Volume 30 Number 1 Article 3 Pages: 7-8
Year 1976 Month 1
Title: Fire Blight Resistance and Fruit Quality in Pear
Authors: J. Janick and R.L. Bell
Citation
Abstract:
An analysis was made of pear breeding records of the United States Department of Agriculture involving 11,972 seedlings from 395 crosses made between 1961 and 1964 and evaluated between 1965 and 1974 at Baeltsville, Maryland.
The lowest fire blight score ever recorded for each seedling was used as a measure of inherent fire blight resistance; trees that did not show any evidence of fire blight were included only if they flowered.
Fruit quality of 2604 trees was rated on the basis of flavor, grit, texture, russet, and appearance, total score, and a weighted total score which was used as a selection index.
Phenotypic correlation of coefficients (r) between resistance and each of the quality factors were all close to zero.
This was true for the combined data based on all progeny (n=2604), for particular species combinations, and for a sample of individual parents.
THis indicates that blight resistance and quality are not genetically linked or physiologically associated, that selection for resistance and high quality is feasible, and that susceptible seedlings may be discarded with impunity.
A method was established to obtain unbiased estimates of parental blight resistance and mean progeny blight values from breeding records.
Phenotype was estimated from verious trials in which three cultivars ('Bartlett', 'Moonglow' and 'Magness') were included; these were regarded as standards.
The rating of cultivars not common to all trials were adjusted on the basis of their average perfromance, relative to the mean of the three standards in each trial.
In a similar manner progeny mean blight values were estimated.
This was done using progeny means for blight resistance of an elite set of eight selections ('Bartlett', 'Kieffer', 'Magness', 'Moonglow', 'Bradford', 'US 307', 'US 301' and 'US 539') all crossed with the same four testers ('Bartlett', 'Kieffer', 'Magness' and 'Moonglow'). The progeny means of all other crosses involving one or more of these eight elite parents were then adjusted to the grand mean of the elite set on the basis of their relative performance for each cross.
This technique permits a comparison of relative prepotency of each parent.
Heritability for fire blight resistance was determined from the regression of progeny means on the midparental adjusted phenotype; h2 was 0.49. This means that half of the variability between progeny means for fire blight resistance os due directly to additive genetic differences.
The technique of estimating adjusted parental and progeny means should prove valuable for determining the genetics of fire blight resistance and for developing breeding strategy in pear improvement.
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