Inheritance of partial resistance to the brown spot disease in Lupinus angustifolius L

Brown spot (BS) is a damaging disease of narrow-leafed lupins (Lupinus angustifolius L.), particularly in Western Australia. Breeding for resistance to the causal agent [Pleiochaeta setosa (Kirchn.) Hughes] has had some success. Here, earlier data on the extent and inheritance of resistance to BS are presented as a possible guide for future breeding programs. In 1980, 236 Mediterranean wild accessions from the Australian germplasm collection, and 17 cultivars and 20 breeding lines of L. angustifolius and L. albus L., were tested by natural infection in replicated short rows at 2 sites in southern New South Wales. The area of lower leaves covered by lesions was up to 70% lower on partially resistant lines than on commercial cultivars. Twelve wild accessions were consistently more resistant than 5 cultivars at Wagga Wagga in 1980 and in 2 subsequent years, in which 66 wild lines and 6 cultivars were tested. However, the rankings at Wagga Wagga differed from those in coastal Western Australia, suggesting that spatial differences occur in the pathotype composition of the fungus. The absence of sexual reproduction in the fungus suggests that its pathotype spectrum would change only slowly at each location. Unimodal distributions of BS scores for individual plants were found in the F2 of Illyarrie (susceptible) � CPI 67877 (resistant) and in the backcross to Illyarrie. The F2 mean was intermediate between the 2 parental means, and only a few plants had a score as low as the resistant parent. The regression of the mean scores for 15 F2 families from crosses among 7 wild lines, and between these and 2 susceptible cultivars, on the respective mid-parent values gave a narrow-sense heritability estimate of 0.94 ± 0.17 on a family mean basis. Thus, resistance was controlled by many loci with co-dominant alleles and should be robust. There was no linkage of resistance genes to the low alkaloid or white flower/green plant loci. Assuming the absence of dominance, the upper limit of narrow-sense heritability on a single plant basis was 0.50 ± 0.13. Several rounds of selection of partially resistant individuals in F2 families and intercrossing among F3 plants from many different wild � domesticated crosses are likely to produce cultivars with much more resistance than those now available. The applicability of the results at Wagga Wagga to the eastern wheatbelt of Western Australia, where BS is acute, and optimum field selection procedures are discussed.