Spur blight is caused by the fungus Didymella applanata. Spur blight
occurs only on red and purple raspberries. Spur blight has been
considered to be a serious disease of red raspberry; however, recent
studies in Scotland suggest that spur blight actually does little
damage to the cane. The extent of damage caused by spur blight in the
United States is not clearly understood. The spur blight fungus has
been reported to reduce yields in several ways. It can blight the
fruit bearing spurs that are produced on the side branches, cause
premature leaf drop, and kill buds on the canes that later develop
into fruit bearing side branches. In addition, berries produced on
diseased canes may be dry, small and seedy.
Symptoms
The symptoms first appear on young first-year primocanes in late
spring or early summer. Purple to brown areas (lesions) appear just
below the leaf or bud, usually on the lower portion of the stem.
These lesions expand, sometimes covering all the area between two
leaves. In late summer or early fall, bark in the affected area
splits lengthwise and small black specks, which are fungal fruiting
bodies (pycnidia) appear in the lesions. They are followed shortly by
many slightly larger, black, erupting spots; another form of fungal
fruiting body (perithecia). Leaflets sometimes become infected and
show brown, wedge-shaped diseased areas, with the widest portion of
the wedge toward the tip of the leaf. Infected leaflets may fall off,
leaving only petioles without leaf blades attached to the cane. When
diseased canes become fruiting floricanes during the next season, the
side branches growing from diseased buds are often weak and withered.
Causal Organism
Spur blight is caused by the fungus, Didymella applanata. It survives
the winter in lesions on diseased canes. The following spring and
summer, during wet and rainy periods, spores are released and carried
by splashing rain and wind to nearby primocanes. There they germinate
in the presence of water and produce new infections, where the fungus
will again over winter.
Control
All steps possible should be taken to improve air circulation within a
planting, to allow faster drying of foliage and canes. Reducing the
number and duration of wet periods should reduce the potential for
infection. Excessive applications of fertilizer (especially nitrogen)
should be avoided, since it promotes excessive growth of very
susceptible succulent plant tissue. Plants should be maintained in
narrow rows and thinned to improve air circulation and allow better
light penetration. Weeds are very effective in reducing air movement;
therefore, good weed control within and between rows is important for
improving air circulation within the planting.
Wild brambles, especially wild red raspberries, growing in the area
should be removed. They can serve as a reservoir for the disease.
After harvest, remove and destroy all old fruited floricanes and any
first-year primocanes that are infected. It is important that old
canes be removed before new canes emerge in the spring. If spur
blight becomes an important problem in the planting, growers may want
to consider the use of fungicide. Special fungicide sprays
specifically for control of spur blight are generally not warranted.
For the most current spray recommendations, commercial growers are
referred to Bulletin 506-B2 "Ohio Commercial Small Fruit Spray
Guide," and backyard growers are referred to Bulletin 780
"Controlling Diseases and Insects in Home Fruit Plantings."
These publications can be obtained from your county extension agent
or the Extension Publications Office, The Ohio State University, 385
Kottman Hall, 2021 Coffey Road, Columbus, Ohio 43210-1044.
-----------------
All educational programs conducted by Ohio State University Extension
are available to clientele on a nondiscriminatory basis without regard
to race, color, creed, religion, sexual orientation, national origin,
gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and
Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868
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