Common or Powdery scab - both common problems in gardens and allotments
Scab
A potato with scab may be unsightly but is perfectly safe to eat
Amongst the harvest questions that are in my intray, we are getting a few asking about scab. If you grow potatoes you will get common scab at some point. I will try to cover some issues in this lengthy post - if I haven't answered your Q, please leave a comment here or email us.
Scab is tricky - common scab can be caused by not enough water - in particular just after the tubers are formed; and powdery scab is generally only a problem if the ground is too wet. Farmers use irrigation to avoid common scab - but they have sophisticated ways of measuring soil moisture deficit and working out if irrigation is needed or not (generally only ware growers do this).
To translate the above technical farm chat to gardens and allotments, quite simply, if you have scab on your potatoes, the weather conspired against you this year! You have done absolutely nothing wrong. 2022 has been an incredibly hard year for potatoes with late frosts, droughts, heatwaves and then downpours. When the tubers are forming, they need to be kept damp (but not waterlogged).
Common Scab
What is Scab?
Unfortunately, the source of infection isn’t a rare, short-lived pathogen; it’s a soil bacteria that can remain in the ground indefinitely. The bacteria, Streptomyces scabies, thrives in soils with a pH above 5.5 and temperatures between 10-31 C. It is most common on light, sandy soils, low in organic matter.
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Both the common and powdery scab pathogens are sometimes described as fungi, but in fact Streptomyces species are more closely related to bacteria and Spongospora subterranea f. sp. subterranea is related to the slime moulds. Both organisms exist in the soil, either free-living (Streptomyces spp.) or as spores (S. subterranea f. sp. subterranea). They invade the surfaces of potato tubers and the plant responds by growing corky scabs, which actually limit the spread and are a misnomer - the tubers are not carriers.
Potato tubers suffering from scab are covered in circular lesions that may appear dark and corky. When many lesions are present, they sometimes grow into one another, creating irregular patches of damage. Surface scabs are annoying but are usually able to be cut away and part of the potato salvaged. More serious diseases can develop, causing deep pitting and cracking that allows secondary pests and diseases to make their way into the tuber’s flesh.
Are some varieties more prone to Scab?
Some varieties are prone to scab and some have good resistance. Varieties that are prone to scab are generally grown because they have other strengths - for example, in the case of Vitabella it has good blight resistance. It is difficult to find a variety that is good in every aspect, and if you add in other dimensions like soil type, cooking type, heat tolerance, drought tolerance, taste, suitability for storing and cold and not so cold conditions, etc, it is impossible for any variety to be perfect for every situation - that's why there are so many varieties available in general in the UK and with us in particular. Unfortunately we do not have a crystal ball and so cannot predict if the weather will mean blight or scab (or neither or both!) will be an issue.
What about scab free seed potatoes?
Seed is often grown on farms and fields where irrigation is not an option, and since irrigation can cause more problems with bacterial diseases, seed growers tend to avoid irrigation. So sometimes there is scab on seed because it is less of a commercial problem than bacterial diseases. From a farmer's point of view, scab on seed does not make the harvest worthless because the scab does not pass to the next crop hence the higher tolerance levels allowed for seed. Also seed only needs to grow to medium size so irrigation is not required to give optimal yield.
We grow many heritage varieties for the garden market and are proud to keep these going, however, due to the rarity of these varieties, and to keep waste levels (therefore overall cost to customers) low, some heritage seed potatoes may have small amounts of scab. The crop will have passed inspection and so the levels are within the permitted levels (scab is permitted on 33% of the tuber surface for seed potatoes in UK). Remember that scab does not pass to your soil, other tubers, or the next generation - if the next generation gets scab, it really is down to the conditions. If I break a leg at the same time as my daughter it isn't heredity!
See our other post about an experiment with seed potatoes with scab
All our seed potato varieties have a tab with independent disease information including scab resistance. These results are from the Agriculture and Horticulture Development Board official testing. To see our full list of Scab Resistant varieties please see here
See our other post about an experiment with seed potatoes with scab
All our seed potato varieties have a tab with independent disease information including scab resistance. These results are from the Agriculture and Horticulture Development Board official testing. To see our full list of Scab Resistant varieties please see here
What can be done to prevent scab?
Potato scab control is targeted at preventing infection in potatoes; once your potatoes are covered in scab, it’s too late to treat.Â
To control common scab, do not allow the soil to become dry during tuber development. Raise organic matter levels to improve water retention. Water the developing crop if necessary, starting two to three weeks after plants emerge and continuing for about four weeks, applying 20 litres per sq m (4 gallons per sq yd).
To control common scab, do not allow the soil to become dry during tuber development. Raise organic matter levels to improve water retention. Water the developing crop if necessary, starting two to three weeks after plants emerge and continuing for about four weeks, applying 20 litres per sq m (4 gallons per sq yd).
Common scab is worse on alkaline soil, so liming the soil to prevent club root of brassicas will predispose to common scab in potatoes. Apply lime after the potato course of the rotation.
Remember for next year to rotate your potatoes to a new part of the garden as this can live in the soil. It might be helpful for future years if you do a quick note of what you remember in regards to when you planted and the weather this spring (and last spring if your memory is really good!) eg you can state "really dry early June", etc. Growing vegetables is a huge jigsaw and the weather plays a big part in it.
Common scab, although may look unsightly, is perfectly safe to eat. Many people will peel the affected areas and some will just boil them up and eat, if the scab is in small quantities. Leave the skin on until you are ready to eat as this helps to preserve them. It will not get any worse once harvested.
Information taken from and for more background reading, please see here:
If you would like us to confirm if you have scab, or if the issue is something else, please send us a pic. Pics here are from a customer this week with some Maris Piper with common scab (used with permission).
I like to eat my spuds skin on, in past years i’ve peeled the scabbie ones but this year I haven’t with the thought that the bacteria would be denatured in the cooking process….anybody any contrasting thoughts or evidence?