Inheritance of Resistance in Strawberry to Bacterial Angular Leafspot Disease Caused by Xanthomonas fragariae

Bacterial angular leafspot disease (Xanthomonas fragariae Kennedy and King) of strawberry (Fragaria species and F. ×ananassa Duch. cultivars) has become increasingly important to strawberry fruit and plant production. Strawberry cultivars and species vary in susceptibility to infection. However, little is known regarding epidemiology of the disease and resistance to infection. Two octoploid genotypes, a native F. virginiana (US 4808, tested as SG-89) and a F. virginiana (SG 26) x F. ×ananassa ('Earliglow') hybrid (US 4809, tested as 80-4-38), previously were found to be highly resistant to two differentially pathogenic strains of X. fragariae representing two of four genotypic strain groups. Our objective was to determine the number of genes involved with resistance for these two strawberry genotypes, whether strawberry resistance is conferred by dominant or recessive alleles, and whether or not the heritability is high enough for breeders to reliably make selections of resistant individuals in breeding populations. About 120 F1 seedlings from crosses of susceptible 'Sweet Charlie' with each of the two resistant genotypes were clonally propagated and challenged with each of four X. fragariae strains. These strains were selected to represent four genotypes of X. fragariae defined by repetitive element based PCR: ATCC 33239, Xf- 3, Xf-6, and Xf-1425. Plants were quantitatively rated on a scale of 0 (resistant) to 5 (susceptible) in replicated evaluations. High estimates for broad sense heritability support the conversion of the quantitative disease scores to qualitative scores and the classification of genotypes as resistant or susceptible. The qualitative ratings were used to estimate the number of genes involved with resistance. Some segregation ratios fit a 7S:1R ratio, and others fit a 15S:1R ratio, indicating that three or four unlinked loci could explain the inheritance of resistance in these populations. The high estimates for broad sense heritability show that resistant progeny can be selected with confidence, though large populations will be needed to identify enough resistant progeny from which to select for other important traits. shown to provide some protection against further spread of BALD (Averre and Driver, 1994; Roberts et al., 1997). Antibiotics, such as streptomycin sulfate, oxytetracycline also have been effectively used (Averre and Driver, 1994), although they are costly and not registered for use on strawberry. However, none of these materials act to eradicate existing Xanthomonas infections. In addition, multiple sprays at short intervals with copper may be phytotoxic (Roberts et al., 1997) and would likely lead to bacterial resistance to the compound (Cooksey, 1990). Strawberry genotypes vary in relative susceptibility to infection by X. fragariae. 'Daybreak' reportedly was more severely affected by BALD than were 'Florida 90', 'Missionary', 'Tioga', 'Torrey', 'Salinas', and La. 1158, and infection of 'Florida 90' and 'Mission- ary' was minimal (Howard, 1971). Kennedy and King (1962b) found that of 64 cultivars and clones, only Minnesota 1716 (a third generation self of 'Belt' x 'Premier') exhibited higher apparent resistance than the other genotypes in greenhouse inoculation tests. In other greenhouse tests, 'Badgerbelle' and 'Atlas' were found more resistant to infection than were 'Earliglow', 'Guardian', 'Sparkle', 'Stelemaster', 'Raritan', 'Midland', 'Fletcher', 'Surecrop', 'Blakemore', and 'Suwannee' (Hazel and Civerolo, 1980).