Blackcurrants, or currants, are a temperate shrub of the species Ribes nigrum which is grown both commercially and domestically for its flavorful berries. The shrub is of a medium size, reaching a maximum of 1.5 x 1.5 m and possesses simple long, broad leaves each with 5 lobes and a serrated edge. The leaves grow in an alternate pattern on the stems of the plant. When not in flower, the leaves are strongly aromatic and produce the familiar blackcurrant smell. The flowers of the plant are dark brown/red in colour each with 5 petals. In the summer, the round green fruits ripen to dark purple, reaching sizes up to 1 cm in diameter. Each fruit contains several nutrient rich seeds and a single blackcurrant bush can produce up to 5 kg of fruit in one year. Blackcurrant plants originate in Europe and have been under domestic cultivation for over five hundred years. Recent breeding efforts have been focussed mainly in Russia, Sweden and Scotland. The blackcurrant is less popular in the US due to a ban introduced in the 1900s to limit the spread white pine blister rust.
The blackcurrant fruit has a strong, tart flavor and although fruit is available fresh, it is usually cooked and/or sweetened prior to eating. The fruit is renowned for its very high vitamin C content and is widely used in juices and cordials. The distinctive flavor of the fruit also makes it popular for use in jellies, jams and preserves and also as a flavor in sauces and desserts. In Europe, it is commonly used as a flavoring in candies, or sweets, whereas in New Zealand, it is popularly exported to Japan for use in dietary supplements and culinary use.
Blackcurrant plants grow best in a well drained soil which is unlikely to completely dry out. The plants are ideally grown in full sunlight but good results can be obtained in partial shade as long as frost prone areas are avoided as this may damage the flowers and decrease fruit yield. A downward slope is therefore recommended to allow drainage of cold air to lower levels and prevent damage to the flowers. Before propagation, the soil should be completely weed free and it is recommended that the soil is tested for nematodes and a general soil profile is obtained to ensure soil suitability (ideal pH for blackcurrant is between 6.5 and 6.8).
The best time to plant blackcurrant is when the soil is not waterlogged or frozen, typically in early winter but they can be planted between October and March depending on the variety. The depth of planting is very important as blackcurrants will produce several stems just below ground level. The root should be given adequate space to bed and plants should be spaced approximately 1.8 m (6 ft) apart. Plantations are best established by planting of a 1 year old bush produced from virus free root or by planting cuttings which have been prepared from hardwood shoots of a fully grown plant. If planting rooted bushes, the roots should be immediately covered with a layer of soil to ensure that they do not dry out.
New bushes should be pruned in the first year to encourage further healthy growth and pruning should continue on an annual basis to remove any dead or damaged shoots. Blackcurrant plants will produce low yields in the second year of growth and it is only in year 3 that growers can expect to see a return on the initial planting investment.
Pests and diseases
Vine weevil (Otiorhynchus sulcatus)
The initial signs of a vine weevil attack are notches, or holes, around leaf edges. Although the vine weevil causes damage above-ground, it is a more serious problem below-ground where the larval stages feed on the plant roots and can cause damage so severe that it ultimately results in the death of the plant. The presence of adults in the foliage should serve as an indicator that larvae may be present on the root system.
There are 3 species of aphid which can infest blackcurrant plants . In small numbers, the aphid colonies can be well tolerated by the plant but a large infestation can cause distortion of the leaves and stunt plant growth. High aphid numbers will result in ‘sticky’ leaves as aphids excrete a sugar rich substance called honeydew which in turn encourages the growth of black, sooty moulds.
Two-spotted spider mite (Tetranychus urticae)
The two-spotted spider mite feeds on sap and initial symptoms on the plant are hard to detect without the aid of a lens or microscope. Eventually, feeding by the mite will lead to the development of black spots on the leaves before the leaf dies and completely falls off.
Leaf curling midge (Dasineura tetensi)
Feeding by larvae of the leaf curling midge can cause severe distortion of young blackcurrant leaves, leaving them susceptible to infection with fungal pathogens. The severe distortion and twisting of leaves can mask symptoms of Blackcurrant reversion.
Clearwing (Synanthedon tipuliformis)
Clearwings are prevalent in many areas where blackcurrant is grown. The larvae of the moth bore into the stems to feed on the inside of the blackcurrant shoots causing a depletion of nutrients in the plant. The larvae eventually bore through the stem to create an ‘exit window’ before they pupate to the adult moth. This tunneling can cause weakening and breaking of the plant stems and areas of the plant that are infested can suffer from uneven bud-break as a result.
Gall mite (Cecidophyopsis ribis)
The gall mite causes the buds of the blackcurrant plant to become enlarged and swollen as the colony builds up to extremely large numbers. A single bud can contain up to 35,000 mites while a single plant can possess up to 100 galls. The gall mite also transmits Blackcurrant reversion disease (see below). Plants with galls should be removed immediately.
Blackcurrant mildew (Sphaerotheca mors-uvae)
Blackcurrant mildew is a type of fungus which infects blackcurrant leaves, shoots and occasionally fruit. The fungus is present as a white powdery growth on the leaves. Other symptoms include the loss of leaves and misshapen fruit.
Leaf spot (Depranopeziza ribis)
Blackcurrant leaf spot is caused by a fungus, Depranopeziza ribis, and causes the formation of necrotic lesions on the leaf surface. Infection with this fungus can lead the plant to lose its leaves prematurely with a significant impact on fruit yields. Wet growing conditions can promote the spread of fungal spores which are spread by water splash.
Blackcurrant reversion disease
Blackcurrant reversion is caused by a viral pathogen known as Blackcurrant reversion virus. The virus is spread by the gall mite, Cecidophyopsis ribis, and therefore the best indication of the presence of reversion virus is the enlarged buds containing the mites. The blackcurrant bushes will continue to grow normally, but the fruit crop will eventually fail. Other symptoms of the virus include a change in the colour of the flower, from pink to magenta and also a lack of hair on the flower. There is no treatment for reversion disease and the only option is to start the plantation again from healthy stock.
Rust (Cronartium ribicola)
Rust on blackcurrants is caused by the fungus Cronartium ribicola, also known as white pine blister rust. Symptoms on blackcurrant are the development of lesions and uredinia (pustule like structures) on the top surface of the leaves eventually leading to the death of the leaf. Protection against this rust is reliant on planting of resistant blackcurrant cultivars.