Q What is chocolate spot on broad beans?
A This foliage disease (Botrytis fabae) of broad beans is caused by a close relative of grey mould, Botrytis cinerea. Grey mould will also sometimes be found on the plant after a chocolate spot infection.
Caption: Chocolate spot appears from mid-winter onwards
Q How do I recognise chocolate spot on broad beans?
A Chocolate spot causes reddish-brown blotching of broad-bean leaves, stems and pods. It can spread rapidly, becoming brown-black, joining up and covering leaves. In addition, the leaves are covered with a grey mould in wet weather. The flowers and young pods are quickly ruined, and the disease will penetrate older pods to discolour the seeds inside.
Q When is chocolate spot likely to occur on broad beans?
A Chocolate spot may appear from mid-winter onwards. Wet weather is needed for the disease to spread. Autumn-sown broad beans are most likely to be infected, but even in spring the disease can be troublesome.
Q What can I do about chocolate spot?
A Broad beans grown in fertile, well-drained soils resist attacks. So make sure your soil has plenty of phosphate and potash. If your soil lacks these nutrients, add 30g a sq m each of superphosphate and sulphate of potash to the bean plot in winter.
To improve the drainage in your plot, consider making raised beds for vegetable growing. In wet or cold districts, raising overwintered broad beans under cloches may help to prevent disease.
It's also a good idea to plant autumn-sown beans at wide spacings, so airflow through the plants is enough to keep the humidity down. When you sow, leave 50cm between the rows and 10cm between plants.