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

fungus that is seed-borne (Lee et al., 1984), air-borne (van Steekelenburg, 1983), or soil-borne (Bruton, 1998;
Keinath, 1996).
It is difficult to get adequate control of gummy stem blight through fungicide applications (Keinath,
1995; Keinath, 2000) and good cultural practices (Keinath, 1996; Rankin, 1954), particularly during periods of
frequent rainfall when relative humidity remains high for a long period. Genetic resistance to gummy stem
blight has received attention since the 1970s as an alternative to chemical control (Norton et al., 1993; Norton et
al., 1995; Norton et al., 1986). Resistant watermelon cultivars were developed by selecting disease resistant
seedlings from backcrossed families that produced high yield of excellent quality fruit (Norton et al., 1986).
From the program, 'AU-Jubilant' and 'AU–Producer' (Norton et al., 1986), 'AU-Golden Producer' (Norton et al.,
1993), and 'AU-Sweet Scarlet' (Norton et al., 1995) were released with moderate to high resistance to gummy
stem blight. However, they were much less resistant than the resistant parents PI 189225 and PI 271778.
We have developed an efficient screening method for testing watermelon lines (Gusmini and Wehner,
2002; Song et al., 2004), including systems for mass production of inoculum of D. bryoniae for large field
screening experiments (Gusmini et al., 2003), and a disease assessment scale to rate for the presence of foliar
and stem lesions (Gusmini et al., 2002). Between 1998 and 2001, all available accessions (1,274) from the
watermelon germplasm collection of the United States Department of Agriculture, Agricultural Research
Service (USDA-ARS), along with 51 adapted cultivars, were tested to identify new genetic sources of resistance
to gummy stem blight (Gusmini et al., 2005b). Additional efforts to study the inheritance of resistance are
underway and our results indicate that resistance to gummy stem blight may be under the genetic control of one
or more quantitative trait loci (QTLs). The db gene conferring resistance to PI 189225 (Norton, 1979) may
actually be either the QTL with a major effect, or a single Mendelian gene whose expression may be regulated
by minor genes.
It is difficult to test watermelon for resistance to gummy stem blight. The high variability associated
with the tests requires the use of multiple years, locations, and replications of progeny rows (Song et al., 2004).
The need for large replicated tests does not fit well in watermelon breeding programs. Plant breeders need
testing methods that allow early selection, preferably at the seedling stage, to make targeted crosses only among
plants with high resistance. In addition, in segregating generations single-plant selection would be more
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