Pests & Diseases Affecting Raspberries
Raspberry beetle: raspberry beetle is probably the most serious pest you are likely to encounter, and bad attacks may render your crop inedible. The female beetle, which is light brown and 0.4 cm (1/6”) long, lays her eggs in the open flower, in late spring and early summer, and the emerging maggots feed on the developing fruit. One of the worst aspects of this infestation is that it is not likely to be noticed until the raspberries are actually on the table. To control, spray with derris about fourteen days after flowering, at the fruitlet stage.
Aphids: there are several species of aphid which infest raspberries, but the most serious are the raspberry aphid and the rubus aphid, both of which transmit viral diseases. They are present in their largest numbers in late spring and early summer, and their feeding results in the leaves curling a little, but they do not themselves do any great damage. It is their ability to spread virus diseases which is the really troublesome aspect of their infestations. If you have had serious trouble with aphids in previous years, apply a mid-winter spray of tar oil mixture to kill any overwintering eggs. Otherwise, if aphids appear, spray with malathion or dimethoate in spring before the flowers open, and again if necessary, but not while the plants are flowering. Remember to allow the specified time to. Lapse between spraying and harvesting.
Birds: unfortunately, birds find ripening raspberries particularly tempting, and althoughprovide some measure of relief, the only really effective long term protection is growing the crop in fruit cages. Cane midge: this pest affects primarily young canes, which will have brownish-black blotches if infested. The grubs of the cane midge emerge in about early summer from eggs laid in cracks at the base of the canes and then begin to feed on the internal tissue of the cane. There may be two more broods in the season. Besides causing some physical damage, the scars from the cane midge larva make the plants more vulnerable to secondary fungal infections. Some varieties seem more at risk than others; Mailing Enterprise is very susceptible. To control cane midge, spray the young canes with gamma-HCH during late spring, and again two weeks later.
Raspberry moth: if the tips of young lateral shoots appear withered and tunnelled in late spring, then it is likely your canes are infested with caterpillars of the raspberry moth. The best preventive measure is keeping the raspberry bed clean and weed free, and removing withered shoots as soon as seen, because the grub, or chrysalis, is likely to be inside it. In severe cases, a winter tar oil wash applied to theat the foot of the canes should kill the hibernating larvae.
Capsids: these pests vary from year to year in numbers and the amount of damage done. The tips of young canes are most vulnerable to capsid attack; infested tips may stop growing altogether or form branches. Leaves are injured, and develop puckered brown spots and tiny holes, eventually becoming very tattered. As with aphids, a spray of dimethoate or malathion before the flowers open (treating the ground round the plants as well) is usually the most effective method of control.
Grey mould ( Botrytis cinerea ): this is more likely to be troublesome with strawberries than raspberries, but the latter can be infected, particularly if they are growing in wet, overcrowded conditions. Berries with botrytis will be covered with grey, fluffy mould, and the canes can also be infected and even killed outright in severe cases. The best preventive measure is to plant the canes at the correct spacing, so that air can circulate freely. Cut off and burn all infected fruit and canes as soon as yoir notice them. In severe cases, spray with captan or benomyl when the first flowers open and again a fortnight later, but do not spray if the fruit is to be used for preserving in any way.
Cane blight: this fungal infection appears on the fruiting canes in summer; Norfolk Giant and Lloyd George are particularly susceptible. The main symptoms are wilted and withered leaves, and very brittle canes with dark basal patches which easily snap off at ground level. Small, dark, round fruiting bodies may appear on the bases of infected canes. Because raspberries attacked by cane midge larvae are very susceptible, the best precaution is to keep the garden clean, and so free of cane midges. All infected canes should be cut out, below ground level if possible, and burnt. This is particularly important in the case of canes which have snapped off, as the stump remaining in the ground can spread the infection. In the following spring spray the remaining canes with Bordeaux mixture or benomyl at bud burst, and repeat just before flowering to protect the young canes. Make sure the new growth is well spaced out and not crowded. Because cane blight is also soil borne, never use canes from infected beds. Buying certified stock will insure that new canes are free from cane blight.
Cane spot: this fungal infection, although not as serious as cane blight, can still reduce raspberry crops. The main symptoms are small, round, purple spots on the canes in late spring or early summer; occasionally the fruits are spotted or misshapen. Infected leaves will have pale spots surrounded with a dark border. In time, the spots on the canes grow larger, become elongated and change to pale grey in the centre. Eventually the holes 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 serious trouble with cane spot in the past, spray with a 5% lime-sulphur mixture or benomyl at bud burst, and repeat just before blossoming.
Spur blight: this can be quite serious, as the buds from which the fruiting laterals grow can be killed. Buds on young canes become infected first, and the infection spreads back to the stem. The infected cane nearest the bud turns purple and this patch is the first visual indication that infection has occurred. Later this changes to a greyish white. Fungal fruiting bodies appear in the centre of these grey areas as tiny black dots. By the following spring, the infected buds will either have been killed, or will soon wither and die after opening. As with cane spot and cane blight, cut out and burn infected canes, thin the remainder out and spray with Bordeaux mixture or benomyl.
Chlorosis: this physiological disorder is most often seen on plants which are growing in alkaline soils. Leaf, tissue of affected plants will turn yellow, on the youngest leaves first if the trouble is due to a deficiency of iron, and later the older ones; eventually they turn almost white. Lack of manganese shows as a fairly regular, small yellow mottling on the older leaves. Lack of magnesium appears as purple brown patches on the older leaves. The best preventive measure is to correct alkalinity problems before planting. Plants which develop chlorosis can be treated with sequestrenes, applied according to manufacturer’s instructions, or fritted trace elements can be applied. Manganese and magnesium sulphate can be applied as foliar sprays.
Blue stripe wilt: this fungal infection lives in the roots and prevents sap from flowing through the plant normally. In early to mid-summer, leaves of infected plants become striped yellow. Eventually the yellow stripes turn brown, and the leaves wither and die; in severe cases the canes become marked with a bluish stripe longitudinally and eventually die back as well. The only option with blue stripe wilt is to dig up and destroy infected plants, so the disease does not spread.
Viruses: there are several types of viral infections to which raspberries are susceptible; this is further complicated by the fact that the same virus may have different symptoms on different cultivars. All virus infections are very serious, and the infected plants must be dug up and burnt as soon as the diagnosis is made. The most common virus is mosaic, which initially appears as yellow and pale green mottling or spots on the leaves. Eventually, the leaves become smaller, crumpled and distorted, the canes become stunted and cropping diminishes considerably. The plant does not necessarily die at once, but it will never recover completely or crop well again. Some varieties are much more vulnerable than others; Lloyd George is very susceptible. Less common is raspberry yellow dwarf virus; the main symptoms are stunted canes and linear yellow patterns on the leaves. Lastly, stunt and dwarf virus may cause trouble; infected plants may be only 60 cm (2’) tall and cropping nearly non-existent. This virus is spread by leaf hoppers, unlike most other viruses, which are carried by aphids.
With all viruses, the best preventive measure is to control the carriers of the disease: aphids and, very rarely, leaf hoppers. Where soil-living eelworms are the culprits, new stock should not be planted in ground so infested. If you can keep your garden free from these pests, the likelihood of viral infections diminishes. Buying certified, virus-free stock for planting is equally important.