Reactions of Sugar Beet to Powdery Mildew: Genetic Variation, Association Among Testing Procedures, and Resistance Breeding

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Abstract
Studies were done on sugar beet (B. vulgaris) cultivars to obtain an estimate of the genetic variation and heritability for disease reaction to powdery mildew, incited by Erysiphe polygoni, and to determine the association among field, greenhouse and laboratory reactions. S1 and full-sib progeny families were grown in the field to estimate variance and heritability of mildew reactions. Genetic variances were significant, and broad-sense heritability estimates for disease reaction were 54-75%. Consistently severe disease developed on susceptible genotypes in the greenhouse following inoculation of plants 6-10 wk old. Mildew ratings from greenhouse evaluations were significantly correlated with field ratings, indicating that greenhouse tests on young plants can be used to predict field reactions. In greenhouse tests, significant interaction for mildew ratings occurred between cultivars and the age (6.8 or 10 wk) at which inoculation occurred. However, this interaction would cause little difficulty in correctly differentiating cultivars unless their reactions were similar. The evaluation of young seedlings (4 wk at inoculation) in flats in the greenhouse only discriminated lines with wide differences in reaction. Ratings from a laboratory in vitro test with inoculated leaf strips did not correspond with ratings obtained from greenhouse or field evaluations. The occurrence of genetic variation for disease resistance and the association between field and greenhouse ratings suggested that a breeding program based on individual plant or line reaction in greenhouse tests should improve resistance to E. polygoni. From a susceptible, open-pollinated source, 2 cycles of mass selection for resistance in the greenhouse reduced the mean reaction from 8.0 for the parent to 5.3 for the 2nd cycle synthetic in a greenhouse test and from 7.0 to 4.6 in a field test.

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