Pests & Diseases Affecting Broad Beans
By far the most troublesome pest ofis bean aphis. Happily, however, it is quite easily controlled. If your broad beans escape bean aphis few other pests or diseases are likely to be a serious problem.
Bean aphis: sometimes called black-fly, black aphid, black dolphin aphid, black army, or collier, these tiny insects suck the sap from the plants and also foul the leaf surface with a sticky black substance called honey-dew. The earliest crops are usually unaffected as they produce beans before the aphids have become established, but later crops may be heavily infested.
The aphids congregate particularly on the growing point but are also found on the stem and on the undersides of leaves, which may curl up if badly attacked. Taking out the growing points of the beans reduces the likelihood of attack, but is unlikely to protect the plants completely. Spray or dust the plants with derris or bioresmethrin immediately after removal of the tops. Repeat daily if necessary. The best time to spray is in the evening when bees and other pollinating insects will not be harmed by the spray along with the aphids.
There is some evidence that summer savory discourages attack by the aphids. Try sowing some between the rows of beans.
Chocolate spot: brown spots and streaks on the leaves, stems, petioles, and sometimes also the pods, of the plants are a sure sign of an attack of chocolate spot caused by forms of the fungus botrytis. Good healthy plants, growing on fertile and well-drained soils are never seriously attacked, although autumn-sown plants which have been weakened by frost are susceptible. However, even these plants are likely to recover without treatment. The disease is only serious if thelacks cither liime or potash and the season is a wet one. The best defence is to plant only in good, well-manured soil, and to space the plants adequately. Spraying should not be necessary, but Bordeaux mixture or captan can be applied, if the disease becomes trouble-some. Destroy infected plants; do not put them on the heap.
Bean beetle (Bruchid beetle): These beetles are not a serious threat to a growing crop but rather damage seed which is being stored for future planting. The adults, which look similar to weevils, lay eggs either on the pods of growing plants or on seeds in storage; and the legless and curved grubs which hatch out bore into the seed, feed and pupate inside it. Because of the size of the bean seed, germination is not usually affected but the holes made in the seed reduce the amount of food available to the germinating, resulting in stunted plants in severe cases. They also expose the seeds to attack by other borers such as millipedes and wireworms, which may be in the soil at sowing time, and to fungal and bacterial diseases.
Seeds containing live grubs or beetles should be burnt to destroy the pest. They should not be placed on a compost heap, as the beatles may spread from it. As the pest is seed-borne the surest precaution against it is to buy seed only from a reputable merchant.
Pea and bean weevil: the same weevil which attacksalso attacks broad beans. The weevils eat semi-circular holes from the edges of young leaves and also eat the nodules on the roots. As soon as this is seen, dust or spray the plants with derris.
Mice: occasionally seed can be correctly sown in good land, and few or no seedlings germinate. This is probably the result of attacks by mice which take the seed and store it. Set traps at intervals along the rows, and inspect the traps frequently.