09 July 2012
Wet, wet, wet and often muggy at this time of year means potato blight is likely : Look out for greyish black blotches on the leaves and stems of the plants, soon followed by flopping and wilting. If you look closely , especially if the weather is very damp and muggy you may see a slightly fuzzy white fungal growth on the blotches, this is the fungus itself.
Potato blight is the bane of any potato grower's life as it can wreck the crop. After the haulms (stems and leaves) become infected the spores may then be washed down though the soil where they can then infect the tubers, causing them to discolour and later rot. Disaster!
The same fungus is responsible for causing tomato blight on outdoor tomato crops, so check these too.
For the fungus to really become troublesome you need at least two consecutive 24hr periods where the temperature is 10C or above and where each has at least 11 hours where the relative humidity is 90% or more. These are called 'Blight infection Periods'.
You can minimise the risk of tuber infection by earthing the tubers up regularly and deeply and on tomatoes, growing the plants in a greenhouse dramatically reduces the risk, but at this stage of growth there is nothing much you can do to prevent it.
So what can you do? If you see blight in the very early stages (usually just a couple of leaves with discolouration, usually it starts first on the lowermost leaves and on their leaf tips and leaf edges) you can sometimes manage to contain it if you quickly remove all infected or suspect areas...and if the weather becomes drier too. However, it is usually difficult or impossible to control, so you need to do your best to prevent the infection spreading into the soil where it can attack the tubers. I'd advise arming yourself with a bin bag or two and some shears and a rake - cut off all the haulms (stems), rake them up and bag them up ready for the local tip. Do this before the next bout of heavish rain and you may stop the spore wash down. Then, at the earliest opportunity, lift the tubers, dry them off and store/eat. Check stored tubers really frequently in case there are any infected tubers, as if they are their infection soon spreads!