Weed Control
As growers have become more environmentally aware, they have largely replaced chemical weed control between the rows with plantings of grass or clover mixes, controlled by mowing. This has removed the need for wide-spread herbicide use. In addition, the headlands of plantations are grassed down, and mown. For the area directly under the bushes, there are two main control methods available to growers.
Either small amounts of specific herbicides can be used to control weeds, which would otherwise severely restrict the growth of the bush, or polythene mulch can be used at planting time to prevent weed growth. More environmentally beneficial practices are being investigated by using weed burners, to replace the use of herbicides under the bush.
Pest and Disease
Insect pests do occasionally attack blackcurrants, like any other crop, but growers have learned that some infestations can be tolerated and will clear up naturally, as the pests’ natural enemies and parasites eventually bring populations back into balance. Growers have put an enormous amount of research into this area in the past decade, and the war against pests is an interesting one. Some of our successes are described below.
Vine Weevil is known to many gardeners to be a problematic nocturnal pest. The first signs of attack are the ‘notches’ around the edges of the leaves caused by feeding damage. They are not the real problem though as this is mainly cosmetic damage. The real problem is the larval stage that lives underground and feeds on roots – they are not fussy either, as they feed on more than 150 species of plants, but they do like to live in light soils or peat. When numbers build up, Vine Weevil infestations will seriously weaken bushes, even causing bush death in hot weather as bushes struggle with a much-reduced root system.
The main Vine Weevil predators in blackcurrant plantations are predatory beetles – mostly from the Carabid family. The commonest ones are Harpalus rufipes (strawberry seed beetle), and Nebria brevicollis (no common name), but the most obvious is Carabus violaceus (Violet ground beetle). It’s obvious because at 20-30 mm long, they are the one of the largest ground beetles in the UK, and they will consume large numbers of Vine Weevil adults and larvae. Grass alleys between the rows, and more specifically grass “beetle banks” do a great deal to encourage the developments of these beetles, and growers work hard to maximise their numbers. Aphids are also familiar to gardeners, who will know that there are many different species, but there are just 3 that occur commonly on blackcurrants.
Aphids have enormous reproductive potential, and their numbers can build up exponentially under favourable conditions. They damage plants both directly, by stunting through feeding on the sugar rich sap, and sometimes causing distorted growth, but also indirectly. Their food source is plant sap, which is sugar rich but very low in protein. In order to obtain adequate protein, aphids suck up great volumes of sap, extracting the protein and excreting excess syrup through siphunculi (a small pair of tubes on their abdomen that look like ‘twin exhausts’). It is this syrup (honeydew) that causes stickiness on leaves and eventually sooty moulds to grow.
The most common aphid predators in blackcurrants are Ladybirds and their larvae, and Orius predatory bugs, mostly Anthocoris nemorum, which hold small insects motionless in their forelegs and suck them empty with their needle-like mouthparts! Parasitic wasps such as Aphidius matricariae are numerous, and lay their eggs in aphids, and the eggs go through 4 larval stages inside the aphid, which swells and stiffens into an enlarged golden brown ‘mummy’, before the wasp leaves through a small hole in the abdomen.
Two Spot Spider Mite numbers can build up in warm weather. The colonies establish themselves on the underside of leaves that show telltale speckles at first, before going brown and falling off. Damage is from sap feeding, and eventually loss of leaves. Spider mite populations are normally held in balance by predatory mites – they are only really visible with a good hand-lens, so have no common names. The most important ones are Typhlodromus pyri and Amblyseus cucumerus.
Managing plantations to take advantage of natural predators and parasites takes great knowledge, skill and careful monitoring, but is increasingly used as part of a package of integrated fruit production. However, the biggest challenge that has faced, and continues to face, our industry is the Reversion Virus. This disease is carried by the Gall Mite (or Big Bud Mite) which lives in large and overgrown buds, and has proven to be almost impossible to kill. If reversion takes hold of a plantation it can spread very quickly and eventually all reverted bushes will carry no crop.
Fortunately through our Breeding Programme varieties have, and are being developed, that are resistant either to Gall Mite or to reversion. This has been done over the past 30 years by crossing the gooseberry, which is resistant, with the blackcurrant, and then using conventional plant breeding to breed out the gooseberry element! Resistant varieties are now being widely used.
Other Pests and Diseases include Leaf Curling Midge, Capsids, Red Spider Mite, Mildew and Leaf Spot. Different varieties are more or less susceptible to certain Pests and Diseases, and again conventional plant breeding is being used to select varieties that are resistant, together with the search for natural predators.
With increased resistance to Pest and Diseases on new varieties, there are now many opportunities to reduced chemical applications. With careful crop monitoring and Growers’ commitment to biodiversity, the industry as a whole is increasingly committed to pesticide reduction.
