Picture of Medwyn Williams

Medwyn Williams

Hello. I'm Medwyn Williams – eleven times Gold medal winner at the Chelsea Flower Show, Past Chairman of the Royal Horticultural Society Fruit Vegetable and Herb Committee and President of the National Vegetable Society.

Potato Scab


I wrote the article below for Garden News a few weeks back and I have certainly had a few phone calls since then. Before writing it I did consult with the Chemist at Everris as well as with Dr David Shaw at Bangor University who is a world renowned expert on potato blight. There is no doubt that the common scab problem start at tuber initiation if the soil or compost is dry at that point.  I just wonder if any of you have any comments to make on it. The two potatoes that are pictured have now been planted with all the others in my Special Potato mixture.


There are many growers who are really worried and concerned when they purchase and receive potatoes with scab on them. The reason for their concern of course is fairly obvious because it looks unsightly and furthermore they think that this scab will manifest itself again in the tubers that’s produced from it. I have always thought that this was a cultural problem rather than a disease and more prevalent in soil containing the bacteria than in a peat growing medium.

The scab that is evident on seed potatoes in the main is what is called Common Scab or to give it it’s proper title – Streptomyces scabies, an infectious form of life somewhere between a bacteria and a fungus. It does however have some merits as derived from this is the antibiotic Streptomycin (antimycobacterial) drug, which was the first antibiotic remedy for tuberculosis.

All Scottish raised seed potatoes are inspected before being released on to the market and as nothing is perfect, there has to be a tolerance to allow the growers a lee way should he have scab on his crop.  The Ministry Inspectors can call on the potato suppliers at any time and should they fail to meet their tolerance criteria they will be rejected. For us growers who grow potatoes for the show bench the crop needs to be free from scab at the point of harvest under the judging criteria ‘Condition’.

The potato develops on the end of a string like structure that extends from the stolon.  During tuber initiation it’s imperative that the bags that most exhibitors grow their potatoes  in are never allowed to dry out from that period onwards. This is often easier said than done as watering is really quite a skilful job as well as being a job that needs to be done on a regular basis. The skill is in keeping the bag uniformly moist throughout the growing season as too much water can also be bad for them. The potato breathes through the pores or lenticels in their skin and too much water can cause fungal infestations or rot.

Some growers think that when they see a shower of rain that the bags get enough water which is rarely the case. Growing in bags is a different scenario to growing in the soil where the moisture in the ground can be more evenly distributed.  Last season on the whole was reasonably good for the commercial potato growers, however we did have a dry period that coincided with the potato initiation, hence the scab developing.

The Level of PH in the ground is also an important factor as the potatoes generally thrive on a PH somewhere between 5 and 6. This of course is much lower than for other vegetables such as brassicas, leeks and onions, hence we grow them in a peat based growing medium. The very Fine Potato Compost that we sell has been formulated organically to produce potatoes that are free from scab with PH of between 5.3 and 5.7.  This is dependant of course on the bags never being dry at any point, particularly at the initiation of the small potato.

Another very important point is the PH of the water you are using, rain water gathered in tanks from the roof etc will have a PH of around 5 whilst a recent PH test on mains water in the Oxfordshire area was 7.35. This of course means that the more mains water you use, particularly during a dry period, can increase the PH of you growing medium and as a result increase the possible incidence of common scab.  I water all my vegetables on the land from a bore hole and I have just ordered a water PH testing kit, I will let you know what the pH is as soon as I know

In order to try and prove that scab doesn’t pass on from the planted potato to those that you harvest in that year, I intend to plant up the two shown in the pictures in my special potato mixture.  One has severe scab on it and looks terrible whilst the other has just a few scabs on.  I will let you know after I harvest them what the result will be.

Potato with slight common scab

Potato with severe Common Scab, the variety in both cases was Kestrel
Potato with severe Common Scab, the variety in both cases was Kestrel



Growing Parsnips

25th July 2007 I am often asked what is the best method of growing both long carrots and parsnips for the show

Read More »

1 thought on “Potato Scab

  1. I have planted 5 Arran Pilot seed potatoes I obtained from you which had mild scab – they went into the ground on March 23rd on an area which had been covered with horse manure in November and left untouched for 4 months over the winter. When I prepared the ground for planting it was necessary to perform quite a bit of tilling as the clay soil here had been saturated by the enormous rains of the winter. However the worm population was thriving when the soil was dug so we will see how things progress.

    As the grass is growing precociously in the warm spring I intend using a mixture of that and horse manure as a top dressing once the plants come through to retain the moisture in the ground. This seems to have worked the past 5 weeks with the garlics/winter onion/spring radish planting area, where the top surface has dried out during 5 days of warm sunshine, but when drills are prepared to sow radish seed, the subsurface is still containing a good dose of moisture. The plants are well ahead in size compared to last year, but rust started in early February, which I have never seen before!! It doesn’t seem to be stopping the plants rapid growth and the new shoots at the moment appear free of the pest.

    Another combination I read about for potatoes which apparently worked well is a 1:1 mixture of
    Ericacious compost and horse manure (a young grower with his grandfather in Poole grew a magnificent crop of Sarpo Mira using that mixture in a vertical tower, along with traditional tomato feed as a potassium additive through the season). I am giving this a try this year, with a small amount of peat moss added to add structure to the horse manure. I am assuming that this mixture will create a slightly acidic pH, although I”ll have to do a test to confirm that.

Leave a Reply

Your email address will not be published. Required fields are marked *