Potato blight and Bordeaux Mixture

by Lila Das Gupta

I'm in a quandary over what to do about potato blight (Phytophthora infestans), a fungal infection that causes foliage to develop dark patches and can result in the rotting of the tubers.

Chitted potatoes ready for plantingI'm in a quandary over what to do about potato blight (Phytophthora infestans), a fungal infection that causes foliage to develop dark patches and can result in the rotting of the tubers. We didn't seem to suffer from it at all on my last plot, but the new site I'm on has suffered from it terribly.

A blight warning - sometimes called the Smith Period - occurs when there have been two consecutive days with temperatures of at least 10? and relative humidity has been above 90%.

I try to grow vegetables organically, so my main line of defence up till now has been to plant mostly first early potatoes (blight tends to strike later in the season). I have grown 'Sarpo' varieties which have a greater resistance to blight, but when a warning is in force or signs appear on the plot, you either have to spray or sacrifice your crops. Tomato blight can also strike at this time (tomatoes and potatoes are in the same botanical family): one day they appear thriving and healthy, the next time you visit, tell-tale brown spots are covering the leaves and your hard work has been lost.

This year I'm growing 10 different varieties of salad potato for a taste trial for Gardeners' World magazine, some of them, like 'Pink Fir Apple', are late maincrop varieties, so there doesn't seem much point in growing them unless I have a plan of action when there is a blight warning. I still have a bottle of Bordeaux Mixture powder (a mixture of copper sulphate and calcium hydroxide) on my shelf, which I've never used. Although approved for use in organic growing, Bordeaux Mixture has a great many detractors.

When I spoke to Anthony Carroll from Carroll's Heritage Potatoes he seemed alarmed that I intended to use Bordeaux Mixture. He classes this as far more poisonous than the conventional fungicide Dithane 945, which breaks down in the soil after a few days. Similarly Iain Barbour of JBA seed potatoes in Scotland also recommends Dithane 945 for the same reason. Copper sulphate is a poisonous substance which neither bees nor worms care for and can take a long time to break down in the soil.

On a more positive note, my other half has pointed out that this year the rooks are nesting much higher in the trees, which, according to folklore, means we're in for a dry, hot summer. Less humidity equals less blight, so let's keep our fingers crossed.

I would be very interested in hearing your opinions on which of these two fungicides you prefer using and why.

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Gardeners' World Web User 16/04/2010 at 14:15

I've used Bordeaux mixture but not Dithane simply because GO reluctantly recommend it as the fungicide of last resort. I'm never happy about using it though. I've also tried Sarpo Mira and Axona varieties - they're phenomenonly resistant to blight, but I've found their taste is nothing to write home about. I'm trying some of the other Sarpo varieties this year to see if at last there might be that happy combination of resistance and taste. I also grow lost of earlies to get round the problem. Of course most of these don't store for as long as maincrop, but I've found that Harlequin does seem to be better than most. And it tastes wonderful :)

Gardeners' World Web User 16/04/2010 at 22:20

Thanks for the tip on Harlequin VP. My friend in Ireland says the Sarpo varieties still succumbed to blight. Agree there are other potatoes I like more flavour-wise.

Gardeners' World Web User 17/04/2010 at 12:27

Bristol has been bad for blight recently so i am trying some resistant varieties but also growing more earlies and making dinners to freeze with them so I can still have their taste even past their keeping date. I grow all my tomatoes in the conservatory now because of the blight.

Gardeners' World Web User 17/04/2010 at 16:33

I know this is nothing to do with potatoes but can someone help me with my grape vine. Its 3 years old now and I know I have to prune it but how hard do I prune and is it safe to prune now? You all helped me last year with my raspberries for which I am extremely gratefull - they turned out to be lovely yum!

Gardeners' World Web User 17/04/2010 at 17:35

My dahlias have been reduced to a pulp. I did some exploring beneath the surface to check there was some growth only to find the tubor turned into a hollow mush. There was a series of tunnels leading off to other parts of the garden. Any ideas?

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