PGRO CROP UPDATE: 8 MAY 2015

Becky Ward, PGRO principal technical officer, comments on the latest situation for pests and for diseases in pulse crops:

PESTS
Pea moth

Growers should remember to order their pea moth traps ready for monitoring later in May. Although emergence may be delayed following the cool spring this year, traps should be placed in crops by mid-May.

The monitoring system consists of a delta trap containing a pheromone attractant. These should be placed in crops just prior to commencement of flowering. Traps are available from OECOS Limited, 11a High Street, Kimpton, Hertfordshire SG4 8RA. An alternative monitoring system is available from Agrarian Ltd., The Old Brickyard, Ashton Keynes, Swindon, SN6 6QR.

Adult moths emerge from the soil of previous year’s pea crops and fly to flowering crops in early to mid-summer. The females lay eggs on the foliage and eggs hatch between 8 to 17 days later depending on temperature. The larvae move over the surface of the plant towards the pods where they bore into the pod and feed on the peas for about 3 weeks. Pea moth caterpillars damage pea quality, particularly in areas of intensive combining pea production, where locally high populations can develop.

Control of the larvae must be carried out following egg-hatch and before they reach the pod, and the monitoring system will accurately predict spray date. When traps are in place they should be monitored three times each week and a threshold is reached for combining peas when 10 or more moths are caught in either trap on two consecutive occasions. Timing of sprays is related to egg development and this is affected by temperature. Starting from the day on which the threshold is reached, record the daily maximum and minimum temperature and set these on the outer scales of the calculator supplied with the Oecos traps. The figures exhibited in the window are added each day until the total reaches 90. Alternatively, a spray date can be obtained from the PGRO website (www.pgro.org), based on a computer prediction, 3 - 4 days after reaching a threshold.

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.

Where the acceptable level of damage is much lower in vining peas, the threshold catch for combining peas is not suitable and therefore the traps should be used as a guide as to the presence of moths. In areas where damage has been a problem in the past, crops which are at the first pod set stage should be treated with a single application.

Silver Y moth
Silver Y moth caterpillars feed on the foliage and pods of peas and beans, but most of the economic impact is felt through contamination of vining pea produce at harvest, causing crop rejection by the processors.

The moths are large, grey-brown, day-flying Noctuid moths with distinct silver ‘Y’ markings in the middle of each forewing. They migrate from North Africa during early to mid-summer and are attracted to a wide range of plant hosts. The caterpillars are bright green with a white strip along each side of the body and a darker line along the back. When disturbed the caterpillars roll into a ball.

A monitoring system consisting of traps containing a pheromone attractant is available. The system is available from Agrarian Ltd. In peas the trap is placed at crop height in the field in late May and moths, caught in the base, are counted on three occasions during each week of monitoring.

A threshold is reached when a cumulative total of 50 moths has been reached by the time that the peas have reached the first pod stage. When the threshold has been reached, a single spray of a pyrethroid insecticide, approved for pea moth control, should be applied 10 - 14 days later. This application will control both large and small caterpillars, which fall off the plants before the crop is harvested.

Aphid monitoring
Aphid monitoring is carried out in the UK by Rothamsted Research, SASA and FERA. Updates on suction trap catches and activity of crop aphid species can be found in the summaries provided by AHDB on the PGRO website at www.pgro.org or at www.hgca.com.

The AHDB Aphid News bulletins provide information about when aphids are migrating at key times of the year. Information in the newsletters should be used to optimise the use of insecticides, time treatments better and reduce harm to beneficial insects. This will also lower the risk of selection for insecticide resistance by reducing unnecessary or wrongly timed sprays.

Support for this project is provided by BBSRC, The Lawes Agricultural Trust, PGRO, BBRO, AHDB and other industrial supporters.

DISEASES
Chocolate spot in Field Beans

Disease pressure in field beans has remained low due to the settled and dry weather experience over the last few weeks. However, while the weather forecast is less settled for a period, chocolate spot may start to develop in crops.

Small, round discrete chocolate coloured spots develop on the lower leaves of plants. These lesions may coalesce to form unevenly shaped grey-brown patches on the foliage as the disease develops. Both winter and spring beans may develop aggressive chocolate spot and brown streaks may appear on stems. In severe infections plants may defoliate.

The disease is favoured by overcast, humid conditions and particularly when long periods of wet and overcast weather are experienced during the summer. Fungicides applied at the first signs of disease help to prevent rapid infection, and in winter beans a spray at the onset of flowering and repeated at first pod set has been shown to be an effective programme.