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Crop Health update: Winter beans

18 December 2018

Nitrogen fixing crops such as winter beans obviously have advantages in a rotation, adding to soil fertility as well as adding diversity to rotations. Yields can be variable between seasons – those of the winter bean crop tend to be more stable than spring legumes as it is less vulnerable to short periods of stress compared to the spring crop. However,  2018 was not a good year for winter bean yields. A hard winter and the very hot summer did not suit the crop, and there were significant issues with quality to contend with on top of the disappointing yields. Yields in the UK were sufficiently poor that seed was in short supply and there was a temporary derogation to accept lower germinating seed just to have sufficient. Lower yields have led to higher prices though so beans have remained a consistent crop choice this back end. Growers were urged to check germination levels if they were home-saving and adjust rates accordingly because of the concerns over seed quality. Winter beans are a relatively minor crop in Scotland but despite the worries about seed quality emergence generally looks to be even and young plants are going away well. Drier conditions in the south of the UK have lead to patchier crops, but winter beans have been a second choice option for some growers in the south where oilseed rape has failed to establish.

In terms of crop protection winter beans are a relatively low input crop which is appealing. It usually requires some plant protection inputs to manage chocolate spot caused by Botrytis in wet summers. However, if beans are grown as part of an Environmental Focus Area (EFA  NFIX)  then no plant protection products are permitted between drilling and harvest of the crop so diseases such as botrytis must just be allowed to run their course and earlier senesce has to be accepted. Given that the crop has to harvested in an EFA by the 1 August to favour ground nesting birds, early senescence is a positive rather than a negative feature. Symptoms of chocolate spot are more likely in wet summers and in thick crops and fungicide treatment at early flowering is usually required (in non EFA NFIX crops). A second spray is sometime required but should be judged at the time and only applied if wet conditions persist.

At this time of year the main risks are seedling blights and foot rots, often caused by Fusarium species. Beans are less prone to these than peas which is another reason that they tend to be more resilient in Scottish conditions We also tend to be at lower risk in the north as rotation lengths between beans are good and these trash borne issues have less chance to build up. Downy mildew can be a risk in wet years but none is reported so far this season. Downy mildew is poorly controlled by most bean fungicides and control, if needed, tends to be with off-label approvals (EAMUs) and these change frequently and often apply only to specific batch numbers of a product so carful advice and checking is needed. Don’t assume if you’ve used a product before that the approval will still hold.

The winter bean crop can be more prone to attack by Ascochyta leaf spot especially in wet conditions but this is a nice example of where seed health standards can nip off a problem at source as the disease is mainly seed borne. There is some risk from volunteers but in sensible rotations these should not be an issue.

One of the major causes of reduced quality for the 2018 bean crop was bruchid beetle damage, the larvae of which leave little round holes in the beans. These are not as common in Scotland as in England as they like warm temperatures around pod set. As with other examples of integrated pest management, pyrethroid sprays should only be used where there is a history of damage and when temperatures at pod set are greater than 20oC.

Fiona Burnett, SRUC for the Farm Advisory Service

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