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Bitter gourd (Momordica charantia L.) is a commercially and nutritionally important market vegetable in Asia cultivated mainly by smallholder farmers. Cucurbit powdery mildew (CPM) caused by Podosphaera xanthii (Px) is a nearly ubiquitous and serious fungal disease of bitter gourd. Five bitter gourd breeding lines (THMC 113, THMC 143, THMC 153, THMC 167, and THMC 170) were selected at theWorld Vegetable Center for resistance to a local isolate of Px in Kamphaeng Saen, Thailand.We evaluated the resistance potential of these five inbred lines against local isolates of Px at 12 locations in five Asian countries. Plants were inoculated with the respective local Px isolate 15 and 30 days after transplanting and additional Px-infected plants of the inoculated control were interplanted throughout each test. Plants were rated 60 days after transplanting for CPM reaction using a 0 (no evidence of infection) to 5 (>75% infection evident on individual leaves) disease severity scale. THMC 153 and THMC 167 were resistant to the local race of Px in all locations, whereas THMC 143 was observed resistant in all test locations except one in China. THMC 113 was resistant in each location except one in India. THMC 170 was susceptible in three locations in India. The multilocation tests revealed four unique Px races on bitter gourd in different Asian countries and sources of resistance for breeding CPM-resistant bitter gourd cultivars. Six strains of Px isolated from other cucurbits (Cucumis and Cucurbita) and representing five melon CPM races were unable to infect the susceptibleM. charantia accession THMC 144 and the five resistant breeding lines, indicating pathotype differences between them and an isolate of M. charantia origin typed as race 1 on melon. THMC 143 and THMC 167, which originated from India, exhibited good yield potential in trials conducted in Thailand, Myanmar, Vietnam, and Bangladesh. HORTSCIENCE VOL. 53(3) MARCH 2018 337 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 betacarotene, 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 K rístkov a, 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 singlespore 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. Materials and Methods Germplasm and field test sites. Five bitter gourd CPM-resistant inbred lines and a susceptible bitter gourd check line were evaluated against the respective local Px isolates at 12 locations in five countries in 2013 and 2014 (Table 1). Each field test was planted in a randomized complete block design with three replications of five plants per plot. Entries were planted on raised, 1.6-m wide beds covered with black plastic mulch. Plots were 5-m long on a single bed; each consisted of five transplants spaced 1-m apart. Plants were trellised on the plastic net erected on vertical bamboo poles. Field test. At each location, a spore suspension was prepared by detaching heavily sporulating leaves of susceptible THMC 144 and washing them with a spray of 100 mL of water and filtering through a double layer of cheesecloth. The suspension was diluted to a concentration of 4 · 10 conidia/mL of water as determined by a hemocytometer. This was freshly prepared as required for each inoculation. Seedlings were inoculated 15 and 30 d after transplanting, at the threeleaf stage of growth, at each location. The spore suspension was sprayed over the plants until runoff, by using a pressurized sprayer. THMC 144 plants with abundantly sporulating CPMwere used as spreader plants, placed between rows as additional sources of powdery mildew inoculum. Disease severity was rated on leaves of individual plants 30 d after the second inoculation using a 0–5 visual rating scale, where 0 = no symptom; 1 = 1% to 10%; 2 = 11% to 25%; 3 = 26% to 50%; 4 = 51% to 75%; and 5 = >75% of leaf surface covered by mycelium. Plant ratings of 0 and 1 were considered resistant. The susceptible check had a mean rating of 5.0 at all test locations. The 0–5 scale was converted to percentage usingmidpoints: 0 = 0%, 1= 5.5%, 2= 18%, 3= 38%, 4 = 63%, and 5 = 97% and the data were subjected to analysis of variance (ANOVA) using SAS general linear model (GLM) procedure (SAS Institute, Cary, NC). Mean separation was performed using Fisher’s least significant differences (LSD) at P# 0.05. Growth chamber tests. There were two growth chamber tests. The first evaluated the five breeding lines against European and Mediterranean CPM isolates, whereas the second evaluated various cucurbits with a single-spore CPM strain from Kamphaeng Saen, Thailand. Five plants, each of the five bitter gourd CPM-resistant inbred lines, and a susceptible check (Table 2) were inoculated at two-leaf stage, similarly as explained previously, with six CPM strains isolated from cucurbits in Europe and the Mediterranean area and typed for race on melon: Sm3 (race 1), S87-7 (race 2F), 00Sm39 (race 3), 98Sm65 (race 5), and 04Sm2 and 08Sm9 (race 3.5). A singlespore CPM strain isolated from M. charantia grown in an open field in Kamphaeng Saen, Thailand, was inoculated on five plants each of ‘Marketer’ cucumber, ‘Diamant’ summer squash, ‘Sugar Baby’ watermelon, and melon CPM race differentials [‘V edrantais’, ‘PMR 45’, ‘PMR 5’, WMR 29, PI 124112, 90625 (PI 313970), and AR Hale’s Best Jumbo]. Plants were raised in a glasshouse until the second leaf stage and then incubated after inoculation in a growth chamber (16 h day 26 C/8 h night 20 C) at GAFL, INRA, Montfavet, France. Disease severity was rated on leaves of individual plants after 10–14 d of inoculation, using 0–3 visual rating scale, where 0 = no visible Received for publication 2 Oct. 2017. Accepted for publication 11 Jan. 2018. Funding for this research was provided by the Federal Ministry for Economic Cooperation and Development, Germany (BMZ), Japan Ministry of Agriculture, Forestry and Fish

Cucurbit Powdery Mildew-resistant Bitter Gourd Breeding Lines Reveal Four Races of Podosphaera xanthii in Asia