Field Beans

Although pulses are known for relying on their symbiosis with Rhizobium for fixed N, they can also rely on soil mineral N (SMN). Typically, 50-60% of the total N uptake by pulses is from fixed nitrogen, but can reach as high as 70-80% in forage legumes. However, high concentrations of SMN (200-250 kg N/ha) will inhibit N fixation, which can reduce total N uptake from fixed N to 35-50%.

pH 6.5 is the optimal pH for N fixation to occur. Avoid soils becoming lower than pH 5.5.

N fixation is most effective when soil temperatures are between 7°C and 20°C. N fixing nodules develop around 3-5 weeks after plant emergence, so it’s worth bearing this in mind when thinking about sowing dates to time nodule development with the best temperatures for N fixation.

Deficiencies in mineral nutrition can reduce N fixation, especially P and Mo.

N fixation requires lots of energy, using about 30% of the energy captured by the plant through photosynthesis (Figure 3). It is essential to continue N fixation after flowering to supply enough N required for seed growth. This is dependent providing enough energy through photosynthesis during grain filling by maintaining green leaf area.

Any other factors which limit either root exploration of the soil or nodulation will limit N fixation, such as poor soil structure (i.e., compaction, dry soils, waterlogging) and salinity.

Determined by seed rate and establishment.

Determined during stem extension and influenced most by plant spacing and water supply. Winter beans often branch more than spring beans.

Related to number of nodes per shoot which may be related to plant height and the conditions during stem extension. Flowers from these nodes may be aborted under stressful conditions such as high temperatures, drought or dull conditions. Variety may affect the importance of self- vs insect- pollination for pod.

The potential number for seeds per pod is set following flower fertilization and is sensitive to the amount of photosynthesis.

The stem & bulb nematode has become a major pest in field beans, causing severe problems in wet seasons particularly where infected farm-saved seed has been used. The pest can be seed or soil-borne, therefore ensuring seed is tested and a long rotation is maintained are important IPM techniques to reduce the risk of infection.

Damage from the stem & bulb nematode is not initially apparent, usually becoming most noticeable around the flowering stage. Plants may appear stunted with thickened and twisted stems. Leaves can be thickened and brittle with a bronze discolouration in the leaf petioles. Later, stems may turn a browny red colour and swell, twist and break. Pods tend to fill unevenly and seeds are poorly developed, becoming black and shrivelled as they mature. Affected plants may be isolated or in larger patches of the field. Isolated occurrences are more indicative of an infested seed source whereas patches throughout the field are more likely to suggest a soil infestation. Seed and soil testing for stem & bulb nematode are now available from PGRO.

Temperatures, light and rainfall are critical in the four days following flowering for flower retention and pod set. High temperatures at fertilization can reduce self-pollination and cause flower abortion. Having multi-noded stems with more pods will prolong the period of flowering, helping to insure against adverse conditions.

Although bean crops can self-pollinate, fertilization is improved by a greater abundance of pollinators. It is also possible that there are varietal differences in fertility.

Water limitations can have negative implications on many aspects of yield formation. Encouraging deep rooting is key to good water capture, which can be aided by limiting soil compaction, good seed bed conditions and good soil structure throughout the soil profile.

Maximising green canopy duration is key to maximise the capture of sunlight.

• achieve full ground cover by early May through good crop establishment (e.g., good seedbed, appropriate sowing date, depth and seed rate)
• provide sufficient nutrients, especially P and S, to allow the canopy to expand and fix N.
• Make use of resistant varieties and fungicides to prevent early canopy senescence commonly caused by chocolate spot development.
• Make use of fungicides with physiological greening effects to prolong canopy life.

Downy mildew is predominantly an issue on spring beans, where it causes greyish-brown, felty growth on the under-surface of the leaves. The disease favours cool, humid conditions which are often felt in late spring. It is important to look at the disease rating on the PGRO Descriptive List of Varieties, which ranges from 1-9, with some varieties having high resistance to the disease.

Leaf and pod spot is primarily a seed borne disease, so using certified or tested seed is the best way to prevent infection. Whilst the disease is seed borne, air borne spores can also cause infection, so if growing winter beans in close proximity to the previous year’s crop then it is important to look out for early signs of infection.

Lesions are greyish-brown, circular to oval in shape and often develop a lighter grey centre. Small, pin-prick sized dark pycnidia form in the centre of the lesion. As the name suggests, lesions can spread from the leaf onto the stem and pod surface, where they appear sunken and dark in colour. This type of infection can also blemish seeds inside the pod.

Pea and bean weevils are active in early spring when they migrate in from grassy, uncultivated field edges of fields previously cropped with legumes.

They begin feeding on plants, leaving characteristic ‘u’ shaped notching around leaf edges. Whilst feeding, eggs laid by the female weevil are washed into the soil around the stem base and produce larvae. It is these larvae which begin feeding on the root nodules which can be particularly damaging.

Insecticide sprays can be applied at the first sign of leaf damage and repeated after 7 – 10 days.

The stem & bulb nematode has become a major pest in field beans, causing severe problems in wet seasons particularly where infected farm-saved seed has been used. The pest can be seed or soil-borne, therefore ensuring seed is tested and a long rotation is maintained are important IPM techniques to reduce the risk of infection.

Damage from the stem & bulb nematode is not initially apparent, usually becoming most noticeable around the flowering stage. Plants may appear stunted with thickened and twisted stems. Leaves can be thickened and brittle with a bronze discolouration in the leaf petioles. Later, stems may turn a browny red colour and swell, twist and break. Pods tend to fill unevenly and seeds are poorly developed, becoming black and shrivelled as they mature. Affected plants may be isolated or in larger patches of the field. Isolated occurrences are more indicative of an infested seed source whereas patches throughout the field are more likely to suggest a soil infestation. Seed and soil testing for stem & bulb nematode are now available from PGRO.

The black bean aphid can be very damaging to field beans if colonies develop prior to flowering. Spring-sown crops are usually more likely to suffer

damaging attacks than winter beans. Damage is usually as a result of direct feeding; plants fail to develop and pods do not fill normally.

The honeydew produced from the aphid feeding can also result in secondary moulds and diseases infecting the plant, including chocolate spot. The black bean aphid can also act as a vector for viruses such as the bean leaf roll virus.

The winged aphids migrate into the crop in early summer and once established in the crop, can produce several wingless nyphs each day. Whilst easy to control with aphicides, due to their concurance with the crop flowering, it is important to consider options which are not harmful to pollinating insects. Applications made at early flowering or when 5% of plants are infested reduces infections of aphid-transmitted viruses.

BLRV is aphid-transmitted and both the black bean aphid and pea aphid are vectors. Usually infection is more obvious where aphids infest the crop before flowering. The pea aphid is more likely to move into beans at this time. Control of early invading aphids is essential to prevent virus infection.

BYMV develops in beans at any time before flowering. Leaves are crinkled and may become pointed. Vein clearing can develop and the plant is slightly stunted. It is similar to Pea Enation Mosaic Virus (PEMV) but without the translucent spotting and streaking on the leaf surface. The virus is aphid transmitted and common in field beans. Infection may be on individual plants or groups of plants and, depending on severity, yield can be severely reduced.

Pea aphid is the principal vector, although black bean aphid can also transmit the virus. Early control of aphids in flowering bean crops is the most effective means of preventing virus infection.

Chocolate Spot can be an extremely yield robbing disease. It is estimated to commonly cause yield losses of 25%, but in conducive conditions yield losses can reach 90 – 100%.

Thriving in long periods of overcast and humid weather conditions, the disease can spread rampantly, especially where plant populations are high. ADAS research suggests that temperatures of 15 - 22ºC and high relative humidity of >80% are most favourable for the disease, with rainfall and wind also helping spore dispersal and disease spread. Winter beans are most susceptible, however spring beans can also develop Chocolate Spot during humid conditions.??The disease develops as small, circular, chocolate coloured spots on infected leaves. These become larger and may coalesce to form a greyer coloured lesion extending over the leaf surface. Stems and pods can also develop a covering of spots or flecks.

Severe infections can result in defoliation. Protectant fungicides should be applied as soon as the first lesions appear in the crop or by mid-flowering at the latest. A second spray should be applied 3-4 weeks later if spotting continues to develop on the upper parts of the plant. A third spray is seldom required as sprayer damage can cause more yield loss than late infection of Chocolate Spot.

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. Various fungicides are effective for Rust management and may improve yield etc. 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.