Genetics of Aphanomyces disease resistance in sugarbeet (Beta vulgaris), AFLP mapping and QTL analyses

Aphanomyces cochhoides causes damping-off and root-rot diseases in sugarbeet. As a seedling disease, Aphanomyces severely reduces stand establishment in the fields. Since there is no effective chemical control, resistance breeding has been considered the best approach to overcome this problem. All domestic varieties are more or less susceptible to this disease, and the genetic basis of reduced susceptibility is poorly understood.; Evaluation of Aphanomyces resistance has been problematic for resistance breeding, since no reliable disease screening method has been available. Five different inoculation methods have been tried in this research, and a novel disease evaluation system "box inoculation" was developed, which could effectively reduce experimental error. Active zoospores were found to play a major role in the infection process. By using active zoospores as the inoculum, resistant and susceptible varieties could be reliably discriminated. In addition, submersion of seedlings in water facilitated resistance evaluation by creating anoxic conditions, which are often associated with severe symptoms in the field. Twenty sugarbeet accessions were tested for disease resistance. None was immune, but obvious differences in relative susceptibility were observed. Lab test results were shown to be consistent with field data, implying the feasibility of using lab test results to predict relative field performance.; A segregating F2 population was generated from a cross between Aphanomyces susceptible C869 (sugarbeet) and resistant wild beets (Beta vulgaris ssp. maritima). From 145 F2 plants, an AFLP (amplification fragment length polymorphism) genetic linkage map was constructed, which contained 163 markers on 9 linkage groups. The total map length was 507.1 cM.; Since resistance evaluation is destructive, Aphanomyces resistance of each F2 plant was estimated by the average performance of self-pollinated F3 families in a progeny test. Composite interval mapping analysis of Aphanomyces disease resistance identified one major QTL (quantitative trait locus) on linkage 9 (p < 0.01), and one minor QTL on linkage 2 (p < 0.05). The model combining both QTLs explained 63.2% variation of relative AUDPC (Area Under Disease Progress Curve, rAUDPC). Resistance introduced from wild beet (Beta vulgaris ssp maritima, PI540625) was heritable, and the broad sense heritability of rAUDPC was estimated to be 40.1%.