Potato scab is a common disease and it is caused by a bacteria Streptomyces scabies. It lives in the soil free and attacks the potatoes in ideal conditions. Normally scab does not affect the yield but the skin becomes ugly and you might have to peel your potatoes thickly. The same bacteria attacks radish, beetroot, turnip and swede too.
The bacteria invades the surface of the forming tubers and the plant develops the scabs in response, which are defend the tubers from further infection. So the scabs are basically the defence system of the plants.
Resistant varieties
If the infection is severe and you don’t have the space to grow potatoes elsewhere or your whole plot is badly infected with the bacteria, it is worth to grow resistance varieties. King Edward, Pentland Javelin and Arran Pilot for example shows some degree of resistance to common scab.
Soil pH
High pH, alkaline soil helps the bacteria thrive. The best soil PH if you want to avoid scab is between 5.0 and 5.2. Not too many vegetables like acid soil so this could be a downfall, but with adding a bit if lime the following year will bring the Ph level up again a bit. Applying manure makes the soil alkaline a bit in the first year so make sure you have the poatotes in the second or third year after putting manure on the plot.
Moisture level
The soil moisture can affect scab attacking the forming tubers dramatically. Make sure that after the foilage appear your plants don’t dry out for 4-6 weeks. I know it is not an easy task; do you remember that hot spell we had in May, that is when my potatoes got infected badly I think. The best is not to let the potatoes dry out at all throughout the growing season. It is hard but if you only grow a small row or in bags then it is achievable.
Crop rotation
Crop rotation is important too if you had scab on your spuds in one year, make sure you don’t grow them on the some place for at least 3 years. And don’t grow any other root vegetable on the spot either for three years. Don’t plant any tubers which show signs of scab.
Potato scab is not deadly, you can grow and enjoy your potatoes even if you have this bacteria in your garden or allotment plot. Some dry years the infection can be bad though and have to peel the tubers really thickly. Belive me I do know this as I think my whole big plot is full of this bacteria as anywhere I grow my spuds the scab appears some years bad some years not so bad, depending on the weather and on the actions I take to avoid this disease.
