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

needed to obtain a desired fruit weight. Recurrent selection for population improvement seems to be a valid
breeding method, even though lower gain per cycle would be obtained, due to the lower selection intensity
(typically equal to 20%). Nevertheless, it may be easier to introgress desired qualitative traits into breeding
lines of desired fruit weight by pedigree or backcross breeding, rather than trying to change the fruit weight of
otherwise acceptable cultivars. Low heritability, quantitative inheritance, and high environmental variance are
important limiting factors that may greatly reduce the realized gain from selection in populations of cultivated
watermelon.
Resistance to gummy stem blight in our studies was shown to be determined by quantitative trait loci,
rather than the inheritance of the single gene db from PI 189225, as previously described. Few QTLs may be
involved in the expression of resistance, and the db gene may be a QTL with a major effect or a single
Mendelian gene, under epistatic influence of other regulatory QTLs. In addition, we measured a large
heritability and high additive variance for resistance. Watermelon breeders interested in the development of
resistant cultivars should use breeding techniques that make the best use of additive variance, such as recurrent
selection. Greenhouse testing of the breeding material would be preferred to field testing, based on the larger
heritability for the greenhouse test. Field testing may be used at later stages of selection to confirm resistance of
improved material under field conditions. Multiple locations and replications, and large plots to simulate
commercial production fields should be used. A precise assessment of the causal agent for the loss of plants
also would be necessary before classifying them as susceptible.
Based on the inheritance of resistance to gummy stem blight in our populations (few effective factors
involved, and heritability high in the greenhouse and low in the field), we suggest that QTL mapping would be
a reliable way to: 1) identify molecular markers closely linked to resistant genes (or QTLs), and 2) clarify the
genetics of resistance. The identification of QTLs and the estimation of their average effect could be useful for
the development of a proper MAS breeding strategy for this trait in watermelon. In addition, markers and QTLs
identified through greenhouse testing of the segregating populations could be verified in field tests under natural
epidemics of gummy stem blight on adult plants. The availability of codominant SSR markers in watermelon is
still very limited, thus making it difficult to easily saturate linkage maps and proceed to fine QTL mapping by
random pairing of markers. Our efforts to identify markers linked to resistance to gummy stem blight, so far,
167