Melon production is severely constrained by several soil-borne disease pathogens. Of these pathogens, Didymella bryoniae (Auersw), Rehm that causes gummy stem blight (GSB) is one of the most destructive resulting in substantial economic losses (Bruton, 1998; Crosby et al., 2002; Frantz and Jahn, 2004; McCreight, 2002; Wako et al., 2002). The pathogen attacks and inflicts heavy losses in quality and yield in several other genera of the cuburbitaceae worldwide, e.g., squash (Cucurbita L.) (Zitter and Kyle, 1992), cucumber (Cucumis L.) in the United States (Gusmini and Wehner, 2002; St. Amand and Wehner, 1991) and on watermelon (Citrullus Neck.) (Keinath et al., 1995). The disease has also been reported in Europe (Frantz and Jahn, 2004; van Steekelenburg, 1985), Asia (Wako et al., 2002), and elsewhere (Bruton, 1998).
Although chemical control has had great success, repeated use of fungicides is not advisable as a long-term solution as a result of the negative impact of pesticides in the environment. Moreover, development of resistance to some systemic benzimidazole fungicides in D. bryoniae from several cucurbit production areas has been reported (Kato et al., 1984; Keinath and Zitter, 1998; Malathrokis and Vokalounakis, 1983). Crop rotations are only partially effective; wind and other agents can easily bring in new inoculum to start new infections (Tullu et al., 2002). This illustrates the necessity to use host resistance in conjunction with good cultural practices and judicial use of fungicides. Use of resistant cultivars is the most strategic, environmentally friendly, accepted, and economic means of GSB management (Vokalounakis, 1993; 1995; Wehner and St. Amand, 1993).
Several greenhouse, plastic tunnel, and field evaluations were previously conducted to identify sources of resistance to D. bryoniae. A limited amount of resistance was reported in some cultigens. Sowell et al. (1966) tested over 1000 accessions of melon and reported resistance in plant introduction 140471 to be near immunity. On the contrary, McGrath et al. (1993) contends no sufficient resistance to GSB is available in melon. Furthermore, resistance in plant introduction 140471 has recently been described as variable (Tsutsumi and da Silva, 2004) or not effective at all (Takada, 1983). Several other sources of resistance to gummy stem blight in melon have been reported over the years (McGrath et al., 1993; Sakata et al., 2000; Sowell, 1981; Takada, 1983; Tsutsumi and da Silva, 2004; Zhang et al., 1997). Zhang et al. (1997) found plant introductions 157076, 157080, 157081, 157082, 157084, 482393, 482398, 482399, 482402, 482403, 482408, 255478, and 511890 to be as good as plant introduction 140471.
Most GSB-resistant melon varieties and breeding lines released to date derive resistance from plant introduction 140471 (McGrath et al., 1993; Norton, 1971, 1972; Norton and Cosper, 1989; Norton et al., 1985; Sowell, 1981). However, plant introduction 140471 has failed to provide sufficient resistance (e.g., Sitterly and Keinath, 1996; Zhang et al., 1997). Moreover, plant introduction 140471 was been reported to be ineffective against a Japanese isolate of D. bryoniae (Sakata et al., 2000; Takada, 1983). Zhang et al. (1997) also reported resistance derived from plant introduction 140471 appears to diminish when deployed into commercial, large-fruited varieties. There is still interest in developing melon varieties with higher GSB resistance than existing ones; hence, new sources of resistance should be sought and validated. New sources of resistance will be useful in melon production areas where existing sources of resistance are not effective.
Information on the genetics of resistance to the GSB pathogen in most melon cultigens reported as resistant is limited (Pitrat et al., 1998). Two genetic systems have been proposed, monogenic-dominant and monogenic-recessive. One dominant gene (Mc) confers high resistance to GSB in plant introduction 140471 (Prasad and Norton, 1967; Sowell et al., 1966); another gene, Mc-2, controlling an intermediate level of resistance in breeding lines C-1 and C-8, was reported by Prasad and Norton (1967). Frantz and Jahn (2004) confirmed the monogenic-dominant status of Mc in plant introduction 140471. Plant introduction 157082 and 511890 were both reported to carry monogenic-dominant genes for GSB resistance, whereas resistance in plant introduction 482399 was monogenic-recessive (Frantz and Jahn, 2004; Zuniga et al., 1999). Frantz and Jahn (2004) reported a monogenic dominant gene for GSB resistance in plant introduction 482398. The genes in plant introductions 140471, 157082, 511890, 482498, and 482399 were renamed Gsb-1, Gsb-2, Gsb-3, Gsb-4, and gsb-5, respectively (Frantz and Jahn, 2004). The five genes are independent of each other (Frantz and Jahn, 2004). Resistance in a sixth accession, Jmu-15 (C melo var. agrestis), is conferred by incomplete dominant gene(s) (Wako et al., 2002). However, allelism in this accession has not been verified nor has the gene(s) been named. The genetics of resistance and allelic relationships of most melon plant introductions reported as resistant to GSB is still unknown, incomplete, or has not been verified, e.g., in plant introductions 296345, 266935, 436533, 157076, and others. Knowledge of the genetic basis of resistance to D. bryoniae is essential for the efficient development of resistant melon cultivars. The evaluation, reevaluation, crossing, and backcrossing processes can be time-consuming and tedious. Often because the genetics of resistance of resistant genotypes are unknown, the introgression process becomes one of trial and error. There is need, therefore, to determine the mode of inheritance to the GSB pathogen in other sources and possibly establish if these sources of resistance share the same genetic factors.
The objectives of the present study were: 1) to critically examine the resistance of melon germplasm to D. bryoniae and 2) to characterize the genetics of resistance in plant introductions 157076, 420145, and 323498. These plant introductions were confirmed to be highly resistant to D. bryoniae and are fairly well adapted to Nanjing conditions.
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