Résumé
In Cameroon, Mycosphaerella fijiensis was reported for the first time in 1981 and the control against black leaf streak disease represents the highest production cost which can attend up to 10% of total production cost. In fact, the epidemiological context which exists in Cameroon is particularly favourable to the disease notably in the rainy season from April to November. Towards the end of the 80s, a forecasting system using biological descriptors was elaborated and applied with success, reducing the number of treatments to 12 - 14 per year. Since 1996, the development of fungicide resistance lead, even if the levels of resistance fluctuate, to the progressive abandon of this rational strategy at the expense of systematic control methods. Thus in 2005, about 40-50 treatments were done on most of the plantations and the control programme was based on the ratio of 80% of contact fungicides and 20% of systemic and penetrant fungicides. This evolution has lead to an important increase of the cost of disease control, but also to an increase of negative environmental effects. In those conditions, research conducted in Cameroon is aimed to: - Experiment fungicides having less negative environmental effects. - Adapt control strategies to the situation of fungicide resistance. Experiments realised in 2005 have shown that chlorothalonil enabled a very good control of the disease, even in the rainy season. New strategies where the proportion of chlorothalonil would be rationalized to a minimum are presently evaluated. - Evaluate the possible reversibility of fungicide resistance, in order to reintroduce a the more sustainable forecasting strategy. The hypothesis is that the fluctuations observed in the resistance frequency to systemic fungicides could result from a lower fitness of resistant strains, which could be counter- selected in the absence of the fungicide selection pressure and/or gene flow between the treated and the untreated areas. (Résumé d'auteur)