Gummy stem blight is a major disease of watermelon [C. lanatus (Thunb.) Matsum. & Nakai]. It is caused by three genetically distinct Stagonosporopsis species, S. cucurbitacearum (syn. Didymella bryoniae), S. citrulli, and S. caricae (Stewart et al., 2015). The three species are pathogenic to cucurbits, but S. caricae also causes leaf spot and stem and fruit rot in papaya (Carica papaya) (Stewart et al., 2015). This disease was first observed in 1891 by Fautrey and Roumeguere in France on cucumber (Cucumis sativus L.) and in Delaware in watermelon (Chiu and Walker, 1949; Sherf and MacNab, 1986). In 1917, GSB was reported in the Southern United States, affecting watermelon fruit in Florida (Sherbakoff, 1917). Gummy stem blight remains an important limiting factor for watermelon production in Florida (Keinath, 1995; Power, 1992). Gummy stem blight on watermelon plants is evident as crown blight, stem cankers, and extensive defoliation, with symptoms observed on the cotyledons, hypocotyls, leaves, and fruit (Maynard and Hopkins, 1999). Stagonosporopsis cucurbitacearum is seedborne (Lee et al., 1984), airborne (van Steekelenburg, 1983), and soilborne (Bruton, 1998; Keinath, 1996).
Adequate control of GSB through fungicide applications (Keinath, 1995, 2000, 2016) and appropriate cultural practices (dos Santos et al., 2016; Rankin, 1954; Keinath, 1996) is difficult, particularly during rainfall when relative humidity remains high for extended times (Café-Filho et al., 2010). In addition, there is concern among pathologists and breeders for the development of resistance by S. cucurbitacearum to fungicides (Avenot et al., 2012; Kato et al., 1984; Keinath and Zitter, 1998; Li et al., 2016; Malathrakis and Vakalounakis, 1983; Miller et al., 1997; Thomas et al., 2012; van Steekelenburg, 1987). Resistance to GSB has received attention since the 1970s as a possible alternative to chemical control (Lou et al., 2013; Norton et al., 1986, 1993, 1995).
Differences in GSB resistance among commercial cultivars of watermelon (C. lanatus) were reported, with ‘Congo’ the least susceptible, ‘Fairfax’ intermediate, and ‘Charleston Gray’ the most susceptible (Schenck, 1962). Resistance assays by controlled inoculation of watermelon plants using spore suspensions of S. cucurbitacearum identified PI 189225 and PI 271778 as the most resistant accessions available in the USDA-ARS watermelon germplasm collection (Sowell, 1975; Sowell and Pointer, 1962). In crosses with susceptible ‘Charleston Gray’, a single recessive gene db was determined to confer resistance in PI 189225 (Norton, 1979). Resistant watermelon cultivars were developed from two crosses (‘Jubilee’ × PI 271778 and ‘Crimson Sweet’ × PI 189225) by selecting disease-resistant seedlings from backcrossed families that had a high yield of excellent quality fruit (Norton et al., 1986). ‘AU-Jubilant’, ‘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 resistance to GSB. However, they were found less resistant to GSB than the resistant parents PI 189225 and PI 271778. To date, no cultivars of watermelon have been released that have a high level of resistance to natural epidemics of GSB.
The expanding watermelon industry in the southeastern United States and the increasing losses due to GSB outbreaks in the last decade led to a new set of studies for the use of genetic resistance to control GSB in watermelon (Gusmini et al., 2005; Li and Brewer, 2016). The watermelon breeding program at North Carolina State University developed an efficient screening method for testing watermelon germplasm (Gusmini and Wehner, 2002; Song et al., 2004), including systems for mass production of inoculum of S. cucurbitacearum for large field screening experiments (Gusmini et al., 2003), and a disease assessment scale for rating foliar and stem lesions (Gusmini et al., 2002). Available PI accessions (totaling 1274) from the USDA-ARS watermelon germplasm collection, along with 51 adapted cultivars, were tested to identify new genetic sources of resistance to GSB (Gusmini et al., 2005). A total of 59 new accessions were identified that had resistance to GSB as good as or better than PI 189225 and PI 271778 at the field and greenhouse tests. Two of the best were PI 482283 and PI 526233.
The objective of this study was to determine the inheritance of resistance to GSB in watermelon accessions PI 482283 and PI 526233, along with the previously identified accession PI 189225. Because of the unsuccessful breeding history for this trait, we hypothesize that resistance to GSB is due to a more complex mode of inheritance, which will be tested by validating the monogenic inheritance of db gene in PI 482283 and PI 526233.
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