Verticillium wilt is one of the most serious diseases of raspberry.
This disease is caused by a soil-borne fungus and reduces raspberry
yields by wilting, stunting, and eventually killing the fruiting cane
or the entire plant. The disease is usually more severe in black than
in red raspberries. Blackberries are also susceptible to the disease,
but seldom suffer severe losses. Verticillium wilt is usually a
cool-weather disease and is most severe in poorly drained soils and
following cold, wet springs. The appearance of symptoms on new canes
frequently coincides with water stress caused by hot, dry mid-summer
weather.
Symptoms
The symptoms usually appear on black raspberries in June or early
July, and on red raspberries about a month later. The lower leaves of
diseased plants may at first appear to have a dull green cast as
compared to the bright green of normal leaves. Starting at the base
of the cane and progressing upward, leaves wilt, turn yellow and
drop. Eventually, the cane may be completely defoliated except for a
few leaves at the top. Black raspberry canes may exhibit a blue or
purple streak from the soil line extending upward to varying heights.
This streak is often not present or difficult to detect on red
raspberries. The final effects of the disease are observed on
fruiting canes that were infected the year before. In the spring,
many of the diseased canes will be dead. Others will be poorly
developed and have shriveled buds. The new leaves are usually yellow
and stunted. Infected canes may die before fruit matures, resulting
in withered, small, and tasteless fruit.
Casual Organism
Verticillium wilt is caused by the fungus, Verticillium albo-atrum. It
is a very common soilborne fungus, and has been reported to cause
wilt on more than 160 different kinds of plants including
strawberries, eggplant, tomatoes, potatoes, stone fruits and peppers.
The Verticillium fungus overwinters in the soil and plant debris as
dormant mycelium or black, speck-sized bodies called microsclerotia.
The fungus can survive in the soil for many years. When conditions
are favorable, the microsclerotia germinate and produce threadlike
fungus filaments (hyphae). These hyphae can penetrate the root
directly; but invasion is aided by breaks or wounds in the roots.
Once inside the root, the fungus grows into the water-conducting
tissue (xylem). The destruction of water-conducting tissues prevents
the movement of water from the roots to the rest of the plant; thus,
the plant eventually dies. The fungus produces microslerotia in
infected tissues. When these tissues die and are returned to the
soil, the disease cycle is completed.
Control
Applications of fungicides are ineffective in control. Soil fumigation
has provided excellent control in some locations but is generally
very expensive. Reintroduction of the pathogen into fumigated soils,
accompanied by a rapid buildup in pathogen populations, is a major
concern with using soil fumigation. Rotations (3- to 4-year) with
nonsusceptible crops have been recommended for control in Canada but
were not effective in California.
Only disease-free nursery stock from fields known to be free of
Verticillium should be used to establish new plantings. Satisfactory
resistance in commercial raspberry cultivars is not available. It is
generally recommended that raspberries not be replanted in an area
where the disease has been a problem. If they are replanted in an
infested site, soil fumigation should be considered.
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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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