America banned the blackcurrant for decades to protect its pine forests
Blackcurrants are a European staple for juice, jam and cassis, yet for much of the 20th century the United States outlawed growing them. The shrub hosts a rust fungus that barely harms it but devastates North American white pines. The federal ban ended in 1966; some state bans outlasted it.
Ribes nigrum is a strongly aromatic shrub about 1.5 metres tall and wide, native to central and northern Europe and northern Asia, where it likes damp, fertile ground. European cultivation is thought to have begun in the late 17th century. The raw fruit is especially rich in vitamin C and polyphenols, and it is used for jams, syrups, drinks and even dyes. An established bush yields around 4.5 kilograms a year.
Pollination is a delicate business. Flowers hang in chains called strigs of 10 to 20 blossoms, opening from the base outward and visited mostly by insects. After a pollen grain lands, its tube takes about 48 hours to reach the ovule in warm weather but up to a week in the cold, by which time the ovule may have passed its window. A berry with under roughly 35 fertilised ovules may fall before ripening. Frost below about minus 1.9 degrees Celsius damages the blooms, although those near the base, sheltered by leaves, fare better.
The rust behind the American ban, white pine blister rust, needs two hosts to complete its life cycle. On blackcurrant leaves it causes pale patches, orange pustules and early leaf fall, with little effect on the fruit. On white pines that did not evolve alongside it, it causes serious disease and death. Europe has its own scourge: reversion disease, spread by a microscopic gall mite whose colonies swell winter buds into so-called big buds holding thousands of mites. Because control is weak, badly infected bushes are destroyed.
Fruit forms mainly on one-year-old shoots, so growers cut out about a third of the old stems each year to keep new growth coming. Commercial crops are picked by straddle harvesters that shake the bushes as they roll along the rows; a modern machine can gather up to fifty tonnes a day with one operator and two tractor drivers.
Source: Blackcurrant
