W.O. Cline, Extension Plant Pathology
The stem blight fungus causes a rapid wilt with browning or reddening of leaves on individual branches, often followed by death of the entire plant as the fungus spreads downward through vascular tissue to the base of the plant. Most infections can be traced to a wound as the initial point of infection. In the field, the most obvious symptom is called 'flagging'; stems recently killed by the fungus do not drop their leaves, resulting in a brown-leafed 'flag' which stands out against the green healthy portions of the bush. Further diagnosis can be accomplished by removing a wilting stem that has both dead and healthy portions and splitting it longitudinally. A stem blight-infected stem will have a uniform, light brown discoloration in the wood extending down the infected side of the stem.
In a normal year, stem blight infections become evident in June, soon after harvest in southeastern NC. New infections can be observed throughout the summer months. Infections are usually associated with cold injury on succulent shoots that were damaged near the end of the previous growing season. Infections can also occur via other wounds such as mechanical, chemical or insect damage. Infected stems quickly wilt and die. The infection can also develop in wounds at the base (crown) of the bush in susceptible cultivars, resulting in rapid plant death without the typical flagging symptom associated with infections on individual stems. Below-zero temperatures (-0 degrees F) have also been observed to cause cracking in the forks of blueberry stems, which has resulted in wound-related epidemics in March and April.
Pruning to remove infected stems is the best method of reducing disease in established fields. Pruning serves two control functions: 1) It removes infections from bushes, preventing eventual death of the individual stem or plant, and 2) it reduces the number of spores released in the field by removing dead, spore-bearing stems. Pruning can be done anytime infected stems are observed, but care should be taken to cut well below the infected area. After a stem is removed, examine the cut end of the remaining stem. If any brown areas are visible in this cross-section, the cut must be made again further down the stem until all infected tissue is removed. Otherwise, the disease will remain in the stem and continue on down to the crown, possibly killing the plant. Infected stems should be removed well away from the field and burned or shredded.
Fertilizer management is necessary to prevent formation of succulent shoots late in the growing season. Infection of cold-injured shoots around the base of the bush is a primary means by which this fungus enters blueberry plants. Fertilizer should not be used after mid-summer, especially on young bushes. This will allow bushes to enter a natural dormancy and will reduce the chance of fall cold injury. On soils with a high organic content (>5%), new plantings can be established without the use of fertilizer.
Cultivar resistance is available and should be a primary consideration in the establishment of new plantings; remember that young bushes are the most susceptible. Cultivars known to be very susceptible to stem blight should be avoided in areas where stem blight has been a problem. Bluechip and Bounty are the most susceptible cultivars. Croatan, Reveille, Bladen, Duke and the rabbiteye cultivars Premier and Powderblue are considered susceptible, but have been grown with losses averaging less than 10-20%. Once established (3-4 yr), these cultivars tend to survive fairly well, unlike Bluechip and Bounty. The most resistant cultivars, O'Neal and Legacy have only rarely been observed to die due to this disease, although they will become infected on occasion.
Site selection appears to play a part in the severity of stem blight. The worst cases of stem blight in commercial fields occur on drought-stressed bushes in soils that are either 1) extremely sandy, resulting in poor growth and chronic drought stress, or 2) on the black, heavy muck soils that promote excessive growth.
Avoid wounding bushes unnecessarily. Since stem blight is most damaging to young plantings, heavy pruning to promote rapid growth should be avoided in 1- to 2-yr-old plantings; pruning in young plantings should be limited to removal of stem blight -infected canes. Another wounding phenomenon which occurs in new fields is caused by termites. In recently cleared fields where old stumps, trunks and branches have been left buried in the field, termites have been observed to wound and even kill new bushes. This can be avoided by thorough field preparation prior to planting.
South Region Small Fruit ConsortiumNC Ag Chemicals Manual Horticulture Information Leaflets (HIL) Home Page HIL-201 Suggestions for Establishing a Blueberry Planting in Western North Carolina HIL-201-B Principles of Pruning the Highbush Blueberry HIL-201-E Blueberry Freeze Damage and Protection MeasuresNorth Carolina Insect NotesNCCES Educational Resources
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.**December 2007