ANTHRACNOSE OF BRAMBLE FRUITS
John R. Hartman and Donald E. Hershman
Anthracnose, caused by the fungus Elsinoe
veneta, causes severe damage each year to blackberries, to purple and
black raspberries and to a much lesser extent, to red raspberries in Kentucky.
When left to develop unchecked, anthracnose can significantly reduce overall
yields as well as limit the longevity of bramble plantings.
Anthracnose can be present on both
current and second-year canes. Initially, infected canes develop circular,
light gray spots about 1/8 inch in diameter. Later, these spots enlarge,
become sunken, and develop a purplish border with a light gray center.
Spots often grow together to form large cankers, especially on older canes.
Severely infected canes may be girdled or cracked, thus reducing the supply
of water and mineral nutrients to the top. When this situation occurs,
the fruit associated with the diseased canes often ripen abnormally and/or
"dry-up" before maturation. The abnormal berries may have an "off" flavor.
The anthracnose fungus can also cause
similar symptoms to on the leaves. Leaf spots are at first yellowish, turning
to a distinct light gray with a red-purple border. Spots are about 1/16
inch in diameter. Leaf tissue showing anthracnose infection often drops
out, leaving holes in the leaves called "shot holes." Only rarely does
foliar anthracnose result in significant defoliation.
Cane blight and spur blight, two other
potentially serious raspberry diseases, are also caused by fungi which
can produce cane cankers like anthracnose. Although the symptoms can sometimes
be confused these are distinctly different but less common diseases. Control
measures are similar, however.
The anthracnose fungus overwinters
primarily in old, infected canes produced the previous season. Primary
infection in the spring occurs when spores (produced in lesions in these
canes) are spread by splashing rain and wind to susceptible tissue. Once
infection has occurred and the fungus is established, the disease continues
to spread as additional spores are produced in what is known as the secondary
disease cycle. Generally, severe disease is favored by prolonged wet weather
during the growing season.
Since the anthracnose fungus overwinters
in old infected tissue, sanitation is important for effective disease management.
Before setting new plants, cut off and burn to below ground level the stubs
of old canes attached to the young plants. This procedure limits the infection
of new shoots by any anthracnose that may be present in the old canes.
In addition, remove and destroy old fruiting canes after harvest to reduce
overwintering inoculum levels of the fungus.
Fungicide sprays can supplement sanitation
for control of anthracnose. Generally, a delayed dormant application of
liquid lime sulfur (10-12 gal per 100 gal dilute spray) is essential for
good disease control. Although red raspberries are less likely to be seriously
damaged by anthracnose than black or purple varieties, sprays should be
applied if the planting has a history of anthracnose. Completely cover
all canes with the fungicide to receive the spray's maximum benefit. Timing
is also important as sprays applied after new shoots are 3/4 inch long
may "burn" the leaves. This treatment will also aid in controlling spur
blight and cane blight.
In severe disease situations, additional
sprays may be required for acceptable disease control in new canes, even
after harvest. However, in most cases, good disease control early in the
season will reduce the need for later sprays. For a listing of additional
fungicides and usage rates, consult Cooperative Extension publications,
ID-94, "Small Fruit and Grape Spray Guide" or ID-21, "Disease and Insect
Control Program for Home Grown Fruit in Kentucky".