Taking an environmentally sensitive approach to pest management
Anthracnose is a common disease on blackberry, as well as purple, black and red raspberries. This disease is also known cane spot, although it occurs on flower buds, fruit, leaves, and canes. Anthracnose is caused by the fungus Elsinoe necator (formerly E. veneta). Severe infections can cause defoliation and cane mortality. Infected berries are off-flavored and unmarketable.
Symptoms of anthracnose become visible on primocanes (new canes) in late spring and early summer, especially after periods of rainfall and warm temperatures. Reddish-purple, elliptical spots often occur on the internodes of canes. Over time, the center of the lesion becomes gray with the margin of the spot remaining purple (Figure 1). As lesions mature, they coalesce and eventually encircle the stem, causing cane mortality beyond the affected area. Infected canes have poor winter survival or can also develop malformed lateral shoots on floricanes (two year-old canes) with poor quality fruit.
The first symptoms of anthracnose on leaves are small, purple spots (1/16 inch-diameter) (Figure 2). Later, the center of each lesion becomes gray and eventually drops out, causing a "shot-hole" appearance on leaves. Infected berries have undersized druplets that become brown and shriveled.
The best way to prevent anthracnose is to select a resistant cultivar. Unfortunately, disease resistance of cultivars has not been evaluated for many years. Older reports list Heritage, Willamette, Chilcotin, Nootka, Meeker, and Autumn Bliss red raspberry as anthracnose resistant. However, these most of these cultivars are not well-adapted to areas with hot summers. Heritage, a primocane-fruiting cultivar, can be grown in Missouri with fruit harvested in the fall. Jewell is considered the least susceptible black raspberry to anthracnose, while other black and purple raspberry cultivars are considerably more susceptible.
Little is also known about blackberry resistance to anthracnose, but some of the cultivars released from University of Arkansas are listed as being moderately-resistant, including Natchez, Quachita, Apache, and Osage. Anthracnose has not been observed on Navaho, Arapaho, Caddo, and Ponca in Arkansas breeding plots.
Before establishing a new planting, completely remove any existing wild bramble as they serve as a reservoir for anthracnose. Choose a location for the new planting that will be in full sun and space plants three to four feet apart to enhance air circulation and facilitate plant drying after rainfall. Follow University of Missouri fertilization recommendations to avoid excessive use of nitrogen. Avoid wounding primocanes (new canes) as injured tissue can become a site for infection. When summer pruning blackberries, only remove tips of canes by pinching. Avoid overhead irrigation as it wets the foliage and enhances dispersion of anthracnose spores. Remove any infected canes from the planting and burn them. During winter pruning, thin canes to improve airflow among canes and destroy prunings.
For chemical control, liquid lime sulfur was traditionally used as a delayed dormant spray to prevent anthracnose. However, due to its caustic properties, lime sulfur is no longer available for residential use. Products containing copper hydroxide can be used to prevent disease infection in the home garden when the new growth from buds is less than 3/8 inch-long. For commercial growers, Sulforix (lime sulfur product) and copper hydroxide are labeled for use on brambles.
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REVISED: May 6, 2020