Tomatoes and potatoes are so closely related (compare their flowers) that they are both susceptible to Late Blight. Of all the fungus diseases that attack tomatoes, blight is the most severe. In warm, damp weather in July the tiny fungus spores drift in through open doors and vents, and germinate quickly where they land on damp plants. The fungus spreads out to make large dark spots on the plants that join together and blacken whole leaves or stems. Brown sunken patches appear on the fruits and they rot. Mould then appears on the surface, releasing more spores to spread infection. There are no approved sprays for blight control on tomatoes, so use cultural controls instead.
The spores are spread by wind, hands and water-splash. Close doors and vents on cool days to keep the spores out. On warm days open glasshouse vents and tunnel doors to air out the greenhouse. Keep leaves dry by watering onto the roots; avoid overhead watering. Water in the mornings so the plants are dry by nightfall. Remove the lower leaves by breaking them upwards (not cutting, as the wound heals slower) and break out side-shoots to improve air circulation around and through the plants. Remove all potato crops from the greenhouse by mid-June to stop them infecting the tomatoes. Remove and destroy infected leaves and fruits immediately, before they infect their neighbours, but wash your hands before handling healthy plants. Don’t compost infected material as some spores can survive composting. Grow tomato varieties selected for blight resistance such as ‘Ferline’ and ‘Legend’: they are not fully immune but it helps.