Grey mould of potato
A common potato disease
What is it?
This common potato disease is caused by the fungus Botrytis cinerea. Botrytis is favoured by wet or humid conditions and has a very wide host range. It produces numerous airborne spores that may land on a host plant and germinate to create a new infection. The fungus usually first colonises damaged or senescent plant parts, but once it has a foothold it can spread rapidly into adjacent healthy tissue. Grey mould is not a particularly damaging disease on potatoes, but it can be very common if factors causing extensive damage to the foliage (e.g. hail, strong winds) are followed by prolonged wet weather. It is, however, important to distinguish grey mould from the devastating disease late blight, and this can be very difficult visually if subsequent dry weather causes the affected tissue to dry out temporarily, with little or no obvious fungal growth.
What does it look like?
Under wet or humid conditions, Botrytis can be identified as a dense grey-brown fungal growth on soft, decaying leaves, petioles, and stems. Under dry conditions the affected plant parts may appear light brown in colour with little fungal growth visible. Tuber symptoms are rare but can develop during storage, where the surface becomes wrinkled and tissue becomes flabby, eventually developing a watery, brown decay.
How is it spread?
The fungus can overwinter on diseased plant tissue or as small (a few millimetres), resilient resting structures called sclerotia. New infections can arise from crop debris from the previous season, from germinating sclerotia or from spores produced on the many other plants this fungus can attack.
How can I control it?
Control of grey mould on potatoes is not usually required, although it may be best to dispose of severely affected plants. As mentioned previously it is, however, important to determine whether leaf spots and lesions on stems are caused by grey mould or late blight – if the latter disease is responsible prompt action will be required.To prevent tuber infection by grey mould avoid damage at lifting, do not store tubers wet, and store under cool conditions.
When do I test?
Symptoms of grey mould are likely to be observed from mid-summer onwards (although they could appear earlier if spring and early summer are very unsettled). Testing should be carried out as soon as symptoms are detected and monitoring for symptoms should continue until cropping is complete.
What do I test?
Where possible, select areas of leaf or stem which show symptoms of disease. For best results include material where healthy and diseased zones meet. Do not use completely dead plant material. As a general guide, samples of leaf material should be approximately 25mm x 25mm. Samples of stem should be small, and no larger than 5mm across.