Leafhopper resistance mechanisms in common bean and mapping of associated quantitative trait loci
The leafhoppers 'Empoasca fabae' Harris and 'Empoasca kraemeri' Ross & Moore can be very damaging to field beans in North America, and Latin America, respectively. Development of molecular markers for leafhopper resistance will allow plant breeders to make selections in the absence of leafhopper populations and in early generations. The first part of the study examines the damage caused by leafhoppers in the small white-seeded, 'E. kraemeri' resistant line EMP 419 and the large tan-seeded leafhopper susceptible line Berna, and a population of 108 (F5:6-8) recombinant inbred lines (RILs) derived from these parents. Damage scores for leaf burn and leaf curl scores were significantly correlated. In the RILs, growth habit explained up to 26% of leafhopper damage scores for both 'E. fabae' and 'E. kraemeri', and variation for seed-coat colour explained approximately 15% of ' E. kraemeri' resistance. The second part of the study investigated the utility of the leafhopper resistance in the RILs. There was a positive correlation of visual damage scores with percent reductions in seed count, seed yield and seed weight for both 'E. fabae' and 'E. kraemeri'. Nymph counts were positively correlated with leaf curl scores (r = 0.66) in Ontario but not in Colombia. Nymph counts on a subset of lines from the RIL population revealed that resistant RILs hosted fewer 'E. fabae' nymphs than the susceptible RILs. The aim of the third part of the study was to partially sequence a set of 79 bean RFLP clones to develop PCR-based markers for bean. Fifty-nine of the clones had homology to either known or predicted gene sequence. In addition, 16 STSs were developed. The fourth part of the study used the RIL population to identify molecular markers associated with leafhopper resistance. Bulked segregant analysis using RAPD markers was used to identify markers linked to leafhopper resistance. Quantitative trait analysis using a linear model detected 15 and 12 markers associated with 'E. fabae' and 'E. kraemeri' resistance, respectively. Composite interval mapping positioned quantitative trait loci for leafhopper resistance to 'E. fabae' and ' E. kraemeri' resistance on bean core linkage groups B1, B3, and B7.