Phytophthora Root Rot (Phytophthora cinnamomi)

     This disease is usually associated with poorly-drained areas of a field. The fungus thrives in wet (saturated) soils and survives for long periods of time in soil. Symptoms are observed on the roots and on the above-ground portions of the plant. The very fine absorbing roots turn brown to black; larger diameter roots may also be discolored. In severely infected bushes, the entire root system is reduced and totally black. Above-ground symptoms include chlorosis and reddening of the leaves, small leaves, defoliation, branch dieback, death of entire canes, stunting, and death of the entire bush. The disease may be present in a few infected plants scattered throughout the planting or localized in a group of plants in a low lying area of the field. The disease is most severe where plants are growing in heavy clay soils.

     Phytophthora cinnamomi, in addition to attacking blueberry, attacks a number of additional susceptible Ericaceous hosts, including rhododendron, azalea, and cranberry. Lowbush blueberry appears to be immune. This species of Phytophthora is not an important pathogen on any other small fruit covered in this guide.

Management

The disease is avoided through careful site selection before planting. Heavy soil which becomes waterlogged or has a high water table should be avoided. Internal and surface water drainage should be improved before planting. Plants can be grown on raised beds if desired. Manage irrigation to avoid prolonged periods of saturated soil. Most cultivars are susceptible to the disease, although some cultivars may better tolerate some degree of infection better than others. "Bluecrop" and "Weymouth" are two cultivars which have shown promise.

Bacterial Crown Gall (Agrobacterium tumefaciens)

This disease is caused by the bacterium Agrobacterium tumefaciens. Since blueberries are grown on acid soils and the crown gall bacterium does not grow well in an acid environment, the disease is uncommon. Globose, pea-size to large galls occur on low branches, twigs, and at the base of canes near the ground. Injured tissue is more likely to contain galls.

Management

Sanitation, purchasing healthy nursery plants, and maintaining proper soil conditions (pH 4.5 to 5.2) are the most reliable controls. A closely related bacterium, Agrobacterium radiobacter, produces an antibiotic called Agrocin and is available as a biological control agent for use either as a soil treatment or for dipping the root system of bushes prior to planting. This control measure is not recommended unless the planting has a history of crown gall problems.