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

9 = plant dead. Plants with a disease rating greater than 5 had lesions on the stem, thus being prone to death
from subsequent development of the disease. Plants with a disease rating of 5 or less had lesions only on the
leaves (no stem lesions). Leaf ratings are important, because plant yield and survival is affected by leaf area,
which is reduced by severe disease outbreaks. Stem ratings are important, because large, localized lesions can
kill the plant, especially if located near the crown (base) of the plant. Nevertheless, our rating scale allowed a
quantitative assessment of the amount of disease present on each plant, because stem lesions are typically
accompanied by major leaf damage and appear after leaf lesions during the epidemic.
Experimental Design and Data Analysis
We conducted all our tests in the greenhouses at North Carolina State University in Raleigh, North
Carolina, and in the field at the Horticultural Crops Research Station at Clinton, North Carolina. The two
families 'Allsweet' × PI 526233 and 'Calhoun Gray' × PI 482283 were tested in 2002, while the other two were
tested in 2003. The experiment had two sets in 2002 and four sets in 2003 (equally divided in field and
greenhouse tests), each set including all six generations.
The inheritance of the db gene from Norton (1979) was tested after classifying each plant as
susceptible or resistant based on their rank relative to the mean value of the disease assessment scale adopted
(4.5). We performed segregation analysis and goodness-of-fit tests with the SAS-STAT statistical package
(SAS Institute, Cary, NC) and the SASGene 1.2 program (Liu et al., 1997). All χ 2 tests were performed at the
95% confidence level. Since there was strong evidence against the single gene hypothesis, we verified the
distribution of the F 2 data for each family using the UNIVARIATE procedure of SAS-STAT and by plotting the
disease ratings against their frequency, prior to analyzing resistance to gummy stem blight as a quantitative trait.
We tested the F 2 data for homogeneity of variances using the Bartlett's method (Ostle and Malone,
1988; Steel et al., 1997). Since variances were homogeneous only among tests (field vs. greenhouse) within
family, we pooled the data by family. We also analyzed the data for each family and test, to highlight possible
differences among tests.
Phenotypic (P), environmental (E), genotypic (G), and additive (A) variances were estimated from
generation variances as follows (Warner, 1952; Wright, 1968):
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