Resistance to gangrene (<i>Phoma exigua</i> var.<i>foveata</i>) and dry rot (<i>Fusarium solani</i> var.<i>coeruleum</i>) in potato tubers. 2. Evaluation of genotypic differences in pectin-bound magnesium and calcium for screening purposes

The levels of incorporation of Mg and Ca into tuber cell walls and middle lamellae were compared with the intensity of attack by Phoma exigua var. foveata (P.e.f.) (gangrene) and Fusarium solani var. coeruleum (F.s.c.) (dry rot) in six genotypes varying in their levels of resistance. The Mg:Ca ratio was higher in those genotypes more resistant to P.e.f. , while the cation effect on attack by F.s.c was variable. Although genotype resistance may be improved by extra Mg bridges in the pectin structure the genotypic differences in susceptibilty to gangrene could not be explained by differences in Mg content assessed from alcohol-insoluble solids. Analyses of breeders' selections showed no general relationship between Mg or Ca contents and disease incidence. Therefore, genotypic differences in incorporation of these cations could not be used for screening for improved resistance.