Citrullus lanatus (Thunb.) Matsum. & Nakai - Cucurbit Breeding ...

test. The large genetic variance found in our study indicates that the hypothesis of a quantitative trait is more
likely than the hypothesis of a single gene with large environmental variation.
Additive genetic effects were estimated, but a comparison with dominance effects was not possible.
With our experimental design, dominance variance could be estimated by subtraction of genetic and additive
variances from the phenotypic, but such an indirect estimate would not be precise. Additive effects in our
experiment were large in the greenhouse tests (mean = 4.45), but small in the field tests (mean = 1.32).
The broad-sense heritability was high for field and greenhouse tests (0.68 vs. 0.73, respectively). In
the family PI 482283 × 'Calhoun Gray' the broad-sense heritability was largely different among tests (0.21 in
the field test vs. 0.82 in the greenhouse test). The narrow-sense heritability was much larger in the greenhouse
than in the field tests (1.13 vs. 0.54, respectively), except for the family PI 526233 × 'Allsweet' (1.14 vs. 1.93,
respectively).
Our data indicated that broad-sense heritability for resistance to gummy stem blight in watermelon can
be high, indicating more importance of genetic than environmental variability in many tests. Nevertheless,
greenhouse testing should be used to capitalize on the higher additive components and increase the narrow-
sense heritability for population improvement. In addition, the overall large heritability estimates confirm that
the genotype has a larger effect than the testing environment, even though the use of more uniform and
controlled environments, as in greenhouse tests, helps to enhance the genotypic effect and to allow more precise
selections of resistant parents for the next generation.
Our analysis could not estimate dominance and epistatic effects. Thus, the estimates of the minimum
number of effective factors (genes) for resistance may be biased. We used five estimates, but only those most
consistent among families are presented (Table 4). These estimates indicate that few genetic factors may be
involved in the inheritance of resistance to gummy stem blight in watermelon.
Our analysis showed that almost no progress can be done by field selection. On the contrary, selection
based on greenhouse data could lead to a gain of at least three points (on a 10 point scale) per generation even
under the lower selection intensities (i.e., 20%) typically used in recurrent selection programs.
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