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
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