Blackberry plants that look normal and healthy may sometimes flower
profusely but fail to set fruit. This failure may be complete, with
no fruit set at all, but more often it may be partial, with the
production of misshapen berries. The appearance of such berries
may range from nearly normal to some with only a single drupelet.
The condition may be the result of infection by a virus or fungus,
insect damage, hereditary abnormalities, or a combination of these
Failure to set fruit in blackberries is a symptom of one or more
virus diseases that affect the entire plant. Diseased plants produce
new canes that are more vigorous, with rounded and glossier leaflets
than normal. The leaves also develop a brilliant, premature reddening
in the fall. Although flowers appear to be normal, they only produce
a few drupelets per receptacle (fruit). The production of fruit
buds for the next season is also reduced. This disease does spread
in a planting, but its means of transmission (other than by root
suckers) is undetermined.
There are no chemicals to control virus-induced blackberry sterility.
The following measures are suggested:
1. Purchase only virus-free plants from nursery producers
who will certify that their plants were produced from fruitful stock.
Do not use root suckers to propagate plants from fields or
gardens where the sterility virus has previously been found.
2. Where possible, destroy neglected plantings and wild
bramble patches within 500 feet (152.4 meters) of new plantings.
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Erect blackberry, infected with sterility virus.
Anthracnose, a common fungus disease also affects fruit development.
This disease is seldom severe on the fruit of erect blackberries, but
is often serious on fruit of trailing blackberries. When immature drupelets
are infected, ripening is prevented. Infected fruit is small, brown, dry,
and woody. Drupelets that are attacked when they are more mature become
brown and shrunken.
Anthracnose and other less common fungi that may be injurious to the
fruit are generally controlled by following a regular, pest control schedule
and by removal of fruiting by pruning canes immediately after harvest.
These canes die after fruiting and should be destroyed as soon after harvest
as possible to minimize the spread of any diseases present to the new
Mites, thrips, tarnished plant bugs, and adult beetles of the raspberry
fruit worm may sometimes cause fruit malformation. These insects feed
on the flower buds, stamens, or pistils. Extensive damage from mites and
insects is not common. Special sprays to control them are generally not
warranted, particularly when the regular pest-control schedule is followed.
Commercial growers should contact their nearest Extension office or the
Dixon Springs Ag Center, RR 1, Box 251, Simpson, IL 62985, for the latest
pest-control recommendations as given in the current Proceedings of
the Illinois Small Fruit and Strawberry Schools.
Poor fruit set can also be the result of gene or chromosome combinations
that do not permit effective self-pollination. Plants and flowers look
normal, but the pollen produced does not fertilize the ovules. This process
is necessary for normal fruit development. Commercially important cultivars
are generally self-fruitful and usually are planted in large blocks without
concern about this type of sterility.
Sometimes plantings in which a few or many plants produce little or no
marketable fruit are simply mixtures of wild blackberries. Such plants
should be removed and destroyed as soon as they are identified since they
are usually more vigorous than the productive plants and tend to replace
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