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Potato blight questions
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Martin
Posted 23/7/2010 10:11 (#256)
Subject: Potato blight questions


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Tommy mentioned to us that some black spots on the leaves of some of the early potatoes at Netherton indicated blight, and we cut down the affected parts. We also found similar marks on our early potato plants here, but the descriptions of blight all seem to talk about brown discolouration of the edges and tips of leaves. So, I had a look on the Garden Organic forum, and found a suggestion there that the black spots are a symptom of early blight, or target spot, a different fungal infection from late blight, which is the nasty one. The Garden Organic forum said that early blight "rarely causes significant loss of yield and no treatment is necessary". Can anyone confirm/ clarify?
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Agric
Posted 23/7/2010 11:00 (#258 - in reply to #256)
Subject: RE: Potato blight questions



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I conversed with Tommy on this, I wasn't so sure it was blight on the Mimi potatoes at Netherton - they showed a lot of small brown lesions which wasn't late blight on their leaves last season (and the seed was bought new last year). The first earlies that were planted mid April should be about ready now so their yield won't suffer much from beheading so better safe than sorry.

These pages show blight:

http://www.ruralni.gov.uk/index/crops/potatoes/blight_net/potato_bl...

http://www.sac.ac.uk/consulting/services/c-e/cropclinic/cropadvice/...

http://www.sac.ac.uk/consulting/services/c-e/cropclinic/cropadvice/...

http://www.sac.ac.uk/consulting/services/c-e/cropclinic/cropadvice/...

http://www.gardenaction.co.uk/fruit_veg_diary/fruit_veg_mini_projec...

http://www.dgsgardening.btinternet.co.uk/potato.htm

http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=217

http://www.growveg.com/growblogpost.aspx?id=43

I have a couple of photos of blight from Netherton, will try to upload here. There are blight watch sites, There isn't a recorded outbreak of blight in the IV7 area (but that doesn't mean much):

http://www.potato.org.uk/department/knowledge_transfer/fight_agains...

Blightwatch shows a recent full 'Smith period' ( = 2 consecutive days of full Smith criteria) on 20th & 21st July so expect blight appearing about Sunday:

http://www.blightwatch.co.uk/content/bw-History.asp?y=2010&stid=281...

There is a long series of PDFs about blight here:
http://www.potato.org.uk/department/knowledge_transfer/fight_agains...

Dithane (the only adequate chemical for amateurs) is being withdrawn, highly unlikely to find in any garden shop etc. Have found 1 supplier online, a big packet (300gms) for £14 inc P&P, enough for Netherton for 2 seasons or more. Tommy has bought some so if you're going to spray I recommend you do it NOW - it's much more effective applied prophyllactically:
http://www.amazon.co.uk/gp/product/B003M4VA8M/ref=olp_product_detai...

There are now no reasonably effective chemical treatments against blight for non-commercial growers. Here's a list of current blight fungicides for commercial growers:
http://www.potatoreview.com/technical.htm

It might be worth seeing if any local growers have supplies we could blag.
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Martin
Posted 23/7/2010 22:35 (#264 - in reply to #258)
Subject: Re: Potato blight questions


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None of these photos look like the black spots we saw, both at Netherton and here. I think it must be early blight/ target spot.
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nutrientz
Posted 19/8/2010 20:46 (#310 - in reply to #256)
Subject: Re: Potato blight questions


We definitely had blight on our main crop ( Nicola) pots. So I cut off the tops and lifted one- an excellent crop. I have now lifted all my main crop, so what do I do in the tatty hols in October? Does anyone know what happened to Bordeaux mixture - the traditional treatment for blight .
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Agric
Posted 20/8/2010 17:46 (#311 - in reply to #310)
Subject: Bordeaux mixture



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Is still available and is sold in garden shops etc, it's about the only chemical that's permitted for organic potato crops, but it needs to be spayed prophyllactically and frequently, and isn't an impressive remedy even then.

It seems that copper compounds are the only anti-blight chemicals sold in retail outlets for amateurs nowadays, Bayer have replaced their Dithane fungicide with a copper oxychlorate chemical I think.
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