STEM CANKER OF BLUEBERRY

 

Stem canker caused by Botryosphaeria corticis is a major factor limiting the production of blueberries in the southeastern United States. It occurs on cultivated and wild highbush blueberries and on the wild rabbiteye blueberry in North Carolina, Georgia, Florida, Alabama, and Mississippi. The use of infected wood for propagation has contributed to the increased importance of this disease. Disease incidence is often so great that new plantings are severely reduced in productivity or entirely destroyed before they come into full production.

Illustrations:

  • Early stages of canker infection
  • Close-up of cankers
  • Mature canes with canker

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    Symptoms


    Mature cankers on older stems are readily identified by the pronounced swelling caused at the point of infection. Cankers are noticeably thicker than adjacent areas of the stem and have deep cracks running lengthwise with the stem. Some very susceptible cultivars such as Wolcott and O'Neal can have stems that are cankered along their entire length (2-3 feet) when multiple cankers form close together on the stem. Spore-producing fungal structures (perithecia and pycnidia) emerge through the bark and appear on the cankers as patches of black specks. On one-year-old canes of the sort used for hardwood cuttings, canker infections can be separated from other spots or lesions on the wood within a few months after infection because only canker infections produce raised, round lesions, often with longitudinal cracks in them.

     

    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. The young and actively growing shoots are more susceptible to infection than are the older stems. The infection process, spore germination, penetration, and establishment of B. corticis in blueberry stems occurs rapidly; however, the development of the visible canker is a relatively slow process. Canker symptoms first appear as small, red lesions on succulent shoots. Stem lesions slowly enlarge and become swollen 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 on susceptible cultivars and the stem finally becomes girdled, weakened, and eventually dies. Cankers are scattered and restricted in size or absent on the more resistant cultivars. The stem canker fungus, unlike stem blight,attacks only blueberry (Vaccinium) species. The canker fungus has developed races in response to the use of canker-resistant cultivars. These races have greatly complicated efforts to maintain resistant cultivars.

     

    Control


    Control of canker relies on sanitation, avoidance and the development and use of resistant cultivars. In general, the use of fungicides to control stem canker has proven to be ineffective.

    Sanitation consists of collecting cutting wood from canker-free mother plants and planting only canker-free cuttings. The use of softwood cuttings appears to greatly reduce transmission of the disease via propagation. In established plantings, selective pruning to remove visible cankers can greatly reduce the number of cankers present and will extend the life of infected bushes.

    Avoidance can be accomplished by planting canker-free plants in an area where there are no adjacent blueberry plantings or where no canker has been observed. Even canker-susceptible cultivars can be grown successfully in areas isolated from the disease.

    Resistance development is an ongoing process. North Carolina has an active canker screening program and has released a number of cultivars that are resistant to one or more races of the stem canker fungus. Among highbush cultivars (Vaccinium corymbosum), the cultivar Croatan is resistant to canker races 1 and 6, and this cultivar accounts for 60% of the highbush blueberry acreage in NC. No serious canker problems have been observed on Croatan. The cultivar Murphy is resistant to all races of canker except race 8, a new canker race identified at a single site in Bladen County in 1984. The planting where race 8 was discovered has been destroyed.

    Among recently released highbush cultivars, only O'Neal and Cape Fear have exhibited serious canker problems. Growers who insist on planting O'Neal must rely on avoidance and the use of clean propagating stock in order to produce this cultivar successfully. The cultivar Cape Fear is no longer recommended for planting due to problems with soft berries, a disorder unrelated to its canker susceptibility.

    Cankers have been observed on rabbiteye blueberries (Vaccinium ashei) in NC and in Georgia, but no significant loss of productivity has been associated with the disease on this blueberry species.

    Last updated: 27 May 1997
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