Blackcurrant

The blackcurrant (''Ribes nigrum''), also known as black currant or cassis, is a deciduous shrub in the family Grossulariaceae grown for its edible berries. It is native to temperate parts of central and northern Europe and northern Asia, where it prefers damp fertile soils. It is widely cultivated both commercially and domestically.

It is winter hardy, but cold weather at flowering time during the spring may reduce the size of the crop. Bunches of small, glossy black fruit develop along the stems in the summer and can be harvested by hand or by machine. Breeding is common in Scotland, Poland, Lithuania, Latvia, Norway, and New Zealand to produce fruit with better eating qualities and bushes with greater hardiness and disease resistance.

The raw fruit is particularly rich in vitamin C and polyphenols. Blackcurrants can be eaten raw but are usually cooked in sweet or savoury dishes. They are used to make jams, preserves, and syrups and are grown commercially for the juice market. The fruit is also used to make alcoholic beverages and dyes.

Description

''Ribes nigrum'' is a medium-sized shrub, growing to . The leaves are alternate, simple, broad and long with five palmate lobes and a serrated margin. All parts of the plant are strongly aromatic. The flowers are produced in racemes known as "strigs" up to long containing 10–20 flowers, each about in diameter. Each flower has a hairy calyx with yellow glands, the five lobes of which are longer than the inconspicuous petals. There are five stamens surrounding the stigma and style and two fused carpels. The flowers open in succession from the base of the strig and are mostly insect pollinated, but some pollen is distributed by the wind. A pollen grain landing on a stigma will germinate and send a slender pollen tube down the style to the ovule. In warm weather this takes about 48 hours but in cold weather it may take a week, and by that time, the ovule may have passed the stage where it is receptive. If fewer than about 35 ovules are fertilised, the fruit may not be able to develop and will fall prematurely. Frost can damage both unopened and open flowers when the temperature falls below . The flowers at the base of the strig are more protected by the foliage and are less likely to be damaged.

In midsummer the strigs of green fruit ripen to edible berries, very dark purple in colour, almost black, with glossy skins and calyxes at the apex (the calyxes being persistent), each containing many seeds. An established bush can produce about of fruit each year.

Plants from Northern Asia are sometimes distinguished as a separate variety, , of which ''R. cyathiforme'' is considered a synonym.

Phytochemicals

Polyphenol phytochemicals present in the fruit, seeds and leaves, are being investigated for their potential biological activities. Major anthocyanins in blackcurrant pomace delphinidin-3-''O''-glucoside, delphinidin-3-''O''-rutinoside, cyanidin-3-''O''-glucoside, and cyanidin-3-''O''-rutinoside, which are retained in the juice concentrate are among other polyphenols.

Distribution and habitat

The blackcurrant is native to northern Europe and Asia.

Cultivation

Cultivation in Europe is thought to have started around the last decades of the 17th century.

Site selection and planting

Blackcurrants can grow well on sandy or heavy loams, or forest soils, as long as their nutrient requirements are met. They prefer damp, fertile but not waterlogged ground and are intolerant of drought. Although the bushes are winter hardy, frosts during the flowering period may adversely affect the yield and cold winds may restrict the number of flying insects visiting and pollinating the flowers. A soil pH of about 6 is ideal for blackcurrants and the ground can be limed if the soil is too acidic. Planting is usually done in the autumn or winter to allow the plants to become established before growth starts in the spring, but container-grown stock can be planted at any time of year.

Two-year-old bushes are usually planted but strong one-year-old stock can also be used. Planting certified stock avoids the risk of introducing viruses. On a garden scale the plants can be set at intervals of or they can be set in rows with planting intervals of and row separations of or more. In the UK, young bushes are generally planted deeper than their initial growing level to encourage new stems to grow from the base.

Manures and fertilizers

The blackcurrant requires a number of essential nutrients to be present to enable it to thrive; nitrogen provides strong plant growth and stimulates the production of flower sprigs; phosphorus aids growth, the setting of fruit and crop yield; potassium promotes growth of individual shoots and increases the weight of individual fruits; magnesium is a constituent of chlorophyll and helps increase yields through interaction with potassium; calcium is required for cell division and enlargement and is particularly important for young plants and buds.

An annual spring mulch of well rotted manure is ideal and poultry manure can also be used but needs prior composting with straw or other waste vegetable material. Spent mushroom compost can be used but care should be taken as it often contains lime and blackcurrants prefer slightly acidic soils. The blackcurrant is a gross feeder and benefits from additional nitrogen, and phosphatic and potash fertilisers should also be applied annually. A balanced artificial fertilizer can be used and a 10-10-10 granular product can be spread around the bushes at the rate of per plant. Weed growth can be suppressed with an organic mulch such as sawdust, bark, mushroom compost or straw, heavy plastic topped with an organic mulch cover or landscape fabric.

Pruning

Blackcurrant fruit is borne primarily on one-year-old shoots. Newly planted bushes should be pruned severely, cutting all shoots back to two buds above ground level. This gives the plant a chance to get properly established before needing to put its energy into producing fruit. The general rule when pruning is to remove all weak shoots and those growing out sideways which may get weighed down when fruiting. The remaining branches should be thinned to remove old unproductive wood and to encourage new shoots. An established bush should not be allowed to become overcrowded and should have about one third of its main branches or stems removed each year. When harvesting by machine, plants with an upright growth habit are encouraged.

Harvesting

On a garden scale, the berries should be picked when dry and ripe. Commercially, most harvesting is done mechanically by straddle harvesters. These move continually down the rows, straddling a row of bushes, shaking the branches and stripping off the fruit. The blackcurrants are placed into half tonne bins and to minimise stoppage time, some machines have cross conveyors which direct the fruit into continuously moving trailers in the adjoining row. A modern machine can pick up to fifty tonnes of blackcurrants in a day using only one operator and two tractor drivers. The bins should be stored in a cool place. Some fruit is still picked by hand for use in the fresh fruit market.

Diseases and pests

''Ribes'' plants are susceptible to several diseases and a number of insect pests. However, new varieties have been or are being developed to overcome some of these problems.

Reversion is a serious disease transmitted by the blackcurrant gall mite (''Cecidophyopsis ribis''). It causes a decline in yield and is quite widespread in Europe but is rarely encountered on other continents. Symptoms include a modification of leaf shape in summer and swollen buds ("big bud") in winter, each housing thousands of microscopic mites. As pest control has limited effectiveness, severely infected bushes should be destroyed. All new plants purchased should be certified as virus-free.

White pine blister rust (''Cronartium ribicola'') needs two alternate hosts to complete its life cycle. One host is plants in the genus '' Ribes''. On the blackcurrant, it causes the leaves to become pale and later develop tiny orange pustules and sometimes a yellow filamentous coating on some leaves. The fruit crop is little affected but the leaves fall early and growth is slowed the following year. The other host is any of the white pines, in which it causes serious disease and mortality for the North American species that have not co-evolved with the rust. As a result, the blackcurrant was banned in the United States as a disease vector for much of the 20th century, and even after the federal ban was lifted in 1966, several U.S. states continued their own bans, some of which remain in force as of August 2021. The effectiveness of these restrictions is questionable, since other ''Ribes'' species also host the disease, some are native to North America, and others such as red currants and '' Ribes uva-crispa'' were never banned.

American gooseberry mildew and powdery mildew can infect the leaves and shoot tips, and botrytis may cause the fruit to rot in a wet season. Currant and gooseberry leaf spot (''Drepanopeziza ribis'') is another disease of blackcurrants, but it is not usually a serious problem as most cultivars now have some resistance.

The blackcurrant leaf midge can cause browning, crimping and distortion of leaves at the tips of shoots but it is seldom a serious problem. The blackcurrant sawfly (''Nematus ribesii

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