‘Late blight’ control tactics
I’M writing about the article ‘How can we beat blight’ (AG, 16 September). It does give quite sound advice but I thought I would add a little to it. Properly known as ‘late blight’, the fungus affects both potatoes and tomatoes with outdoor tomatoes being affected more than glasshouse crops. Contrary to your article, the blight spores are most unlikely to overwinter in soil or compost in the UK. Spores will not overwinter in blighted foliage in the compost heap. The fungus can overwinter, however, in reject potatoes left in the ground. I would destroy any blighted potatoes and not allow them to survive over winter. If you notice rogue potatoes appearing next year, get rid of them.
The advice regarding good ventilation is crucial to good blight management but is easier to do for tomatoes. Infection can often start on the stems, dark brown patches being seen followed by the rapidly spreading foliar black/brown rot if the weather is warm and wet/humid. Keeping free air movement around the plants can do much to stem spread; remove basal leaves as they begin to get old and even remove younger leaves and side stems up the plant if by so doing you can create better ventilation around the stems. A copper based fungicide for edibles will help but a contact treatment will only protect the plant surface it is sprayed on. It is imperative to get your first spray on before disease appears. I apply to my outdoor tomatoes when they are 45cm high, making sure you treat the lower stems. When young fruits appear I stop any chemical treatment and rely on ventilation to keep the fungus at bay. I still expect some blight very late on, but by then I will have reaped the rewards already. Prof. P E Russell via email