Topic: Potato infection

[gavin]

Hello there all - does anybody know more about this?

One bed of my potatoes were almost all affected by small black flat spots - I'd lift them (several varieties - second early, salad, and main-crops) and they'd clumps of soil sticking quite stubbornly.  Washing them to use, they'd black spots which were very hard to wash off - I'd to scrape them with a knife.  The spots are slightly raised, but not at all rough like scab.

Reading and exploring - it seems to be Black Scurf; a cosmetic fungal infection which doesn't affect eating quality.   (And the Yukon Golds we had tonight were beautiful!  :) :) :) )

A few questions though.

- Reasonable diagnosis?

- Does it affect storage qualities?

- Is it a hot and dry weather problem - might figure with this summer?

- The only bed affected was the one with the seed from our local potato day - several different varieties affected.  Is it therefore something which is most likely introduced by seed?

All best - Gavin

[Hugh_Jones]

Gavin, to try and answer your questions (although not necessarily in the right order):-

Impossible to confirm your diagnosis without seeing them, but your descriptiion seems accurate.

Arises from fungal pathogen rhizoctonia solani - fairly widespread soil pathogen, but unless it is allowed to build up by repeated potato growing on the same soil it does not usually attack.

Unless the bed has been used year on year for potato growing, the attack has almost certainly arisen from use of infected seed.

Any attack may be exacerbated by the presence of potato cyst eelworm in the soil

There is no treatment available to the amateur gardener in UK (although the commercial boys have acces to some)

Doesn`t materially affect the crop in storage

Best prevention - change your seed supplier, and try to use a 4 year rotation.

Dry weather conditions are unlikely to have had much effect; prolonged wet weather might make it worse

[gavin]

Thanks Hugh, for confirming suspicions and for additional information.  Seems I might have got off lightly withthe hot dry summer?

It was a bed that hasn't had potatoes for six years or more, and I have a four year rotation for the tatties.  And it was only the varieties bought from the potato day - at least I'll have several questions ready and pat to ask Alan Romans this year, if he comes back!!!!

Hope it's gone by the time the potatoes come back to this bed!

Thanks again - all best, Gavin

[campanula]

you guys are scaring me.......potato novice!!

[Beer_Belly]

Here in Holland it's the law to have at least a three year rotation for spuds (we have to keep records) I myself have grand plans to have at least a five year rotation on my plot for everything, maybe even six If I can manage it :-)

[Hugh_Jones]

On my previous posting on this subject, I forgot to mention that some time ago I saw a report indicating that Black Scurf is fairly widespread in Germany - maybe that`s where your seed potatoes came from.

Field trials have been going on since 1999 in Germany involving the use of Bacillus Subtilis to combat the Rhizoctonia Solani fungi strains.  The early reports indicated that the B.Subtilis was indeed having considerable effect in repressing the fungi, but I have seen no reports of more recent trials.  Trouble is that if it works the EU will probably ban it.

[gavin]

Thanks, Hugh - another question on the list for the potato day people!!!  (and yet another - what else? :( )

All best, Gavin