Purple blotch of caneberries—Septocyta ruborum =Rhabdospora ramealis
Purple blotch causes lesions and girdling on the canes of blackberry, blackberry hybrids, and occasionally other caneberries. It is a serious disease in the wetter areas of Oregon and Washington but is an uncommon problem in California where the weather is mostly dry during the growing season.
Identification
Dark green, irregularly shaped lesions form late in the growing season on canes, mostly on blackberries and their hybrids. In winter and spring the lesions become brown, purple, reddish, or a combination of these colors. Affected areas are commonly 1/2 to 2 inches long and develop into cankers that can girdle the cane. Mature lesions become pale brown to tan and develop small, black fungal fruiting bodies (pycnidia) in the spring. Purple blotch does not cause leaf spots.
Lookalikes. Anthracnose (Elsinoe veneta =Sphaceloma necator), cane and leaf rust (Kuehneola uredinis), and Septoria leaf spot (Mycosphaerella rubi =Septoria rubi) also cause cane lesions and dieback of caneberries. However, unlike purple blotch, these other fungal diseases cause obvious, discolored spots on leaves.
Cane blight (Paraconiothyrium fuckelii) also causes red to purplish lesions on canes. It does not produce leaf symptoms until foliage dies after canes are killed. Cane blight is a more likely cause of disease because purple blotch is uncommon in California. Properly collected samples may need to be submitted to a plant pathogenic diagnostic laboratory to confidently distinguish the cause of cane death.
Life cycle
The purple blotch fungus overwinters and persists in infected canes. In spring during rainy weather, and also during summer if infected canes are wetted by sprinklers, pycnidia form on floricanes (second-year or older canes). When wetted, the pycnidia exude a tiny pale tendril of spores about 1/25 inch (1 mm) long. The spores are spread by splashing water and infect primocanes (first-year canes). Stomata (natural openings for gas exchange) are the primary location of infections. Cuts or deep wounds in canes can also become infected.
Damage
The purple blotch fungus causes discolored lesions and cankers on canes. The lesions can grow and completely girdle canes, killing them. Severely affected canes generally die in the spring.
Solutions
Where purple blotch has been a problem, consider growing less susceptible cultivars, such as such as boysenberry or loganberry. Grow caneberries in macrotunnels (hoop houses covered with clear plastic); this prevents free moisture from forming on the plants and reduces or eliminates some cane and leaf diseases of caneberries.
Adequately space plants, grow caneberries on a raised berm or mound of soil, control weeds, and prune primocanes early in the growing season to thin plants and maintain open rows. These practices improve air circulation, which can reduce the incidence of purple botch and some other diseases of caneberries.
Avoid using overhead sprinklers. Use drip irrigation. Direct any hand-watering to soil around the base of plants and keep the canes dry.
Trellis canes by early September to reduce winter disease and injury that causes wounds that become the sites of pathogen infection. Use an alternate-year fruiting program, where canes are trained up as they grow. After harvest and before fall rains, prune out and destroy old wood. Do not allow old canes to lie on the ground overwinter or this will worsen purple blotch disease.
Applying Bordeaux mixture or another copper fungicide may provide control. Spray after harvest and before fall rains begin. Spray again in spring at budbreak. Fungicide will not eliminate existing infections but can prevent new infections from developing.
Adapted from Blackberry (Rubus spp.)-Purple Blotch, Pacific Northwest Plant Disease Management Handbook and California Pest Rating Proposal for Septocyta ruborum (Lib.) Petrak. 1967 (syn. Rhabdospora ramealis (Roberge ex Desm.) Sacc.) (PDF), California Department of Food and Agriculture. |
