Tuesday, August 23, 2011

Blueberry Stem Canker

Blueberry stem canker caused by the fungus Botryosphaeria corticis is common on cultivated and wild blueberries in North Carolina.  The disease spreads by spores and through the use of infected cutting wood for propagation.  Canker is important because it can weaken and kill susceptible or stressed bushes.

'O'Neal'  canes dying out due to canker infections

Symptoms: Cankers are noticeably thicker than adjacent portions of the stem, forming raised areas with deep cracks.  Usually only short sections of the stem are affected, though canes of some very susceptible cultivars can have multiple infection points along their entire length.

Swollen area (at center) with deep cracks is a fully developed canker

Disease cycle:  Infection of stems is by spores (ascospores and conidia) that are released during wet weather and disseminated by wind from April through September.  Young, succulent, actively growing shoots are infected, and symptoms appear within 4 to 6 months after infection.  As the fungus continues to grow and invade the wood, large cankers with deep fissures and cracks develop that girdle, weaken and kill the stem.

Cankers on a young stem, exuding spores

Control:  Fungicides are partially effective but not practical. Control relies on maintaining general plant health,
pruning to remove canker-weakened canes,use of resistant cultivars, and use of disease-free planting stock.  Cutting wood should be selected from undiseased plants, or the plants sourced from disease-free nurseries.  In areas where canker is not present, the use of disease-free planting stock is critical to avoid introducing this pathogen to an new field.

Highbush blueberry:  Among the popular southern highbush cultivars (Vaccinium corymbosum) grown in eastern NC, O'Neal, Blueridge and Legacy are susceptible -- however all three are successfully grown where plant health is maintained via proper site selection, fertility, pruning and irrigation.

Cankers on a one-year-old 'Legacy' stem

Rabbiteye blueberry: Cankers can be numerous on rabbiteye blueberries (Vaccinium virgatum, synonym V. ashei) but generally do not cause yield loss on this vigorous blueberry species.