Stem Blight of Blueberry

Fruit Disease Information Note 9

W.O. Cline, Extension Plant Pathologist




[General Information] [Symptoms] [Disease Cycle] [Control]
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General Information

Blueberry stem blight, caused by the fungus Botryosphaeria dothidea, is the primary disease limiting establishment of blueberry plantings in southeastern NC. Both highbush and rabbiteye cultivars are susceptible to this disease, which enters the plant through wounds and causes rapid death of individual canes and entire bushes. The disease is especially severe on 1- and 2-yr-old plantings of susceptible cultivars.

Symptoms

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 a wound caused by mechanical damage or insect damage, or can be related to late-season cold injury on succulent shoots that occurred during the previous growing season. 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.

Disease Cycle

This fungus overwinters in dead and infected stems. The disease also occurs on many other wild and cultivated plant species (including alder, holly, wax myrtle, blackberry and willow) which contributes to the survival and spread of the disease.

Spores are carried by wind and rain from infected stems to wounds on healthy plants. These spores germinate and invade the vascular tissue of the host, causing a pecan-brown discoloration which extends up and down the stem from the infection point, eventually killing the stem. Stems killed by blight eventually drop their leaves after a few weeks and turn dark brown to black in color; these dead infected stems are noticeably darker than stems dying due to other causes. Fungal fruiting bodies are produced all along the stem just under the surface, and spores are released which spread to wounds on adjacent bushes. These spores are released year-round with the exception of a few weeks in winter; however, the greatest numbers of infections occur in early summer.

Control

Control of this disease depends on cultural methods; fungicidal chemicals do not provide adequate protection.

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 cut off, 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 prunings should be removed well away from the field and burned or shredded.

Fertilizer management is neccessary 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 which are 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, Harrison, Bladen, 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 are Murphy, O'Neal and Cape Fear, which 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 soils which are extremely sandy, resulting in poor growth, or 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.

Other Resources

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.

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Web page last updated on Oct. 2000 by A.V. Lemay.