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Winter beans and their diseases

8 January 2018

Winter beans showing slightly uneven emergence and growth in poor autumn drilling conditions.

Winter beans usually produce strong vigorous plans and can be so healthy and green that late season chocolate spot caused by botrytis is almost welcome or they might stay green until Christmas. Compared to other combinable crops they suffer relatively few diseases and need correspondingly less in terms of fungicide inputs. Sprays are usually targeted at managing chocolate spot but there might be a few other problems to factor in. There has obviously been a lot more interest in growing pulses for greening reasons but it seems that a key legislative change for 2018 is that beans grown on EFA land will not be able to receive any plant protection products so must be grown pesticide free.

There are a few diseases to watch for. We’ve seen a little bit of ascochyta leaf spot over the years in the SRUC crop clinic , especially in wet years. It shows itself as small brown lesions often with a grey bleached centre with characteristic dark dots, but it is relatively rare because, being mainly seed borne, the best control is not to home save and to buy seed that has been tested at less than 1%. It is also worth noting that home saving increases the risk of stem nematodes (Ditylenchus) so that is another driver for buying quality seed. Aschochyta can transfer from volunteers too but again that is a problem easily avoided through good rotations. Many winter bean varieties such as Wizard have good resistance to ascochyta so it has all the hallmarks of a disease best controlled by cultural and agronomic measures rather than through fungicides.

Downy mildew in winter beans is less common than chocolate spot and not well controlled with the same chemistry.

Downy mildew on beans is a trickier one to manage. It is favoured by damp conditions and is poorly controlled by the fungicides commonly used to control chocolate spot. It produces a greyish white mildew symptom and, where it appears early it can be necessary to control it. Generally this requires the use of fungicides with special off-label approvals (EAMUs) and because these change regularly and often apply to specific batches of product only it is worth getting specific advice if you see the problem. Varieties differ in susceptibility so selecting varieties with high ratings like Karioka or Blueman will help where you often see the problem

Botrytis in beans causes chocolate spot – well named as the foliage turns a deep dark brown when spots start to join up. It starts as small reddish-brown spots and in wet conditions quickly becomes worse. It enjoys Scottish cool damp summers and thick crops where humidity is high are also more prone. Where it appears early then fungicides at early flowering are applied with a range of effective actives available such as azoles, strobilurins and SDHIs but labels must be carefully checked for approvals for beans. Sometimes a second spray is necessary up to a month after the first to continue to manage the disease but much beyond the end of flowering it is seldom worth controlling. Given the crop has to die of something eventually, if the disease arrives late it can just be left to do its work.

There are a few other diseases that appear occasionally in bean crops. Rust in beans is unusual in our clinic samples but is managed by the azoles often applied to manage chocolate spot. Sclerotinia affects beans too so it is worth considering that risk when planning rotations and thinking of other susceptible crops like potatoes, oilseed rape or vegetable crops like lettuce or carrots. If it appears in beans there is no chemical means of control.

Fiona Burnett (SRUC) for the Farm Advisory Service

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