The downside is that moist, humid weather in July is prime time for potato blight. Spores are spread by wind and rain these conditions when temperatures are 10oC (50oF) or above. Brown or black patches appear on the tips and margins of leaflets, that curl and wither, it then spreads rapidly to the leaves and stems before the plant finally collapses. The spores are then washed down to the tubers, the skin becomes discoloured and a reddish-brown rot appears, this, alongside secondary infection reduces the tuber to a foetid, liquefied mass and it rapidly devastates crops that are growing or stored.
Tuber infection can be reduced by ‘earthing up‘ or mulching with a thick layer of organic material like hay or straw. Remove infected foliage immediately can slow its spread and stop spores from spreading to the tubers then don‘t harvest the crop for at least 3 weeks, after this the tubers will have thicker skins and spores on the surface will have died. Lift even the tiniest infected tubers and dispose of them away from the garden and always plant ‘seed’ from reliable sources. Early maincrop varieties are more prone to blight, plant early so that they mature before it appears. Planting alternate rows of different resistant varieties can improve cropping. Check tubers in store regularly; remove any that are rotting.
Another option is to apply Bordeaux mixture, copper oxychloride or mancozeb before plants become affected. Listen for ‘Blight Infections Periods‘ are mentioned on Farming Programmes or weather forecasts and hope that the weather becomes dry and sunny, halting the spread of ‘blight‘. There are several ‘resistant’ varieties like Colleen, Premiere, Cosmos, Lady Balfour, Pomeroy, Orla, Remarka, Cara, Milva, Valor, Verity, Arran Victory. Sarpo Mira, the first to score 100% in commercial evaluations against blight is ushering in a new era of blight and virus resistance, more varieties, including ‘Sarpo Axona‘ following; roll on the bight resistant revolution!