Stem Blight of Blueberry
Fruit Disease Information Note 9
W.O. Cline, Extension Plant Pathologist
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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.
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.
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.
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 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.
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.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.
Published by the North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Web page last updated on Oct. 2000 by A.V. Lemay.