Smallholder farmers are the main producers of cabbage and kale – popular crops for the periurban markets in Kenya. Virus diseases are, however, a major problem as they can cause severe reduction in yield and can sometimes result in a crop being completely destroyed. In a survey conducted in 2000 by CPP Project R7571, viruses were considered by the farmers to be the most important disease constraint. Almost 90 per cent of farmers, however, do nothing about virus infection.