A horrible disease has ruined my potato crop. Brown marks developed on the leaves and eventually whole plants collapsed. When I dug up the potatoes, they had dark patches too. What caused this damage? G.W by e-mail
I’m quite sure that your potatoes are suffering from blight. The fungal spores that cause this are carried by rain or travel on air currents. During moist, humid conditions they infect the leaves, causing the symptoms you have described. Spores from the leaves wash down to infect the tubers, which show dark, sunken patches and discoloured flesh. This is why, if you see damage on the top growth, it is worth removing it straight away in the hope of saving any potatoes that have already developed underground. Dispose of infected top growth away from the garden and never compost it. The disease can affect tomatoes, which are also members of the family Solanaceae. If it seems to be a recurring problem, choose potatoes with some resistance, such as `Romano`, `Valor`, `Estima` or `Cara`. You can also spray the foliage with a fungicide, such as Bordeaux mixture or copper sulphate when the weather is favourable to stop the blight spreading. Earthing up the potatoes deeply will also help to reduce the chances of the infection developing on the tubers.