Yellow Sigatoka disease caused by Mycosphaerella musicola (anamorph Pseudocercospora musae) is one of the most important threats the banana industry has to face in Guadeloupe. The disease causes leaf spot, and heavy attacks can reduce considerably the number of leaves and, ultimately, bunch weight. Nevertheless the most important effect of this disease is indirect. Heavy spotting results in a reduction in the greenlife of the fruit, which considerably depreciates its export marketability. At least ten viable leaves at harvest are necessary for good fruit marketability and therefore excellent control of this disease is essential. From 1937, when yellow Sigatoka disease was first reported in Guadeloupe, 40-50 fungicide treatments per year have been used to control the disease, and treatments have been applied systematically according to a pre-established `calendar' programme. The main objective of CIRAD-FLHOR (Centre de Coopération Internationale en Recherche Agronomique pour le Développement - Département Productions Fruitières et Horticoles) research has been to reduce the number of treatments to the minimum necessary for ensuring good fruit quality, so decreasing the cost of control, as well as minimizing risks of fungicide resistance and environmental effects. Key to this was gaining a better knowledge of the disease, so that a forecasting system could be developed for optimizing fungicide applications