Variabilite du pouvoir pathogene chez Septoria nodorum Berk. (Leptosphaeria nodorum Müll)

Les résultats consignés montrent que ies amplitudes de variation du pouvoir pathogène de Septoria nodorum, évaluées par la réduction du poids de 1 000 grains ou par des éléments de la résistance partielle, sont du même ordre de grandeur entre isolats, entre pycnides issues d’un même isolat, qu’entre monopycnidiospores issues d’un même cirrhe. Les souches de S. nodorum sont donc des populations faites d’éléments aux pouvoirs pathogènes différents. Le noyau et le cytoplasme de l’hôte exercent sur ces populations des pressions de sélection permettant aux éléments les mieux adaptés à l’hôte de passage d’être sélectionnés. Suivant le niveau des perturbations qu’ils provoquent sur les blés sensibles, on peut classer les cytoplasmes dans l’ordre suivant: Triticum timopheevi > Secale cereale > T aesfivum. Sur le plan de la dynamique de l’épidémie, ces perturbations peuvent se traduire par une moindre compétitivité des souches. La réponse de lignées de blé à haut niveau de résistance générale ne semble pas perturbée. Toutefois, le gain d’agressivité qui s’exprime sur l’hôte de passage, dès le premier passage, tend à s’amenuiser au fur et à mesure des passages successifs.

The parasite can adjust its pathogenicity by means of two mechanisms: one is at the level of ascospores differenciation, the other is the result of heterogenicity among the spores resulting from vegetative multiplication of the fungus. These two mechanisms participate in the initiation and spreading of epidemics. This study will deal with the second mechanism. The following material was used: 1) host range (Table I) : two wheat cultivars, two series of triticales and one rye cultivar; and 2) fungal isolates : isolate 6 (wheat adapted), isolate 42 (triticale adapted), monopycnidiospore cultures derived from 6 and 42 and subcultures derived from a single spore, isolate collected in Lyon’s area-strain "Lyon" (Rhône-Poulenc), isolates collected in Israel: strains IS1 and .IS2- The following measurements were made: 1) thousand-grain-weight (PMG) or reduction of thousand-grain-weight (RPMG), 2) rate of spread of foliar necrosis (Pmax) 3) rate of spread of epidemics. The selection pressure by host-plants was obtained with 1-4 passages through the host, using inoculation of detached seedling leaves. The parasite was re-isolated from pycnidia and inoculated on ears. Significant differences in aggressiveness, as expressed by thousand-grain-weight, were observed between pycnidia cultures isolated from the same strain (Table II), between monopycnidiospore cultures derived from a same pycnidium (Tables III, IV) or between 2 generation monopycnidiospore subcultures (Table V). Similar observations were made with tests on Triticale "Clercal" (Table VI). The difference between the dynamics of strains 6 and 42 was revealed by two tests: Fig. 1. : on two wheat lines, the one susceptible, the other with a high partial resistance level; and Fig. 2. : on two susceptible wheat cultivars, according to the number of dissemination and contamination periods. Variability of aggressiveness of pathogen strains towards their own hosts, was also expressed by thousand-grainweight in response to selection pressure by several alien hosts (Table Vlla). The range of isolates is not the same after host passaging (Table Vllb). The particular effect of genotype and cytoplasm was illustrated with series of triticale: one isoplasmic with various genomes, the other isogenic and alloplasmic (Table VIII, Fig. 3). The effect of number of passages (1-4) of strain 6 through an alloplasmic list of three Triticale, observed either on two susceptible wheat cultivars (Fig. 3) or on passaging host (Fig. 4), showed that the increase in pathogenicity occurred from the first passage onwards. Concerning the partial resistance we observed that the passage of strain 6 through the Triticale "Clercal" reduced the incubation time (LPI) and increased the rate of spread of foliar necrosis (Pmax). These parasite variations led to an increased pathogenicity on susceptible varieties, but the behaviour of lines with a high level of general resistance remained constant (Table IX).