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Mycosphaerella blight caused by Mycosphaerella pinodes (Berk. et Blox.) Vestergr. is now recognized as one of the major problems limiting yield of pea crops in Algeria. The present work was carried out to study the aggressiveness of 75 M. pinodes isolates collected from different pea-growing areas forming four population groups representing four geographic areas in western Algeria. The latent period, incubation period and disease severity were measured in the greenhouse for each isolate × cultivar combination. All three aggressiveness components differed significantly between isolates and between cultivars. No significant interaction however was noted between isolates and cultivars. Both principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed to analyze the variation pattern within and between population groups. Cluster analysis, which summarizes the relationship between isolates according to their distance of similarity, sorted isolates into six distinct aggressiveness groups. Aggressiveness group 1 was the most represented, with 34% of all isolates. Both PCA and cluster analysis revealed that many isolates were closely related irrespective of the geographic area or the host cultivar from which they were collected. At the same time, and based on the same aggressiveness components, the cv. Onward, Lucy and DP were the most susceptible, whereas the cv. Rondo and MK were partially resistant.

The effect of host leaf wetness duration, Mycosphaerella pinodes inoculum concentration and pathogen isolate on the latent period and the incubation period of the pathogen or disease severity were quantifi ed on pea (Pisum sativum L.). Seedlings of two widely grown pea cultivars, Onward and Merveille de Kelvedon, respectively susceptible and moderately resistant to M. pinodes were subjected to six leaf wetness durations of 6, 12, 24, 48 and 72 h, and inoculated with fi ve inoculum concentrations, 2.5×103, 4×104, 3.5×105, 4x106, and 5.2×107, in order to determine whether the cultivars reacted differently to M. pinodes isolates inoculated under identical conditions. Increasing the duration of leaf wetness and inoculum concentration caused signifi cant (P