CHARACTERIZATION OF POPULATIONS OF PHYTOPHTHORA INFESTANS (MONT.) DE BARY IN MOROCCO USING AGRESSIVENESS, MATING TYPE AND METALAXYL RESISTANCE

Late blight of potato and tomato caused by Phytophthora infestans (Mont.) de Bary has become a serious problem for these crops during some growing seasons. It first appeared in Africa in 1941 (Sedigui et al., 1997). The population structure of this fungus is reported to have changed since 1980's essentially the occurrence of the A2 mating type outside Mexico (Drenth et al., 1993). Also the new populations of P. infestans are more aggressive and genetically diverse from the old populations (Fry et al., 1992; Peters et al., 1999). Until 1997, only A1 mating type of P. infestans had been detected in Morocco (El Ismaili, 1994; Sedigui et al., 1997). In this study 99 isolates from different tissues of potato (92 isolates) and tomato (7 isolates): leaves, stems and tubers, originating from several regions of Morocco during 1997-2000, were characterized by using aggressiveness, mating type and metalaxyl resistance. These isolates were tested for aggressiveness on detached leaflets and tubers of four potato cultivars ('Desiree', 'Nicola', 'Spunta' and 'Kondor') which are the most cultivated in Morocco and on detached leaves of 'Daniela tomato' cultivar. The mating type of isolates was determined by pairing them with English tester isolates of mating types A1 and A2. Resistance to metalaxyl of the isolates was tested in vitro on pea agar amended with different concentrations of metalaxyl (0, 0.1, 1, 10 and 100 mu g ml super(-1)) and in vivo by floating potato leaf disks inoculated with P. infestans on metalaxyl solutions (0, 0.1, 1, 10, 100, 1000 mu g ml super(-1)). This study has revealed that all the isolates from potato were more aggressive on the potato and tomato cultivars tested, but the isolates from tomato were more aggressive only on 'Daniela tomato' cultivar. This may explain the host specificity of the isolates. All the seven tomato isolates were of A2 mating type and metalaxyl sensitive. Pairing and metalaxyl resistance tests for the potato isolates revealed a distribution as follows: 28 isolates were of A1 mating type and metalaxyl sensitive, 9 isolates were of A2 and metalaxyl sensitive, 13 isolates were of A1 and metalaxyl-intermediate, 33 isolates were of A2 and metalaxy-intermediate, 3 isolates were of A1 and metalaxyl resistant and 6 isolates were of A2 and metalaxyl tolerant. These results confirm the presence of the isolates of A2 mating type with a high degree of metalaxyl resistance which can exceed 100 mu g ml super(-1). In spite of the limit