Gummy stem blight (GSB) is a major disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] that leads to significant economic losses. This disease 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). In 1891, GSB was first observed on cucumber (Cucumis sativus L.) by Fautrey and Roumeguere in France, and it was observed on watermelon in Delaware (Sherf and MacNab, 1986). In 1917, GSB affecting watermelon fruit in Florida was reported (Sherbakoff, 1917). Furthermore, GSB remains an important limiting factor for watermelon production in Florida (Keinath, 1995) and South Carolina (Rennberger et al., 2018, 2019). This disease has also affected watermelon production in some important watermelon-producing countries (Basim et al., 2016; Huang and Lai, 2019). Currently, GSB on watermelon plants is as evident as crown blight, stem cankers, and extensive defoliation, with symptoms observed on the cotyledons, hypocotyls, leaves, and fruit (Maynard and Hopkins, 1999). S. cucurbitacearum is seed-borne (Lee et al., 1984), airborne (van Steekelenburg, 1983), and soil-borne (Bruton, 1998).
Adequate control of GSB through fungicide applications (Keinath, 2016) and appropriate cultural practices (dos Santos et al., 2016; Keinath, 1996) is difficult, particularly during rain, when relative humidity remains high for extended times (Café-Filho et al., 2010). In addition, there is concern among pathologists regarding the development of resistance to fungicides by S. cucurbitacearum (Avenot et al., 2012; Li et al., 2016; Thomas et al., 2012). 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).
In previous studies, PI 189225 was identified as the most resistant accession in the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) watermelon germplasm collection (Sowell and Pointer, 1962). Later, PI 271778, PI 500335, PI 505590, PI 512373, PI 164247, and PI500334 were also identified as accessions resistant to GSB (Boylan et al., 1994). When resistant PI 189225 was crossed with susceptible Charleston Gray, a single recessive gene (db/db) controlling the resistance was identified (Norton, 1979). To develop resistant cultivars with a high yield of high-quality fruit, PI 189225 and PI 271778 were chosen as resistant parents in crosses with ‘Crimson Sweet’ and ‘Jubilee’. Several lines with moderate resistance to GSB were released as ‘AU-Jubilant’, ‘AU-Producer’ (Norton et al., 1986), ‘AU-Golden Producer’ (Norton et al., 1993), and ‘AU-Sweet Scarlet’ (Norton et al., 1995). However, they were much less resistant to GSB than the resistant parents PI 189225 and PI 271778. In cucumber, it was found that genetic factors were weaker than environmental factors, and that there were five genes controlling resistance (St. Amand and Wehner, 2001). Recently, watermelon accessions with GSB resistance, PI 189225, PI 482283, and PI 526233, were crossed with susceptible cultivars, and their progeny showed a continuous distribution of the disease phenotype and partial failure of the data to fit the single gene inheritance. Therefore, resistance to GSB in PI 189225, PI 482283, and PI526233 may be controlled by a more complex genetic system (Gusmini et al., 2017).
Additional sources of resistance to and variations in fungicide effectiveness against GSB in watermelon have been reported (Gusmini et al., 2005; Li and Brewer, 2016). Additionally, an efficient screening method has been developed for identifying resistant germplasm (Song et al., 2004), including a system for the mass production of inoculum of S. cucurbitacearum for large field screening experiments (Gusmini et al., 2003). Available plant introduction accessions (total of 1274 accessions) from the USDA-ARS watermelon germplasm collection, along with 51 cultivars, were tested to identify new sources of resistance to GSB (Gusmini et al., 2005). A total of 59 accessions that were at least as good as PI 189225 and PI 271778 were identified in field and greenhouse tests. Two of the best were PI 482283 and PI 526233.
Resistance to pathogens can be qualitative or quantitative. However, quantitative resistance requires more time and resources because inheritance is complex and the levels of resistance often are less distinct. Because efforts to transfer a single gene for resistance to adapted cultivars were not successful, we used a different approach. Resistant accessions were intercrossed multiple times before crossing progenies from advanced cycles to adapted cultivars. After that, we crossed and intercrossed progeny from resistant × elite for multiple cycles. We hoped to improve our chances of transferring resistance genes from Citrullus amarus to C. lanatus.
We used the rating method for GSB resistance created by Gusmini et al. (2002). The scale for leaves and stems was as follows: 0 = no disease; 1 = yellowing on leaves (a trace of disease only); 2 to 4 = symptoms on leaves only; 5 = some leaves dead, no symptoms on stem; 6 to 8 = symptoms on leaves and stems; and 9 = plant dead. The scale was used for screening accessions from the watermelon germplasm collection for resistance to GSB (Gusmini et al., 2005). The scale has been used in genetic analyses of large numbers of plants tested in multiple environments (Gusmini et al., 2017). Another method of measuring disease severity involves the area under the disease progress curve (AUDPC). The AUDPC is a measure of quantitative disease resistance that uses data from repeated ratings (Schandry, 2017; Simko and Piepho, 2012; Yuen and Forbes, 2009). This assessment of disease severity summarizes disease progress over time with a single value (Schandry, 2017; Yuen and Forbes, 2009). We used those rating methods to ranking resistant and susceptible cultigens (Gusmini et al., 2017; Schandry, 2017).
In addition to high resistance to GSB, we were interested in retaining as much external and internal fruit quality characteristics as possible for use in watermelon breeding (Haejeen et al., 2010). Some descriptors are used for morphological characterization of watermelon (Szamosi et al., 2009). However, because the number of morphological characteristics in watermelon is high, the minimum descriptor list is used in breeding studies. Fruit shape is an important characteristic to consider because of the different market requirements. Generally, fruit shape categories are elongate, round, or oval, and the genetic basis is one gene involving the round or elongate shape, with the F1 being intermediate (Wehner, 2008). Similarly, rind pattern and toughness (durability) are important characteristics of watermelon fruit. Rind patterns can be gray, striped, or solid, and rind stripes can be narrow, medium, or wide (Gusmini and Wehner, 2006). Good rind durability (toughness) is important for reducing losses due to shipping damage. Ideally, the rind should be thick and tough on large-fruited cultivars, but a thin and tough rind is desirable on small-fruited cultivars. In general, the thickness should be a small percentage of the flesh diameter to obtain the maximum edible volume. Rind toughness can be measured by driving a spring-loaded punch into the rind, dropping the fruit onto the ground from the height of the knees to determine whether it breaks, and using the “thumb” test, which involves the breeder pressing on the rind (Wehner, 2008). The flesh color of watermelon fruit is an important trait for consumers. Flesh color can be scarlet red, coral red, orange, canary yellow, salmon yellow, or white (Zhang et al., 2017). Scarlet red (YScrYScr) is dominant to coral red (YCrlYCrl), which is dominant to orange (yoyo), which is dominant to salmon yellow (yy). Canary yellow (CC) is dominant to noncanary yellow (cc) and epistatic to (overcomes) the y locus for scarlet red-coral and red-orange-salmon yellow. Coral red is recessive to the white flesh color that is common in citron. Additionally, the seed color and size of watermelon fruit are important for the market. Seed color can be white, tan, brown, black, red, green, or dotted. Seed size can be tomato, small, medium, or large. Black seed color is the most attractive with scarlet red or canary yellow flesh color (Wehner, 2008).
The objective of this study was to evaluate an RIL population and starting materials of watermelon (C. lanatus subsp. lanatus × C. lanatus var. citroides) for resistance to GSB and fruit quality traits. We aimed to combine resistance genes from PI accessions from several cycles of intercrossing with high-quality fruit from susceptible cultivars to identify lines for cultivar development.
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