Pests & Diseases Affecting Green Beans

Bean beetle (Bruchid beetle): because this pest resembles a weevil in appearance, it is sometimes called, incorrectly, ‘bean weevil’. The female lays her eggs on the growing seed pod, or else on seeds which have been dried and stored. Once the legless grubs hatch out, they bore into the seeds, and then bite a round, window-like hole beneath the skin surface; it is through this hole that the adult beetle eventually emerges.

Besides feeding on the seed, the holes that they make allow secondary infections, such as fungal and bacterial diseases, to enter. Millepedes and wireworms also find it easier to attack pods which have been initially damaged by bean beetle. Remove and burn any seeds or pods which are holed, or which contain living grubs. Because these pests are usually seed-carried, the best precaution is to be sure your seeds are from a reliable source.

Slugs: these familiar garden pests feed on a wide variety of plants. Active chiefly after dark, they attack leaves and pods, biting large holes in them. During the day, they hide away in dark, moist, cool places. Slugs are often found in decaying vegetable matter, and on soils which are rich in humus and moisture.

One method of destroying slugs is to trap them. Place wet sacks, or heaps of damp vegetable refuse, such as cabbage or lettuce leaves or orange peels, at the base of the bean plants. Inspect the traps daily and destroy any captured slugs.

Alternatively, control slugs with pellets containing metaldehyde or methiocarb.

Capsids: these are sucking insects whose attacks produce pin-prick holes in the leaves and occasionally distort the pods. They attack the growing point of the plant, resulting in stunting of the plant and, if very young, in completely killing it. Control them by spraying or dusting with derris plus pyrethrum, or dimethoate if an infestation is stubborn. Because capsids drop to the ground when disturbed, remember to treat the soil around the plant as well. Since the damage does not usually show until after the capsids have gone, it is best to start spraying in late spring, when the capsids actually begin to feed, particularly if you had damage the previous year.

Bean aphis (black fly): this insect is most troublesome in late spring, when it completely smothers the growing points of beans. Plants infested with bean aphis stop growing, and the few pods which develop may be covered with a black, sticky substance. Control by spraying with bioresmethrin or liquid derris, or dust with derris powder, and repeat as necessary. Remove and burn infected tops of plants as soon as enough pods have formed.

Bean seed fly: these pests destroy the seeds and the seedlings of newly planted French beans. The pale, legless larvae are most active between late spring and mid-summer. They attack the plants below ground, where they feed on the seeds, roots and underground stems of seedlings.

Because they are more likely to appear on land which has been given manures rich in nitrates, avoid excessively rich nitrogenous fertilizers. A second precaution is to dust the drills with gamma HCH before sowing or planting out.

Red spider mite: this minute sucking insect pest occasionally attacks French beans, especially if grown in hot, dry conditions. Watch for the appearance of leaves heavily speckled greyish-brown or pale yellow, together with webbing and slow plant growth. If it occurs, spray with malathion and give additional water and ventilation. Remember to allow the correct time interval between spraying and harvesting.

Halo blight: this is a seed-borne, bacterial disease which is encouraged by the unnecessary practice of soaking the beans before sowing. The main symptoms are small, transparent spots which are surrounded by a yellow ring. Eventually the spots dry up, and where many are present, so that they coalesce, the entire leaf will wither. Seedlings may be killed outright, and even older plants can wilt completely. The pods can also be infected with round, moisture-oozing spots. Remove and destroy diseased plants as soon as seen. The best precaution is to only use seeds from reliable sources; never sow seeds which are wrinkled or blistered or have yellow spots on them. The variety The Prince is resistant to the disease. Anthracnose: this fungal disease is usually associated with cool, wet growing conditions. The main symptoms are dark brown elongated spots on the stems which result in the leaves withering. The pods can have small, round, sunken spots, reddish-brown in colour, and the seeds inside diseased pods will eventually develop brownish-black markings. A half-strength mixture of Bordeaux (230 g in 46 litres of water, or 2 lb in 10 gallons of water) sprayed onto the infected plants offers some measure of control, but this spraying must stop when the plants begin to flower. The plants should be destroyed after cropping, and dwarf beans grown in a different place for several years. Because the disease is seed-borne, the best precaution against anthracnose is clean seed, obtained from a reliable source.

Root or foot rot: this fungal infection occurs most often on soils which are cold and badly drained. Roots growing in these conditions will be weakened, and thus will be more vulnerable to attack. Unfortunately, the symptoms of root rot are not visible above ground. If a plant is not growing well, and has yellow, wilted foliage for no apparent reason, gently pry the soil away from the main stem. If root rot is the cause, then dark brown or reddish discolourations will be seen on the roots and base of the stems, and the roots will be withered looking. The crop is likely to be greatly reduced. As with all seed-borne diseases, the best precaution is to obtain clean seed from a reliable source. Affected plants should be destroyed after any crop has been taken. Those only slightly affected may be induced to throw out fresh roots by mulching up and around the stems.

Botrytis: this fungal infection, commonly called ‘grey mould’, is usually associated with cold, wet growing conditions; seedlings are particularly vulnerable. As its common name implies, the main symptom is fluffy, grey mould appearing on the stems, leaves and, occasionally, pods.

Sufficient ventilation is important; if you sow your beans under glass, make sure the seedlings are not overcrowded. Thin them as soon as they are large enough to handle, and always remove and destroy weak or damaged plants. If there is an outbreak of botrytis, control by removing badly affected plant parts and spraying the rest with sulphur, or dusting with a fine spray of sulphur dust. Benomyl is also effective.

Virus: two diseases, common mosaic and yellow mosaic, occur on beans. The former produces dark and light green mottling, and distorted leaves. The plants are stunted and the crop reduced. Yellow mosaic produces yellow irregular patches on the green leaves, and the pods can be considerably misshapen. Common mosaic can be seed-borne; both are carried by greenfly. As with all virus diseases, there is no remedy, and infected plants should be destroyed. Do not save seed for future sowing.

31. August 2013 by admin
Categories: Fruit Gardening, Uncategorized, Vegetable Gardening | Tags: , , , , , , , | Comments Off on Pests & Diseases Affecting Green Beans

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