Scab on potatoes

[Woofster]

This year I grew Cara, Desiree and Sarpo Mira ptatoes. I got a great crop with some of the largest potatoes I have ever seen. They grew well in a raised bed but sadly they are all marred to a greater or lesser degree by scab. I know this does not affect how good they are to eat (absolutely delish) but I like to enter local flower and veg shows and I cannot show off my otherwise perfect potatoes covered in scab.
I am looking for an explanation: is it the soil (quite chalky) or is it something I am doing. Are some varieties more resistant than others

[Snadger]

This year I grew Cara, Desiree and Sarpo Mira ptatoes. I got a great crop with some of the largest potatoes I have ever seen. They grew well in a raised bed but sadly they are all marred to a greater or lesser degree by scab. I know this does not affect how good they are to eat (absolutely delish) but I like to enter local flower and veg shows and I cannot show off my otherwise perfect potatoes covered in scab.
I am looking for an explanation: is it the soil (quite chalky) or is it something I am doing. Are some varieties more resistant than others

You've answered your own question........it's the chalky soil thats given you the scab!
You may find though that if you grow the spuds in a less alkaline soil you won't have as good a crop.
It's a balancing act between size of spuds and clean spuds methinks!

[vegnut]

I agree with Snadger. Spuds like a little bit of acid, some gardeners will put a small handfull of grass in the hole before planting the spud, they say it sweetens up the soil and gives just enough acid to the surrounding area so as not to get scab. never tried it myself. It's the first time i got scab this year, so i think its about time for me to try. Roll on the spring

[Two_Sheds]

I had bad scab this year, even though the spuds were enormous.

I mulched them with grass clippings, as I always do ... I just think the ground had been over-limed the previous year (the plot's new to me)

[tattieman]

Common scab can be caused by a fungus in the soil.
Chalky soils also increase scab.
It is a very strange problem that even commercial growers get caught out with.

[Comfreyfan]

I used wood ash on my spud bed this year, which I believe is alkaline, just a light scattering, but this was the first time I've had scab on my potatoes. No wood ash on the spud bed next year!

[Woofster]

Thanks everyone for your comments. Although I said my soil is chalky, I grew the spuds in a raised bed filled with a mixture of manure and mushroom compost as well as the contents of various growbags so there was not much chalk content. The previous year I grew spuds directly in the soil (with some added manure and mushroom compost0 and did not get scab at all. I guess it is one of gardening's mysteries. I shall look forward with interest to see what happens this coming year.

[Two_Sheds]

Maybe it was a bad year for the scab fungus? Seeing as we all got it this year (although it wasn't a dry summer, by any means)

"The cause (of scab) is Streptomyces scabies which resembles a bacterium but produces a mycelium, like a fungus. It is usually found in sandy soil with a high lime content and in recently cultivated areas. Low moisture levels during hot dry summers promote its development.

Do not add lime to ground where potatoes are to be grown. Areas where brassicas have grown recently are more prone to scab. Adding a few handfuls of grass clippings when planting should reduce the damage to the new potatoes, possibly because it increases the acidity as it decomposes. There are resistant varieties, eg. 'King Edward' and ones which are more prone to infection, eg 'Maris Piper' and 'Desiree'. There are no chemical treatments and as the damage is only cosmetic, Scab does not usually cause great concern. " POTATO SCAB- symptoms, cause and treatment

[rana]

One of the constituents of mushroom compost is chalk.

[rustylady]

Any organic material is good to add to the soil. I got scab this year too. Next year I'm going to line the trenches with homemade compost (that's if I've got enough left from the bean trench).