthy plant tissues. Field renovation practices that destroy previously diseased plant parts may contribute to the effectiveness of a chemical control program the following spring. Fruiting canes should be removed from the field as early as possible to prevent development of more spores. Cultivate in late fall or early spring prior to new leaf development to cover fallen leaves and reduce inoculum sources.
Chemical protection starts at bud break, which usually occurs the second or third week of April, and continues through green fruit development or until the weather turns from rainy springs to dry summers. Fungicides are used as a protectant and will not eradicate the disease after it has been established.
Resources
Cornell University, Cornell Fruit Resources: New blackberry rust disease may spread across the U.S.
http://www.fruit.cornell.edu/Berries/newrust.htmlNorthwest Integrated Pest Management, Blackberry Growers Alert: Information on Phragmidium Rust Disease
http://www.nwipm.info/blkrust-05.aspOregon State University Extension, Plant Disease Control: Blackberry Rust
http://plant-disease.ippc.orst.edu/ShowDisease.aspx?RecordID=1476Washington State University Extension, Gardening Friends and Foes: A New Rust of Blackberries
http://whatcom.wsu.edu/ag/homehort/pest/P_violaceum.htm
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