Many factors can affect growth of winter and spring beans, and the notes below describe the main pests and diseases which reduce yield and quality. Further information can be found in PGRO Technical Update TU16 ‘Checklist of Fungicides and Insecticides for Field Beans’.
The pest can cause damage to spring beans if large numbers appear when plants are small. Leaves of attacked plants show characteristic ‘U’ shaped notches around the edges, but the main damage occurs as a result of the larvae feeding on the root nodules. Sprays may be applied at the first sign of leaf damage and repeated after 7 - 10 days. A monitoring system for pea and bean weevil is available from Agralan Ltd., The Old Brickyard, Ashton Keynes, Swindon, Wilts, SN6 6QR. Winter beans, although still prone to attack are usually too advanced in growth for the weevil or the larvae to have any appreciable affect on yield, and spray treatment is justified only when pest pressure is very high and winter beans show retarded growth.
The nematode has become a major pest in field beans and can cause severe problems in wet seasons, particularly where farm-saved seed from an infested stock has been multiplied for several generations.
The pest is seed-borne and can also infest soils, thereby becoming a problem for future crops of beans. Seed should be tested for nematode, and only clean stocks should be sown.
For further information see PGRO Technical Update TU09 ‘Stem and bulb nematode in field beans.
Mildew is prevalent on spring beans, where it causes greyish-brown, felty growth on the under-surface of the leaves. Some varieties have resistance to the disease and 1 - 9 ratings are given in the Recommended List of Varieties. However, metalaxyl M (SL567A EAMU 0917/13) mixed with another foliar fungicide may be may be necessary on the more susceptible varieties if infection begins at early flowering. There are no seed treatments for field beans and varietal resistance should be the first measure to avoid serious losses.
This produces brown lesions containing distinctive black fruiting bodies (pycnidia). Winter beans are more prone to serious attacks which can develop in wet conditions but, since the disease is almost entirely seed-borne, it is advised that farm-saved seed should be tested by PGRO. Some winter varieties which are very susceptible to the disease may develop severe symptoms in wet years, particularly if growing near to previous years’ bean fields where infection can be transmitted from bean volunteers. Resistant varieties are available and information is given in the main variety table. Some fungicides will give partial control of the disease.
Black bean aphid can be very damaging to field beans if colonies develop just prior to flowering. Spring-sown crops are usually more likely to suffer damaging attacks than winter beans. As well as forming dense, smothering colonies on the upper part of the stem, these and the less obvious pea aphid are able to transmit several viruses which add to the yield loss. Aphids can be controlled using pirimicarb as soon as 5% of the plants have been colonised. Care must be taken if using other insecticides, especially when flowers are present on the crop, as there is a serious risk to bees.
Symptoms appear as reddish-brown spots, which eventually enlarge to give a more damaging aggressive phase in cool, wet or damp weather. Winter beans are more likely to suffer yield losses, especially where the plant population is high and the crop becomes tall. Early fungicide treatment is essential if the crop shows symptoms at first bud or early flower. A second spray may be required 3 to 4 weeks later if damp conditions persist. Additional sprays are unlikely to be economic unless prolonged rain is experienced, and losses due to damage caused by the sprayer may be significant. Tebuconazole, azoxystrobin, metconazole, chlorothalonil + cyproconazole, boscalid + pyraclostrobin are effective.
Bean seed beetle, also known as bruchid beetle, can affect both winter and spring varieties. Adults emerge from the seed leaving a circular hole. The beetles do not breed in grain stores, but damaged produce may not be accepted for quality markets. Adults fly to beans during flowering and lay eggs on developing pods. The larvae bore through the pod and into the seed, where they feed until mature. A pyrethroid insecticide approved for use during flowering should be applied using angled nozzles at early pod set following 2 consecutive days when the maximum daily temperature has reached 20ºC and repeated 7-10 days later. For further details see PGRO Technical Update TU10 ‘Bean seed beetle (Bruchus rufimanus)’.
The disease is characterised by numerous reddish-brown pustules on the leaves. It is more serious on spring beans and all varieties are susceptible. Most damage occurs if infection begins during flowering and pod set. Fungicides such as tebuconazole, cyproconazole, azoxystrobin, metconazole and boscalid + pyraclostrobin may improve yield in either winter or spring beans, but treatment is unlikely to be worthwhile if infection begins when pod fill is complete and the crop is beginning to senesce.
This disease occasionally infects winter beans in damp autumn weather, and infections may be associated with preceding crops containing red clover. Plants develop a watery stem rot, which can spread from plant to plant in dense stands. The related fungus, Sclerotinia sclerotiorum infects spring beans and also peas, rape, linseed, lupins and a range of field vegetables. Infection in spring beans is, however, very rare, but the risk should be borne in mind when planning rotations with other host crops.
These can occur on seedlings and on more mature plants, causing browning of the stem base and wilting of the leaves. Foot and root rots in beans appear to be more sporadic than those which occur in peas, and the bean crop in general appears less sensitive to root rots than peas. Nevertheless, growers should avoid over-cropping land with beans.