Panic ensues if a tomato grower discovers late blight (Phytophthora infestans) on his crop. It can destroy an entire crop in a couple of days if weather conditions are favourable, while other fungus diseases at least start off gradually, giving the farmer enough time to get them under control.
Favourable conditions
Late blight becomes active at about 20ºC, with high humidity and cloudy conditions. On the Mpumalanga Lowveld, where I farmed in my youth, we often had late tropical cyclones causing cool, soft rain for days on end, creating ideal conditions for late blight. But once the weather had started or the fungus had already infected the plant, spraying was redundant. And before systemic insecticides, we had to use copper and dithane as full-cover sprays.
The destructive process
The spores enter the leaves, stems and fruit, and exude spores from the stomata. These spores can travel great distances even on a light breeze. Dark brown patches form on the infected areas and a white fuzzy fungus may be seen surrounding the blotches on the underside of the leaves. t does sometimes happen that conditions become unfavourable for the fungus and it dies off, leaving a few dead lesions which may be difficult to diagnose. f the leaves are placed in a closed glass jar, white fungus develops if it’s late blight. C ontrol should then be directed at prophylactic treatment. In regions very prone to infection, resistant tomato varieties should be used. – Bill Kerr ([email protected] or call (011) 366 0616) |fw
