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.
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. 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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