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.