Fire blight on raspberries and blackberries

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

While fire blight is most common in pears and apples, it also affects raspberries and blackberries (Rubus spp.). Summer red raspberries cultivars K81-6 and Boyne are particularly susceptible. Losses result from berry necrosis and from tip dieback of primocanes. Fruit losses of 65 percent or more have been reported on thornless blackberries in Illinois.

Symptoms

The most obvious and striking symptom are blackened cane tips, which bend over and die, resulting in a “shepherd’s crook” appearance. Infections may proceed down the cane for up to 8.0 inches and may produce cream-colored bacterial ooze under high moisture conditions. As the disease progresses down the cane, the veins of leaf veins and portions of the leaf surrounding the midvein turn black. Entire leaves may wither and die. Typically, discoloration and dieback is limited to succulent young growth. In addition, the disease can affect fruit clusters. Infected peduncles (the stalks of fruit clusters) turn black and the young developing berries become brown, dry and very hard. Entire fruit clusters may be infected, but generally a few berries in each cluster remain healthy.

Cause

Fire blight is caused by the bacterium Erwinia amylovora. Although this is the same organism that causes fire blight on pear and apple, it is a different strain. Thus the strain that attacks raspberries and blackberries will not infect apple or pear and vice versa. However, it has been found that ‘Boyne’ raspberries can be infected by the apple strain, but this is an exception. The bacteria are likely spread from plant to plant by insects, wind, and splashing water. Rain, high humidity, and warm temperatures favor disease development. It is not known how and where the bacteria overwinter, although they likely survive in cankers on infected canes.

Cultivar resistance

Fire blight affects both red and black raspberries and blackberries. The susceptibility of purple raspberries is unknown. While there has been no thorough study of resistance to fire blight among commercially available cultivars, Latham, Boyne, K81-6 and Fallgold raspberries are known to be susceptible.

Control

No specific control measures have been developed because of the sporadic nature of the disease. However, the following practices will limit establishment and spread of the disease:

  1. Purchase and plant only certified, disease-free plants from reliable nurseries.
  2. Remove and destroy diseased canes from the planting as soon as you see them. Pruning is best done during dry weather to avoid spread of the disease. Disinfest pruning shears in a 10 percent household bleach solution (containing one part bleach and nine parts water) between each cut to avoid transmitting bacteria to healthy canes. Isopropyl alcohol (70 percent) or quaternary ammonia may also be used, but the bleach solution is more effective.
  3. Manage insect pests to avoid a possible means of moving the bacteria from plant to plant.
  4. Avoid over-fertilization. Vigorous, succulent growth is most susceptible to the disease.
  5. Orient rows, prune and thin plants to maximize air circulation. This will help lower the relative humidity within the plant canopy.
  6. Destroy wild or abandoned brambles growing nearby. These plants may serve as inoculum sources for fire blight and other pathogens, particularly viruses.
  7. Apply copper as a preventative material, starting before or as soon as the first symptoms appear.

Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.

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