There is uncertainty about how climate change might impact on diseases of potato crops but the threats come in two forms. On the one hand, there are several tuber rot diseases which appear infrequently. Consequently they have not been studied as well and it is often difficult to tell whether we are looking at opportunists (organisms which exploit breakdown resulting from another cause) or diseases which are kept in check because conditions are not favourable. On the other hand, there are diseases which do not currently occur in the UK. Some of these, like brown rot and ring rot, are already recognised as serious threats and plant health authorities are doing everything they can to prevent their accidental introduction through imports. Others may not be recognised as threats but may become so, because our climate is gradually warming to meet their temperature threshold.
Of the diseases already established in the UK, pink rot and watery wound rot have been described in earlier editions of Potato Review (September and November 2008). Growers in North America are advised to avoid harvesting tubers with temperatures above 18.5ºC and 21ºC in order to avoid pink rot and watery wound rot respectively. These temperatures are not out of range for current normal English summers and the implications are clear.From the limited amount of information about violet root rot and rubbery rot we would expect warmer weather to favour both diseases. Violet root rot is caused by a fungus, Helicobasidium brebissonii (also referred to as H. purpureum). It can cause symptoms in a wide range of crops but is best known from the damage it causes in carrots and sugar beet. Rubbery rot caused by the fungus Geotrichum candidum, appears to be relatively rare but easily recognised. However, it is also found frequently in association with other types of rot as an opportunist. Before ringing alarm bells it would be useful to know how often these two are actually found. Progressive growers and agronomists can help here by following up problems diligently as they occur and seeking good diagnostic advice.
We know a lot about bacterial wet rots as a result of the serious consequences of blackleg caused by Pectobacterium atrosepticum. Many growers in England will also have come across symptoms similar to blackleg but caused by Dickeya dianthicola (previously known as Erwinia chrysanthemi). It prefers warmer temperatures so there is a clear danger that with climate change the risk becomes greater even in Northern England and Scotland. There are other, unrelated, bacteria which are capable of causing rots. Members of the genus Clostridium should be put on the alert list. When in experiments potatoes were infected with a mixture of two pathogens at 16ºC, P. atroseptica predominated in the rotten tissue. At 22ºC Clostridium was dominant. The question is whether the latter has the same capacity to cause rots or is just as aggressive as P. atroseptica, given the right conditions.
Looking across the horizon there are several tuber rot diseases which could cause harm if they became established here. Stem rot is caused by Sclerotium rolfsii and affects both stems and tubers. It was reported in Northern Italy in 2005 and this serves as a reminder that during extreme weather conditions which may only occur infrequently, diseases will travel. Charcoal rot and stem rot (also known as southern blight) are associated with temperatures in the upper twenties and so there is less likelihood of these becoming established in the UK.
With so many organisms apparently capable of causing rots it is important to remember that tubers are basically robust. Minimising damage, correct procedures for curing and avoiding condensation in stores have reduced rots to present levels – these are general principles which will also protect against future threats. Healthy seed and good hygiene will further contribute towards preventing the spread of unwanted diseases. What is needed, though, is a keen eye for anything that seems out of the ordinary and a willingness to track down the cause. This will ensure that any new threats are detected early and in time for appropriate action.