Resistance to seed transmission of cucumber mosaic virus in narrow-leafed lupins (Lupinus angustifolius)

Abstract

Cucumber mosaic virus (CMV) is seed-transmitted in L. angusti,folius (narrow-leafed lupin) and plants that grow from infected seed are the primary source of inoculum for field epidemics. Resistance to seed transmission of CMV was sought in cultivars, breeding lines and wild types of this species. The field screening method developed was based on spreader rows, where a susceptible cultivar (Wandoo) with high intrinsic seed infection levels was grown on either side of every test row, and CMV spread occurred by natural aphid transmission. Inoculum pressure was so great that spreader and test rows became virtually 100% infected. Harvested seed from each test row was assessed for the level of infection present by testing for the presence of CMV in the radicles of newly germinated seeds or in leaves of seedlings by ELISA. This method was used successfully from 1987 to 1992 to screen lines for relative resistance/susceptibility to CMV seed transmission. Based on their intrinsic CMV seed transmission rates during the experiments test lines were categorized into four groups: moderately resistant (1-6% seed transmission, as in cvv. Danja and Frost), moderately susceptible (6-20% seed transmission, as in cvv. Illyarrie, Yorrel and Gungurru), susceptible (20-35% seed transmission, as in cv. Wandoo), and very susceptible (35-75% seed transmission, as found in some breeding lines and wild types). No lines were highly resistant (<l% seed transmission) or immune (0%). Differences in seed transmission rates between lines were highly significant and stable from year to year. Resistance or susceptibility was not related to alkaloid content or flowering date of lines but appeared to be polygenically controlled. Breeding for low CMV seed transmission rates is recommended for use in L. angustifolius breeding programs. Also, advanced breeding lines with unacceptably high intrinsic seed transmission rates (i.e. >20%) should be culled out.