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Genetic Basis of Resistance to Systemic Infection by Xanthomonas axonopodis pv. dieffenbachiae in Anthurium

The genetic basis of systemic resistance to bacterial blight disease (blight) of anthurium (Anthurium andraeanum) caused by Xanthomonas axonopodis pv. dieffenbachiae was investigated in progenies of 53 crosses involving 31 parent cultivars using segregation analysis. Inoculation of parents and progenies was achieved by injecting the petiole base of the most recent fully expanded leaf with 100 μl of 10⁹ colony forming units per ml of the blight pathogen (strain X4gfp) transformed with the green fluorescent protein (GFP) gene. The time to death and the presence or absence of GFP fluorescence on newly emerging leaves was monitored over a period of 30 weeks after inoculation (WAI), on an individual plant basis. The expected resistance to susceptible ratios based on a digenic model involving two dominant genes, designated A and B, interacting according to a duplicate recessive epistasis model fitted the observed segregation ratios in the crosses. Based on the segregation ratios obtained, the parental cultivars were assigned plausible genotypes. There were significant differences (P < 0.001) in time to death following inoculation between the various genotypic designations. Cultivars with genotypes AABB, AABb, AaBB, and AaBb died within 10 WAI and designated as susceptible; AAbb and aaBB died from 18.8 to 25.6 WAI and were designated as moderately resistant; and Aabb, aaBb, and aabb produced resistant phenotypes. There was also some evidence for dosage effect especially in the highly resistant category. Hence, (AABb = AaBB = AaBb) < (aaBB = AAbb) < Aabb = aaBb = aabb). An approach to fixing resistance to blight in anthurium is discussed.