Pests & Diseases Affecting the Blackberry Plant
The blackberry is a plant native to temperate climates and is relatively free of troubles. In the list that follows the ones most likely to appear are blackberry mite (redberry disease), rust and blackberry dwarf (rubus stunt).
Blackberry mite: these pests are the cause of what is commonly called ‘redberry disease’; hybrid berries, such as loganberries, are also vulnerable. The tiny mites spend winters beneath the bud scales on the canes; once the leaves have opened, the mites form colonics on the undersides, where they feed and breed. The flowers are also attacked, and fruits of infested plants will be misshapen, and unevenly ripened. Some of the drupelets never colour properly, but remain bright red.
The best precaution is to burn old canes as soon as they have finished fruiting, so the mites will be unable to overwinter in the garden. If you have had trouble with blackberry mite in previous years, apply a lime sulphur wash to the new canes at bud break.
Raspberry beetle: although raspberries are usually the main victims of this pest, blackberries are sometimes affected. The female beetle, which is light brown, and 0.4 cm (1/6”) long, lays her eggs in the open flowers in late spring and early summer. The maggots which emerge from the eggs feed on the developing fruit, which may become brown and hard. In a late attack, the maggots feed on the plug, and it is then that the maggots are not noticed until the berries have been picked and are on the table.
If you have had problems with raspberry beetle in previous years, spray with derris about fourteen days after flowering has for the most part finished, usually about the beginning of mid-summer. The young fruits should be forming at this time.
Aphids: there are several species of aphid which infest blackberries, but the symptoms and control of all of them are very similar. Besides physically weakening the plants, they transmit viral diseases and, for this reason, every effort must be made to eliminate these pests from your garden.
They are most damaging in late spring and early summer; they suck the sap from the leaves, which then become curled and stunted. If you have had serious problems with aphids, apply a mid-winter spray of tar-oil wash to kill any overwintering eggs. Otherwise, if they do appear in spring, spray with derris, resmethrin or malathion before the flowers open, and repeat if necessary. Avoid spraying the plants while they are in full flower; remember to allow the specified time to elapse between spraying and harvesting.
Capsids: in some years these insects are very damaging, and other years they are hardly noticeable. The tips of young canes are most vulnerable to capsid infestation; badly attacked tips may stop growing altogether. Leaves are injured, and develop puckered brown spots and tiny holes, and eventually they become very tattered. Spray with dimethoate or malathion before the flowers open. Because capsids will drop to the ground if disturbed, spray theround the plants as well.
Cane spot: this fungal infection attacks raspberries and loganberries, as well as blackberries. The most characteristic symptom is the appearance of small, round, purple spots on the canes in late spring or early summer. If the fruits are infected, they will be spotted or misshapen, and infected leaves will be covered with pale spots which are dark around the edges. Eventually, the spots on the canes grow larger and become elongated and change to pale grey in the centre. In the final stages, the spots become sunken and cankered, with cracking in the centre. Cut out and burn badly infected canes, and spray the remainder with Bordeaux mixture. If you have had trouble with cane spot in the past, spray with a 5”, lime-sulphur mixture, or benomyl, at bud-burst, and repeat just before blossoming.
Blue stripe wilt: this is a fungal infection which occasionally attacks the roots of the blackberry plant, but is more commonly found on raspberries. It prevents the sap from flowing through the plant normally, the main symptoms appearing in early to mid-summer. The leaves of infected plants become striped yellow, and these stripes eventually turn dark brown, the leaves withering and dying. If the infection really takes a hold, the canes become marked longitudinally with a bluish stripe, and eventually they die as well. There is no known chemical control for blue stripe wilt; dig up and destroy infected plants, so that the disease does not spread, and put fresh plants in a different site.
Botrytis cinerea (grey mould): blackberries growing in very cold, wet conditions are most vulnerable to this fungal infection. Berries with botrytis will be covered with grey, fluffy mould, and the canes can also be infected, and even be killed completely in severe cases. To avoid overcrowded conditions, plant the canes at the spacing recommended for the particular variety, and prune annually. If botrytis does appear, cut off and burn the infected berries and canes as soon as you notice them. If you have had severe trouble in the past, spray with benomyl or captan when the first flowers open and again about two weeks later; do not spray if the fruit is to be used for preserving in any way.
Crown gall: this bacterial infection attacks a wide range of plants;, peach, apple, plum and pear trees can be infected, as well as raspberries, loganberries and blackberries. Plants growing in wet soils or damp conditions are most vulnerable. The main symptoms are round, irregular-shaped swellings on the roots, although sometimes galls will appear on the canes. The galls can vary in size, from quite small to about 7.5 cm (3”) or more in diameter. Sometimes the galls join together to form one enormous gall. Young galls are usually soft and white, while older ones are hard and dark in colour.
Because the bacteria enter the plant through wounds, cither caused by hoeing or primary attack by insects, try to keep your garden pest-free, and avoid deep hoeing. If your soil is waterlogged, adequate correction before the blackberries are planted is another sensible precaution. And lastly, if you have had trouble with crown gall, dip the roots of newly-bought plants in a fungicide, such as captan, before planting.
Although infected plants may continue to crop at a reduced rate, it is best to dig out and burn them, to avoid spreading the disease.
Blackberry dwarf (rubus stunt reversion): this is the most serious disease you are likely to encounter. A viral infection, it causes the plant to produce stunted, crowded new shoots, rather like a witches’ broom. The foliage is misshapen and yellow mottled, and eventually all fruiting stops, and the plant dies. The cut-leaved blackberry seems to be particularly susceptible. Dig up and burn infected plants, as there is no effective chemical control.
Rust: this fungal disease is most often seen on wild blackberries, but cultivated cane fruit, such as raspberries and blackberries, are also vulnerable. The symptoms change as the disease develops. Dark red spots appear first, on the upper sides of the leaves in early summer. On the leaf undersides, tiny pustules develop; these are first pale yellow, but towards the end of summer turn black. In cases of severe infection, the disease will overwinter on fallen leaves and will reappear every spring on the young leaves as they unfold.
As with many diseases, weak and poorly cultivated plants are most at risk. Proper care, including adequate watering and mulching, is the best preventive measure. If rust does appear, cut off and burn all infected foliage and canes, and spray the remainder with a fungicide containing zineb. Rake up fallen leaves.
Powdery mildew: this is another fungal infection which attacks a wide variety of plants; outdoors, plants growing in very dry soils are most at risk. The main symptom is a white powdery coating on the plant, usually on the stems or leaves and, sometimes, the berries.
The best preventive measure is to ensure that the plants have an adequate supply of water, and are regularly pruned and mulched. If powdery mildew does appear, cut out all infected canes immediately, well back to healthy growth, and spray the remainder with dinocap. Do not allow infected canes to remain on the plants over winter.