| Blackberry -- Blackberry Rust | |||||||||
| Cause:
Phragmidium violaceum, a macrocyclic, autoecious rust fungus that is a problem on cultivated Evergreen and wild Himalaya blackberries. Although other cultivars of blackberry can be infected, none appear as susceptible as evergreen blackberry which can result in significant crop loss. Among wild Himalaya blackberries there appears to be a range of sensitivity with some clones highly susceptible while other nearby plants seem unaffected. Although the disease was first recorded in 2005, growers and ranchers of western Oregon observed this problem in 2004. The rust has been found in all west coast states.
The fungus overwinters as telia principally on leaves that remain attached to canes and/or those trapped in cane bundles. Teliospores germinate in the spring and produce basidiospores that can infect leaves of primocanes and floricanes. These primary infections pro-duce aecia, which produce aeciaospores, which also infect leaves. These infections produce uredinia, which produce urediniaspores that infect the flower buds and leaves and stems of the primocanes and floricanes. New uredinia are produced from these infections and continue the epidemic, building up disease in rapid, successive cycles. Black pustules (telia) develop among the uredinia in the late summer/early fall on infected leaves. Old floral canes can be a significant source of spores for fall infection of primocanes in the Pacific Northwest. Wet conditions (> 6 hours of leaf wetness) with mean daily temperatures between 49 and 64°F favor disease devel-opment. Spores are easily spread by the wind; young leaves at 3/4 expansion are most susceptible to infection. |
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| Symptoms: Foliar symptoms include circular purplish leaf spots 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. The upper leaf symptoms may resemble Septoria leaf spot while the 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. Cane infections on evergreen blackberries were rare even under severe conditions. Severely infected leaves may curl and dehydrate. Severely infected flowers and fruit fail to ripen. Continuous defoliation as well as stem infections by the pathogen may cause dieback of the canes. Black pustules (telia) develop among the uredinia in the late summer/early fall on infected leaves. | |||||||||
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Cultural
control: 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.
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Chemical
control: Chemical protection needs to start in winter with applications of lime sulfur targeted to infected leaves remaining on canes or trapped in cane bundles. Fungicide protection should begin in late April or early May and continue through flowering in June or until the weather turns from the Pacific Northwest’s typical rainy springs to its typical dry summers. The rust reactivates in fall, and consequently, a fungicide application near mid-September may be beneficial to plant health. Overuse of any single product may encourage the development of fungal resistance. All of these fungicides are primarily protectants and cannot eradicate the disease after it has become established.
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References: Osterbauer, N., Trippe, A., French, K., Butler, T. Aime, M. C., McKemy, J., Bruckart, W. L., Peerbolt, T., and Kaufman, D. 2005. First report of Phragmidium violaceum infecting Himalaya and evergreen blackberries in North America. Online. Plant Health Progress doi:10.1094/PHP-2005-0923-01-BR. |
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| Content edited by: Pscheidt, Mahaffee, Osterbauer on January 1, 2009 | |||||||||
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