Identification de sources de résistance au puceron du pois chez des Medicago annuelles

A standardized seedling test of lucerne (Medicago sativa L.) resistance to the pea aphid (Acyrthosiphon pisum Harris) was applied to several annual Medicago species. A unit consisted in a tray containing a soil mixture in which 50 seedlings of the same accession were transferred at the cotyledon stage. A fixed biomass of 150 mg aphids at D and at D+7 was used to infest the seedlings, and aphid multiplication was stopped at D+13. At the end of the test, at D+27, accessions were ranked according to two criteria, seedling survival and dry matter of the surviving seedlings. We calculated that 6 replicates (6 units) per accession were enough to distinguish accessions differing by 20% in seedling mortality. In a preliminary test, 46 accessions of Medicago truncatula, Medicago littoralis, Medicago italica were tested on the basis of 1 replicate per accession. Ten accessions were then chosen among the most resistant accessions to investigate the effect of aphid host plant (lucerne, pea) on plant resistance. Previous studies had shown the existence of host races in A. pisum. Taking into account allozyme polymorphism at four loci and colour polymorphism, we have shown that aphid populations on pea were composed of one predominant genotype, while aphid populations on alfalfa were composed of 3 to 4 genotypes different from the one observed on pea. Seedling tests with aphids from pea revealed that all accessions were resistant to the pea aphid (seedling mortality < 15%), while variability of accession resistance was high with aphids collected in alfalfa (seedling mortality ranged from 2,7% to 98,6%). Biological tests on the effect of the resistant accessions on aphid survival and reproduction showed that these criteria were both affected by plant resistance. Un test d'évaluation de la résistance des plantules de luzerne (M. sativa) au puceron du pois a été appliqué à différents écotypes et cultivars de Medicago annuelles. La mortalité des plantules notée le 27ème jour d'une infestation aphidienne contrôlée (infestations avec 150 mg d'aphides à deux reprises séparées d'une semaine avec arrêt de l'infestation après 13 jours) et la matière sèche des plantes survivantes à cet instant, constituent les critères de résistance. Nous avons estimé, sur la base de la variabilité des résultats que six répétitions de barquettes contenant 50 plantules permettent de différencier des accessions présentant des différences de mortalité des plantules de 20%. La variabilité génétique étu