Crop Update - 19 March 2019

INSECT PESTS ...

Pea and bean weevil have been found in traps in Cambridgeshire, and at sites monitored by the Rothamsted Insect Survey, since warm weather in late February. A monitoring system is available that detects adults as they migrate in early spring, consisting of five cone traps containing pheromone lures. It can be used to aid decision-making in the following ways:

• Identifies seasons when weevil numbers are low and crops do not require treatment.
• Identifies time of peak activity and best timing of applications.
• Reduces the need for prophylactic spraying.
• May allow the selection of drilling time to avoid peak weevil activity.

Traps are sited on a grass margin or headland of a field that was cropped with peas or beans in the previous year and should be checked three times each week.

A threshold is reached when an average of more than 30 weevils is recorded per trap on any one occasion.  When a threshold is reached, any crops that have just emerged or are likely to emerge within 10 days are likely to be at risk. Insecticides should be applied at first signs of crop damage, seen as u-shaped notching on leaf edges, especially where there is a history of severe damage. A second spray may be required if crops are growing slowly.

If a threshold is not reached or occurs more than 10 days before crop emergence, insecticides are not required. Winter beans seldom require treatment as they are well established before attack occurs.

Traps are available from Agralan Ltd:  https://www.agralan-growers.co.uk/

Aphids - Aphid flights are predicted to start approximately two weeks earlier than would historically be expected, due to warm weather in February https://ahdb.org.uk/aphid-news.

Mean temperature in January and February is the best predictor of aphid flight and abundance, especially for species that overwinter in active stages, rather than egg stage. It’s therefore possible in some regions that first flights of pea aphid could occur in mid- to late-April and that the risk of early virus transmission may be high in 2019.

For information about aphid management in legumes go to http://www.pgro.org/agronomy-guides publications/ 

or contact the PGRO advisory service on 01780 782585.

DISEASES ...

There have been some reports of downy mildew, chocolate spot and Cercospora in winter beans.

Downy mildew can be severe in spring beans but often doesn’t warrant treating in winter beans. The characteristic symptoms include pale patches on the upper surface of leaves with a greyish-mauve, velvety growth on the underside. The growing points may be chlorotic and distorted and pod set can be reduced. The disease is favoured by cool, humid conditions. Several spring bean varieties are resistant to infection: http://www.pgro.org/downloads/A4-rec-list-2019.pdf.

Treatment should be applied when lesions can be found on about 25% of plants and the crop has started flowering. Regional risk forecasts are available from Crop Monitor: www.cropmonitor.co.uk.

Straight Metalaxyl-M (SL567A) can be used in field beans (EAMU 0917/13). Chlorothalonil + metalaxyl-M is available in broad beans only.

Chocolate spot is encouraged by long periods of overcast and wet weather conditions. Winter beans are more susceptible to infection, especially where plant populations are high. Spring beans and broad beans can develop chocolate spot during wet spring and summer conditions. The disease develops as small, circular, chocolate coloured spots on the lower 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 at first pod if spotting is seen on the leaves. If severe spotting is seen earlier in the season, the first spray should be moved forward. A second spray should be applied 3-4 weeks later if spotting continues to develop on the upper parts of the plant.

A range of products and mixtures are approved. 

Cercospora leaf spot has been reported more frequently in UK field beans in the last few years and there have been reports of Cercospora in winter bean crops in early 2019. The disease persists in crop residues in the soil and is dispersed by wind and rain to infect young plants, particularly in fields planted close to previous year’s crops. In some countries the disease can cause between 5 and 10% yield loss when infection is severe. Symptoms are similar to those displayed by Ascochyta, but no pycnidia are present at the centre of lesions. Lesions are dark brown to black with a grey centre, and with a characteristic zonal pattern.

Applications of products containing tebuconazole or chlorothalonil are effective to control the disease.

Ascochyta  There have been no reports so far of Ascochyta in winter beans and the principal means of transmission is by infected seed. Seed screening and the use of healthy seed has led to a reduction in the incidence of the disease in most crops in recent years, although it is reported occasionally.

The disease develops on seedlings shortly after emergence and pycnidia produced within the lesions contain spores that are easily spread by rain splash. The first signs of infection are small round lesions on young leaves, which become visible in winter beans in early spring, or later in the summer in spring-sown crops. Lesions have a light brown to grey centre, often surrounded by a darker grey to black area. Within the lighter part of the lesion, there are small pinpoint-sized protuberances, the pycnidia, which appear as the lesions become older. The disease is favoured by wet weather.

Fungicides including azoxystrobin will give good control when applied to the crop early before the disease becomes well-established.

(Edition:   Number 2 : CB1902)