Wednesday, 6 May 2009

Climate Change and its potential consequences for stored potatoes

This article is based on a literature review by Paul Gans and Glyn Harper (Sutton Bridge Experimental Unit) commissioned by Potato Council Ltd and was first published in Potato Review, March 2009, p30(
With improved storage techniques heavy losses due to bacterial and fungal rots are, thankfully, infrequent events. But what if the balance between stored crops and the organisms capable of causing rots were to change? Could similar losses as a result of rotting become commonplace again? To answer that question we might first think about how climate change would impact on potato crops. Generally the effect on average temperatures may seem moderate, at least in the short term. Predictions vary between 0.1ºC and 0.5ºC per decade but you need only think back and compare the hot summer of 2006 with the wet summers of 2007 and 2008 to realise that it is the peaks and troughs which can change the threat of disease dramatically. It is likely that fewer cold springs would lead to earlier planting in many parts of the UK and consequently warmer temperatures at harvest. Changes in weather conditions around harvest time and early loading may change storage techniques and the use of ambient air and refrigeration to cool potatoes in stores.

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.

Healthy seed reduces risk of Skin Spot

This article was first published in Potato Review, January 2009, p30 (

Skin spot is an old disease with which have to live in spite of our knowledge about the disease. Why the concern? A minority of varieties are especially susceptible to the characteristic skin blemish. And because markets are quite specific about filling certain niches with particular varieties the industry is sometimes committed to growing susceptible varieties. Skin spot pustules usually develop during the later part of the storage season. For example, a crop of King Edward stored for packing can turn from being quite presentable in early autumn to showing serious defects around February. As time goes on, skin spot can also seriously influence suitability for crisping.

There is, however, another less visible effect. The fungus which causes skin spot, Polyscytalum pustulans, can kill the growing tips of buds. In serious cases this can result in failure to emerge. In less serious infections it can lead to uneven stem numbers and subsequent variations in tuber size which will be detrimental to the value of the crop. The extent to which this is a problem today is not clear since it is difficult, when stem numbers are low, to track this back to P. pustulans as one of several possible causes. However, it is worth noting the value of recording stem numbers as a barometer of how well a crop is performing. In any case buds are far less likely to be affected once they have started to develop into sprouts. So even for unchitted seed there is a beneficial effect in making sure that at least dormancy is broken and there is some movement in the eyes before planting.

P. pustulans is seed borne so the best way to deal with skin spot is to purchase uninfected seed. Stocks start off in the first and second generation of multiplication as being free from skin spot but over the generations crops will become infected. At first just a few plants will carry the disease but gradually the incidence will increase. You can see the infection in crops as a light brown flecking on the base of the stems. It is for this reason that when I am inspecting seed crops I always look at the condition of the underground part of the plant. Although I would not go so far as to say that you can predict skin spot from these lesions, clear white stems are a good indicator of a healthy crop. In fact, if skin spot has got into a seed crop it is likely that silver scurf is also present, though it does not leave the same tell-tale marks. It is also clear that the incidence of skin spot is quite variable from crop to crop and is probably much lower than it used to be.

There are several factors that might explain why we can expect to see less skin spot today. First of all there are usually fewer generations between the first multiplication for seed and the ware crop. Secondly, seed producers understand the importance of 'dry curing' which means giving wounds a chance to heal while at the same time ensuring low relative humidity (below 90%). Practices vary between different producers. Some use drying tents, letterbox systems or positive ventilation, others rely on the store ventilators to do the job. The third factor is store hygiene cleaning gets rid of the spores which might infect crops and there are some great vacuum cleaners and approved disinfectants available to help manage a clean store.

But how do you know whether you have done a reasonable job? In order to be sure that all the correct measures are in place you need to monitor the crop. The 'eye plug test, which consists of cutting an eye from each of 100 seed tubers and incubating these at high relative humidity so that dormant fungi will develop, is a well established procedure. The technique helped an earlier generation of researchers to understand the basics of skin spot and a number of other diseases. The eye plug test can give variable results depending on the time of year when it is carried out while the spores which cause skin spot are more difficult to recognise than others. It is encouraging, therefore, to hear that Sutton Bridge Experimental Unit, in collaboration with CSL, will be continuing research on molecular diagnostics. A test has already been developed but the work is being extended to establish the robustness of the assay and to obtain more information relating detectable levels of skin spot at harvest with risk of disease during storage. It is to be hoped that such a test will enable agronomists and seed producers to readily monitor whether their efforts to reduce infection have paid off.

There are a number of factors which can determine the incidence of skin spot. Spores can survive in soils for more than seven years but an interval of four or five years is thought to reduce contamination significantly, at least sufficiently so as not to pose a threat to the ware crop. Low storage temperatures (less than 4°C) will encourage the development of skin spot while the sprout suppressant chlorpropham (CIPC) can make the problem worse. This is thought to be because the chemical suppresses wound healing so the advice not to begin CIPC treatment until the crop is fully cured is also relevant to the management of skin spot symptoms. Skin spot can be reduced in storage by applying imazalil (Fungazil 100 SL) at or immediately after harvest. This measure is, however, not as effective as 2-aminobutane and a suitable replacement has yet be found.

  • Know the susceptibility of the variety you are growing
  • Practice long rotations to minimise soil contamination
  • Avoid poorly drained soils
  • Use healthy, low generation seed
  • Harvest the crop early and ensure that it is well dried
  • Ensure good wound healing
  • Avoid the use of CIPC soon after harvest
  • Avoid reducing the holding temperature below 4°C
  • Fungicide to reduce skin spot in store should be applied at or immediately after harvest

Are you recognising Watery Wound Rot?

This article was first published in Potato Review, November 2008, p28 (

If you look at some text book pictures of watery wound rot, you might think that this is a condition that is easily recognised. The tuber centre often appears grey and wet and the North American name shell rot accurately describes the way in which the rot hollows out the tuber, leaving the outer layer as a shell. However my impression of the disease changed radically when rapid detection kits became available. I found that many difficult to identify rots tested positive for Pythium spp., a group of moulds, one of which, Pythium ultimum, causes Watery Wound rot. I found that often these did not look at all like the text book pictures. One of the reasons we see these complications is because once a tuber starts to rot, other organisms which are lying dormant also become active. One of the principle culprits can be Pectobacterium atroseptica, which causes blackleg. There are several other bacteria which can cause tuber wet rot. The lessons to be learned from this are firstly that we need continuously to carry out tests to corroborate our observations and secondly that watery wound rot may be more widespread than we realise.

In North America, Pink Rot and Watery Wound Rot are often lumped together as “water rots”. This reflects a similarity between the symptoms and also the circumstances under which the diseases occur. Like Pink Rot, Watery Wound Rot is associated with warm weather and damage during harvest. However the critical temperature for Watery Wound Rot is a little higher than that for pink rot. Whereas to prevent Pink Rot, advice in the US is not to harvest until the temperature is below 18.5°C, for Watery Wound Rot this figure is 21°C. Unlike Pink Rot infection appears to be dependent on the presence of wounds. This has a consequence for fungicide protection. Metalaxyl is better at controlling Pink Rot than Watery Wound Rot. The explanation lies in the fact that much initial Pink Rot infection arises from invading zoospores prior to harvest. Most Watery Wound Rot appears to arise from soil contamination of wounds. In this way the disease appears to bypass the metalaxyl barrier. Like for Pink Rot, metalaxyl resistance has been found for Watery Wound Rot. Also like Pink rot there are variety differences offering scope for breeding watery wound rot resistant varieties in the future but there is little choice of variety to avoid the disease today.

Pythium ultimum, the organism which causes Watery Wound Rot, can cause damping-off of seedlings and rotting of roots, stems and fruits of wide range of crops. As far as we know there are no special strains which prefer one crop to another. It forms oospores, sporangia and zoospores and these may be instrumental in long term survival. However it appears to have developed its own special strategy for competing with other soil organisms by germinating whenever there are nutrients available in the soil and then retracting again to a form in which it can survive long term. This strategy may also be the key to manage the disease if only we could understand the positive and negative factors which affect this process. Some soils may simply be unkind to P. ultimum and if we understood why, we could exploit that.

Watery wound rot, on a scale that is reported in North America, is unlikely to be an issue in this country. However the number of cases where watery wound rot has been confirmed raises the suspicion that the organism which causes watery wound rot is around and may be causing more harm than we realise even at below threshold temperatures. Sorting out what has gone wrong, once you are confronted with serious rotting is a complicated business. Fortunately methods for identifying different causal organisms are improving all the time. For some time now there have been kits* which are specific for Pythium spp. and for Phytophthora spp.. This means that they can be used in combination to tell suspected pink rot from suspected watery wound rot, but will not tell the difference between Phytophthora infestans (blight) and Phytophthora erythroseptica (pink rot). The essential measures to manage watery wound rot are similar to those for pink rot. These measures consist generally of damage prevention at harvest and optimal conditions during the early storage period. Additionaly, when things go wrong it is well worth spending some money either on some test kits or on sending a sample to a reputable lab.

  • Be suspicious of any wet rot and test for watery wound rot

  • Ensure good skin set prior to harvest

  • Minimise damage during harvest

  • Remember that risk increases with higher temperatures at harvest (21°C is the recommended threshold for harvest in North America)

  • Prevent infected tubers from going into store

  • Ensure adequate curing to promote wound healing

  • Tuber temperatures should be taken to below 10°C as soon as is practicable

  • Prevent condensation in store
*Pythium and Phytophthora kits are available from Neogen Europe Ltd, Ayr, Scotland, KA6 5HW. and from Forsite Diagnostics Ltd, Sand Hutton, York Y0411LZ.

Tuesday, 5 May 2009

Look out for Pink Rot

This article was first published in Potato Review, September 2008, p30 (

HOW serious is pink rot in the UK? Whenever I ask growers and agronomists this question, the impression I get is that it does not occur very frequently. However, for the individual grower who is affected the consequences can be serious and even if an outbreak has been contained it is valuable to know how to keep it that way. In the United States, pink rot and watery wound rot are considered to be of greater significance than tuber breakdown caused by blight. This appears to be associated with warmer tuber temperatures when crops are being harvested and a tendency towards shorter rotations. Pink rot is caused by Phytophthora erythroseptica, an organism that shows many similarities to P. infestans, which is responsible for late blight.

We have heard a lot about blight in recent years as a result of changes in the genetic make-up of P. infestans and the potential formation of oospores which are able to survive for long periods in soil. P. erythroseptica also has the ability to form oospores but in contrast to the blight pathogen this does not need two mating types to be present. Each strain has the ability to develop both male and female sexual organs and to produce copious amounts of oospores from infected tubers left after harvest and from infected stems and underground parts of the plant. We are not yet sure how important oospores are to the way in which blight outbreaks develop, at least in the UK, but pink rot oospores accumulate in soils which then become infective, especially in damp patches. The reason that these damp patches play a role is because they enable the oospores to germinate and form a sporangium from which zoospores are released which in turn releases zoospores. It is the zoospores that will infect the next crop of potatoes but in order to find their target they need water in which to swim. In this way tubers become infected before it is time to harvest the crop and these become a source of further spread as a result of the harvesting activity.

Pink rot is quite wet and the rotten tubers will contaminate equipment. Healthy tubers may be damaged and the wounds become points of entry for the contamination on the harvester. An understanding of the key points in the life-cycle gives us several clues about how to manage this disease and reduce its impact. Infection of the following generation of tubers depends on zoospores reaching their targets and the chances of this happening can be reduced by avoiding excessive soil moisture. In the United States potato growers have approval to use metalaxyl to prevent tuber infection. The method of fungicide application has to be adapted for this purpose and currently there is no similar approval in the United Kingdom. After many years of successful metalaxyl use there are now resistant strains of P. erythroseptica in the US though because the disease is soil borne and moves relatively slowly from field to field it is likely that the spread of these strains can be monitored and to some extent contained.

The second stage of the disease is its spread from tuber to tuber. This can be reduced or prevented by ensuring a good skin set, by minimising damage on the harvester and by ensuring that wounds have a chance to heal at moderate temperatures of around 8-10°C. Experience in the US suggests that tubers are at greatest risk when the tissue is above 18.5°C. That may seem out of normal range in many instances but it is worth bearing in mind that choosing an earlier harvest date may mean moving closer to this temperature. Lifting at temperatures higher than 18.5°C should be avoided and once they have been harvested, their temperatures should be brought down to 10°C as quickly as possible, observing the usual safeguards to prevent condensation. Longer rotations will help to reduce the build-up of soil contamination and British experience with other pests and diseases, PCN and Rhizoctonia, for example, has already convinced many growers of the wisdom of growing fewer potato crops on the same land. Temperatures may rise as a result of climate change but higher temperatures at harvest may also come about as the result of earlier lifting dates which are aimed at reducing the impact of black dot and black scurf.

• Avoid excessive moisture, especially towards the end of the season.
• Ensure good skin set prior to harvest.
• Minimise damage during harvest.
• Tuber temperatures must be below 18.5°C for harvesting.
• Prevent infected tubers from going into store.
• Ensure adequate crop curing to promote wound healing.
• Reduce tuber temperatures to below 10°C as soon as is practicable.
• Prevent condensation in store.
• Prevent movement of soil from contaminated areas to non-infected fields.

(Photograph by Blackthorne Arable)