Bitter gourd (M. charantia L.) is an important cucurbitaceous market vegetable in Asia, where more than 340,000 ha are devoted to its cultivation annually (McCreight et al., 2013). Its cultivation is gaining popularity in some African countries such as Ghana, Zambia, Congo, and Madagascar for local consumption or for export to Europe and the Middle East to cater the demand of emigrant Asian communities. It is also cultivated to a lesser extent in the southern United States and Australia (Northern Territory, Queensland, New South Wales, and Victoria), where popular Asian hybrid cultivars are cultivated for consumption mainly by ethnic communities from Asia (Morgan and Midmore, 2002).
Bitter gourd fruit is a rich source of beta-carotene, vitamin C, folic acid, magnesium, phosphorus, and potassium (Dhillon et al., 2017; Yuwai et al., 1991). The health and pharmacological properties of bitter gourd have been well documented (Tan et al., 2016). Currently, 422 million people worldwide have diabetes (World Health Organization, 2016) and Type 2 diabetes accounts for around 90% (379 million). Bitter gourd fruit is used in folk medicine to manage Type 2 diabetes (Abascal and Yarnell, 2005; Grover and Yadav, 2004; Lans, 2006).
Cucurbit powdery mildew (CPM) caused by Px is a serious fungal foliar disease of cucurbit production in open fields and greenhouses. Disease outbreak brings reduction in plant growth, premature foliage loss, and reduction in yield and fruit quality (Keinath and DuBose, 2004). CPM on bitter gourd is currently controlled by fungicides, although fungicide resistance has developed in some areas (Lebeda et al., 2010; McGrath, 2006). The pathogen is highly variable in virulence and represented by many pathotypes (Lebeda et al., 2011) and races (Lebeda et al., 2016). The use of disease-resistant varieties is an economical and safe approach for disease management. Accessions resistant to CPM have been identified in melon (Cucumis melo; Dhillon et al., 2012), watermelon (Citrullus lanatus; Thomas et al., 2005), cucumber (Cucumis sativus; Block and Reitsma, 2005), squash (Cucurbita pepo; Lebeda and Křístková, 1996), pumpkin (Cucurbita moschata; Wessel-Beaver, 1993), and bottle gourd (Lagenaria siceraria; Kousik et al., 2008). Resistance to CPM is, however, often race-specific and not durable (Lebeda et al., 2008, 2016).
Commercial cultivars of bitter gourd resistant to CPM are not currently available. We developed five inbred lines resistant to CPM after screening 150 accessions of a global collection of bitter gourd in the World Vegetable Center genebank against the local CPM population at Kamphaeng Saen (Thailand). A single resistant plant was identified in each of five segregating populations derived from five genebank accessions that originated from India, Thailand, Taiwan, and Belize. Multiple cycles of inbreeding and selection led to the development of the five CPM-resistant inbred lines. We evaluated four of these inbred lines in 2011 against local isolates of Px in Thailand, Taiwan, and the United States (South Carolina, Florida, California) (Dhillon et al., 2015). We report here the reactions of the five bitter gourd CPM-resistant inbred lines against local isolates of Px at 12 locations in five Asian countries (China, India, Thailand, Vietnam, and Philippines) in 2013 and 2014. In addition, we sought to relate Asian CPM–bitter gourd interactions to the more developed body of knowledge of CPM–melon interactions, first by challenging these lines with European and the Mediterranean CPM isolates, and second by challenging a set of melon CPM race differentials with a single-spore strain isolated from a local isolate of Px on bitter gourd from Kamphaeng Saen, Thailand. The latter test also challenged representatives of cucumber, summer squash, and watermelon, cucurbit species on which few CPM races have been identified, with the exception of watermelon where four CPM races have been defined (Davis et al., 2007; Kousik et al., 2011; Mercier et al., 2014; Zhang et al., 2011). Furthermore, we evaluated horticultural fruit characters of the CPM-resistant bitter gourd breeding lines in the field test at Kamphaeng Saen, Thailand in 2014, and assessed the yield potential of two of the lines in Thailand, Myanmar, Vietnam, and Bangladesh in 2016, to assess their horticultural value as sources of CPM resistance.
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