CROP UPDATE NO 4 - INSECT PESTS and FIELD BEAN DISEASES - 28 MAY 2020

Insect Pests….

Bruchid beetle

Bruchid beetles emerged from overwintering sites in mid-April and will be active in field bean crops during warm weather, particularly in winter beans that are flowering and forming pods. If you’re growing field beans for human consumption, insecticides should be applied when the temperature threshold of two consecutive days at 20^°C has been reached and around 50% of pods on bottom trusses are 2 cm long. Sign up for the Syngenta BruchidCast® forecasting service by going to www.syngenta.co.uk/bruchidcast.

The forecast predicts when the temperature threshold has been reached in different regions of the UK and indicates suitable conditions for spray applications. Growers should check that first pods have formed on the bottom trusses. To avoid risk to foraging bees spraying should be carried out very late in the evening or at night-time. Due to the warm temperature over the last week or two, spray thresholds may have been, or will be reached for winter beans.

Aphids
Both pea aphids and black bean aphids have been reported in crops and in later drilled crops there may be an increased risk of damage caused by virus transmission.

The active substance thiacloprid is approved in combining peas, vining peas and field beans and will give good control of aphids if products are available. This is an option to control the earlier virus-transmitting aphids and may be used up to 2 times in a programme in alternation with non-neonicotinoid insecticides of a different mode of action. Do not use Biscaya in consecutive sprays. Check with processors before use.

The active substance pirimicarb is approved in vining peas, combining peas, field beans and broad beans, and in French beans, runner beans and edible podded peas as an EAMU (2152/18) and will control large aphid colonies. Use is limited to a single application in any of the above - mentioned crops.

Flonicamid (Teppeki) is approved in vining peas, edible podded peas, French beans and runner beans (EAMU 3739/18) and will give good control of aphids. Do not apply to crops in flower or to those in which bees are actively foraging. Do not apply when flowering weeds are present. Use is limited to a single application in any of the above - mentioned crops.

Pyrethroid products may give partial control of aphids.

Pea moth

Pea moths have been recorded at several locations in high numbers. Predicted spray dates can be obtained from the PGRO website at https://www.pgro.org/pea-moth 3 to 4 days after you reach a threshold in traps.

On the predicted spray date, crops which are at the first pod set stage, or which have flowered should be sprayed, but later crops should only be sprayed when they reach first pod set. Crops with flat pods are susceptible to damage. A second application should be applied 10-14 days later.

For products available to control pea moth please see our technical updates TU14 (checklist of fungicides and insecticides for vining peas) and TU15 (checklist of fungicides and insecticides for combining peas) at www.pgro.org.

Field bean diseases …

Rust
Following recent warm weather and cool nights with high humidity, field bean rust (Uromyces fabae) may start to develop in crops. Both spring and autumn-sown crops are susceptible, although later-sown spring beans may be at higher risk. The disease first appears as pustules on the lower parts of the plant, developing as small, orange-brown spots. Leaf tissue around the pustule appears pale yellow and as the disease develops, large areas of the leaves and stems develop symptoms. In severe infections, plants may start to defoliate, leading to poor pod development and yield loss. Rust can be controlled with a range of active substances and these can be found in the PGRO Technical Updates TU16 (Fungicides and insecticides for field beans) and TU17 (Checklist of fungicides, insecticides and molluscicides for broad, green and runner beans) at www.pgro.org.

For more information about legume crop management you can contact the PGRO advisory service on 01780 782585. Contact details for the advisory team are also available on the PGRO website at www.pgro.org.

Irrigation in peas

The timing of irrigation in peas is crucial, should be related to both specific crop growth stages and soil moisture deficit, and can lead to good yield responses.

The following comments are based on work by P.J. Slater and colleagues at HRI, Wellesbourne (now Warwick Crop Centre).

Before flowering – Soils may be near to field capacity when peas are sown, and irrigation rarely increases yield if applied before the start of flowering, although it does increase haulm. In several experiments, irrigation during vegetative growth depressed pea yields. Irrigation should not be carried out during this period unless: (i) The seedbed is very dry and adequate germination would not otherwise occur. (ii) The crop is severely wilted because of drought.

At the start of flowering – Peas are most responsive to irrigation when the first flowers are opening and if a single application is to be made it should be at this stage. The plant is thought to be most responsive at this stage because the root system has ceased to grow, making the plant more vulnerable to water shortage.
Yield increases from irrigation at this stage are often very substantial, even up to 50% due to more pods contributing to yield and more peas per pod. Haulm weight is also increased.

Pod swelling – Crops irrigated during pod swelling have also shown up to 20% increases in yield. The number of peas per pod and the mean weight of peas are both increased by irrigation at this time. The weight of haulm is not affected.

Effect on yield – Response to Irrigation.

Effect on maturity – Irrigation during vegetative growth and early flowering has been found to have little effect on the rate of maturation, but applications during pod swell can delay maturity. A predictable delay of approximately 2 days is suggested and must be taken into account when growing vining peas.

Amount of water – It is suggested that during vegetative growth and at the start of flowering 25mm can be applied but at the pod swelling stage the amount should not exceed 30mm.

(Edition Number 4: CB20-03)