18 July 1998


Potato skins are now becoming more common on the menu of trendy eating places. So will growers have to raise the standards of tuber skin quality? asks Sarah Henly.

EVERY year is the worst year for something. There is no ideal season when all potato skin diseases stay below the surface.

Last season, the problem was black dot. Although just a superficial blemishing disease, it forced some growers to accept lower processing prices for affected tubers rather than sell them into the premium pre-packing market.

While quality changes with the seasons, consumer expectations dont. The customer wants a consistently high quality of skin finish, whatever the weather, says Tescos technical development manager for potatoes, Sandy Norman.

"Customers are always attracted to a brighter looking, blemish-free product, be it a carrot, parsnip or potato. Tubers free from superficial marks caused by silver scurf and skin spot are naturally the preferred choice, although we arent certain the shopper would pay a premium for them."

Mr Norman knows that consumers are prepared to pay more for potatoes free from major blemishes such as bruising, mechanical damage and greening, and expect a reasonable skin quality for their money. However he is in no doubt that there will always be a ready market for tubers with a good skin finish, and in seasons where quality is difficult to achieve, those may fetch a premium.

Companies such as Tesco have researched what the customer wants and set specifications for suppliers based on achievable standards. "We understand how difficult it is to produce blemish-free potatoes, and we dont set our specifications from an ivory tower. Our quality controllers are not policemen but overseers of general achievable performance, helping suppliers to meet market demand," he explains.

The main outlets for potatoes – ware, salad, and pre-packed – attract different market prices. While more lucrative, the latter is more demanding in its specifications, particularly where skin finish is concerned.

The four big skin diseases are black scurf, skin spot, silver scurf and black dot. With the exception of black scurf, they all penetrate the skin tissue so cannot be removed during washing. The last two are superficial blemishing diseases, nevertheless they are undesirable on a premium product.

Growers targeting the salad and pre-packed markets need to adopt an integrated approach to disease control, says Sharon Black, potato pathologist at IACR Rothamsted in Hertfordshire. That involves starting with virgin potato land, using clean seed, fungicides where necessary, and paying attention to hygiene.

"Using an integrated policy not only allays public concerns about the use of agrochemicals, but it is the best and sometimes only effective way to keep diseases in check. Before planting, growers should consider their market, the disease risks, the fungicides available and their acceptability to buyers, and farm storage facilities," believes Mrs Black.

Rotational position pays a large part in containing the spread of inoculum of soil-borne fungi, and some field operations such as haulm pulling can help. Dry curing, or maintaining higher initial store temperatures, heals tuber wounds and prevents some pathogens developing.

However each disease is encouraged by different conditions, so it is important to first establish the significance of each in view of the end use. For example, if skin spot is more of a worry than silver scurf, keeping the tubers slightly warmer in long-term storage may be pertinent. If the opposite applies, keep the temperature down, suggests Mrs Black.

While she recognises that no store can be totally sterile, nor the diseases eradicated, she stresses the importance of disinfecting chitting trays and equipment before use.

"It is foolish to use half measures when nearly all potato pathogens can survive in stores, on trays and on equipment. You may spend a lot on clean seed and fungicides, then mess up your chances of a premium crop by leaving dust in the store.

"I believe that customers must accept a small amount of skin disease on tubers, which after all, doesnt affect taste or cooking quality. But growers can minimise the problems by understanding how they occur and doing everything they can to prevent them establishing."


THIS seed- and soil-borne pathogen is common and underrated. It causes peaty-looking, flaky blotches to appear on the tubers. Symptoms are worse in dry, drought-prone soils.

The same fungus – Rhizoctonia solani – causes stem canker (right), which can reduce the numbers of tubers produced and thus affect yields. The tubers are often of variable size, making crops unsuitable for the salad market.

Fungal hyphae may be present on the tubers even if they cant be seen by the naked eye. Good coverage with a protectant fungicide such as tolclofos-methyl (Rizolex) before planting should prevent seed-borne infection getting a hold.

Where a fungicide is not considered necessary, ideally plant tubers into warm, moist soil so they emerge quickly and outgrow any potential black scurf problem.

Since there is no chemical for soil application, avoid double cropping and consider haulm pulling before lifting. This reduces the amount of infected material around for the second crop.


LARGE, raised, grey, rough spots are characteristic of skin spot (above), which is unpopular with processors since rots show up in the cut edges of chips and crisps.

By infecting the mother tuber, the fungus can kill the eyes and thus reduce the number of stems, ultimately reducing yield. Although mainly seed-borne, it can remain present in the soil over short rotations.

Stem bases may appear slightly brown at harvest, however skin spot doesnt really develop until the potatoes reach the store. It is encouraged by cool conditions, so dry curing at humidities of 80-85% for about two weeks is advantageous.

Treating seed with a fungicide such as imazalil (Fungazil) either pre-planting or before storing can alleviate the problem. But the cost-effectiveness of that operation will depend upon the market and the storage period.


THIS seed-borne pathogen produces patches of light silvering across the tuber, which are often concentrated towards the stolon end. It is mainly a blemishing problem, but continues to develop further in store and sometimes causes tubers to shrivel.

  There is some evidence that silver scurf (right) may also persist in short rotations in the soil. Irrigation can alleviate the symptoms.

Pre-planting seed treatments such as imazalil and thiabendazole (Extratect Flowable) reduce the risk of silver scurf. They may be particularly cost effective to use on red skinned varieties, since the symptoms stand out more.

Harvesting early is the best way to prevent spread. The longer a tuber is exposed to the fungus in the soil, the more it will be affected.

As for skin spot, dry curing before storage can reduce the chances of the disease establishing. But unlike for skin spot, stores should be kept cool to prevent its development.


ANOTHER superficial blemishing disease, black dot (above) ranks in importance alongside silver scurf. It is predominantly soil-borne, and is very persistent so most potato growers have experience of finding the characteristic microscopic dots on their tubers.

Some varieties seem to be more susceptible to black dot, though no ratings are available. The best way to prevent it is to harvest early and dry cure.

There is some indication that irrigating increases the likelihood of black dot, though schedules should be targeted for other more important considerations.

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