Sources of Resistance to Race 2WF Powdery Mildew in U.S. Watermelon Plant Introductions

Powdery mildew of cucurbits, incited by Podosphaera xanthii (Castagne) Braun & Shishkoff (syn. Sphaerotheca fuliginea auct. p.p.), is an economically important foliar disease. which is now common in watermelon [Citrullus lanatus (Thunb.) Mastum. & Nakai]. This disease occurs in all watermelon-growing areas and can reduce yields by up to 30%. Finding and breeding for resistance to this disease is important to reduce dependence on fungicides and to use in combination with fungicides to limit the spread of fungicide-resistant P. xanthii. This is the first English report that race 2WF of P. xanthii can infect watermelon. It is the prevalent race of watermelon powdery mildew in Beijing. Watermelon PIs resistant to this disease may be valuable in watermelon breeding programs. We evaluated 2100 PI S1 lines, representing self-pollinated progeny of 820 accessions from the U.S. Citrullus sp. PI collection, for resistance to race 2WF powdery mildew. The accessions represent three species, C. lanatus var. lanatus (766), C. lanatus var. citroides (L. H. Bailey) Mansf. (53), and C. colocynthis (L.) Schrad. (1). Thirty lines demonstrated resistance; the most notable resistance was observed in 11 PI lines, PI 482246, PI 482283, PI 500307, PI 482324, PI 482308, PI 482322, PI 482321, PI 482276, PI 482319, PI 482335, and PI 482350. The majority of the resistant accessions were collected in Africa and include 12 C. lanatus var. lanatus and 18 C. lanatus var. citroides. Watermelon production is increasing throughout China as a result of the crop’s economic value; the Food and Agriculture Organization of the United Nations reports a total production for 2008 of 67.2 million MT (FAO of the United Nations, 2011). Over the past 20 years, powdery mildew has become a yield-limiting disease for this crop in China and other parts of the world (Davis et al., 2001; Feng, 1996; McGrath, 2001). This disease diminishes yields through decreased fruit size and fewer fruit per plant (McGrath and Thomas, 1996). Although the disease can often be managed with fungicides, adequate disease control frequently requires systemic fungicides because application to leaf undersides is difficult. The occurrence of powdery mildew populations resistant to fungicides is increasing (McGrath, 2001; McGrath and Thomas, 1996; for a review, see Lebeda et al., 2010). Resistant watermelon cultivars are important for slowing the spread of fungicideresistant Podosphaera xanthii and will offer growers options for disease control. P. xanthii and Golovinomyces cichoracearum (D.C.) V.P. Heluta (formerly Erysiphe cichoracearum D.C.) are the predominant fungi that cause powdery mildew in cucurbits. These organisms differ in virulence against cucurbit species and in their sensitivity to fungicides (Lebeda et al., 2008). Presently, there are at least seven pathogenically distinct races of P. xanthii and these are differentiated using 10 melon (Cucumis melo L.) differentials (Hosoya et al., 1999; McCreight et al., 1987; Pitrat et al., 1998). Recent work by McCreight (2006) showed that there may be as many as 28 races of P. xanthii on melon that include eight variants of race 1 and six variants of race 2. Currently, races 1W and 2W of P. xanthii have been reported on watermelon in the United States and G. cichoracearum was reported to affect watermelon in the Czech Republic (Davis et al., 2006, 2007; Kristková and Lebeda, 2000; Shishkoff and McGrath, 2001; Tetteh et al., 2010; Thomas, et al., 2005). Wang et al. (2006) determined the predominant cucurbit powdery mildew race in Beijing was race 2 Watermelon France (race 2WF) and demonstrated this race infected watermelon. Davis et al. (2006) released watermelon selection PI 525088-PMR resistant to race 1W P. xanthii and later published the results of their screening 1573 PI accessions for the same race (Davis et al., 2007). Eight of these accessions exhibited high levels of resistance, and another 86 demonstrated intermediate resistance. Thomas et al. (2005) found 22 PIs with intermediate resistance to race 2W (unspecified whether the race was 2WUS or 2WF) P. xanthii after screening 266 PI accessions. More recently, Tetteh et al. (2010) reported that eight of 1654 PI accessions and cultivars demonstrated high levels of resistance to race 2W. This race 2W was resistant to ‘Edisto 47’ and susceptible to PI 414723, which did not allow differentiation into races 2WUS or 2WF. There is no information previously published on resistance of Citrullus spp. to powdery mildew race 2WF. The most effective, least expensive for growers and safest method for controlling powdery mildew in crops is the use of resistant cultivars. The U.S. PI collection of Citrullus spp. contains lines collected from around the world, offering a wide diversity of germplasm for screening for powdery mildew resistance. The objective of this study was to evaluate the 2100 PI S1 lines produced from 820 U.S. PI Citrullus spp. accessions against the Bejing P. xanthii race 2WF isolate. We report here on C. lanatus Pis resistant to P. xanthii race 2WF. Materials and Methods Germplasm. In 2005, the National Engineering Research Center for Vegetables (NERCV) imported 820 PI accessions of Citrullus spp. from the USDA, Southern Regional Plant Introduction Station at Griffin, GA. These accessions represent germplasm collected from 57 countries. The majority of the accessions were originally collected from Turkey (153), Zimbabwe (117), India (96), the former Yugoslavia (82), Spain (59), Zambia (48), P.R. China (23), and Syria (19). These PI accessions represent three species, C. lanatus var. lanatus (766), C. lanatus var. citroides (53), and C. colocynthis (1). One generation of controlled self-pollinations of these 820 PI accessions were made to produce 2100 PI S1 lines to increase the likelihood of identifying recessively inherited resistance. The number Received for publication 22 June 2011. Accepted for publication 8 July 2011. The research was supported by Beijing Municipal Science & Technology Commission, China (Z09090501040902, D08070500690803, and 5100001), The Ministry of Science and Technology of the People’s Republic of China (2009BADB8B02, 2010DFB33740, 30972015, and 2010AA10A107), and The Ministry of Agriculture of the People’s Republic of China (CARS-26). We thank Dr. Kai Shu Ling at the USDA-ARS, U.S. Vegetable Laboratory, for his critical review of the manuscript. USDA is an equal opportunity provider and employer. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 46(10) OCTOBER 2011 1349 of PI S1 lines from each original accession ranged from two to five. Each PI S1 line was named with the PI accession number followed by the line number 1, 2, 3, 4, or 5. For lines in which few seeds were produced, seeds were bulked and labeled as the original PI numbers. Pathogen. The powdery mildew isolate was collected from an infected watermelon field at NERCV in Beijing and was maintained on summer squash (Cucurbita pepo L.) ‘ZaoQing’. Heavily sporulating leaves were placed in 0.02% Tween 20 and a sterile paint brush was used to release the conidia. The spore concentration was determined with a hemocytometer and adjusted to 2 · 10 conidia/mL (Block and Reitsma, 2005). Inoculations with the spore suspensions were performed with a hand sprayer. Plants were sprayed until 3 to 4 mL of the spore suspension evenly coated the leaves of each seedling. Inocula were freshly prepared as needed and were used immediately after preparing. To determine the race of the Beijing P. xanthii isolate, 13 melon differentials were inoculated: Iran H, ‘PMR 5’, ‘Edisto 47’, PI 414723, MR-1, and ‘Nantais Oblong’ supplied by Dr. M. Pitrat (INRA, Montfavet, France); and ‘Védrantais’, ‘Top Mark’, ‘PMR 45’, ‘PMR 6’, WMR29, PI 124111, and PI 124112 supplied by Dr. J.D. McCreight (USDA-ARS, Salinas, CA). Seedlings were rated for disease and race 2WF was the only race identified in all three experiments. Seedling screening. The primary seedling screen of 2100 PI S1 lines was conducted in NERCV greenhouses during the summer of 2006. The 52 most resistant lines were retested in a secondary seedling screen conducted at the same location in the summer of 2007. Powdery mildew race verification and germplasm screenings were performed using the same experimental conditions. The melon differentials were included in each experiment. The experiments were arranged in a randomized complete block designs with three replications. Fifteen seeds of each differential and 10 seeds of each PI S1 line were planted per replication. Additionally, 90 inoculated and uninoculated seedlings of the ZYMVsusceptible watermelon inbred line 98R were planted per replication as susceptible control plants. Inoculated checks served as verification of pathogen infection and uninoculated checks indicated no other diseases in the greenhouse that might confound symptom expression. Seedlings were grown in 80 · 80-mm square pots (600-mL volume) and placed on benches in a greenhouse set at 30 day/25 C night temperatures and 80% ± 15% relative humidity. Seedlings were inoculated at 15 d after seeding, when most first true leaves were fully expanded. Symptoms of powdery mildew infection on seedling watermelon manifested as white powdery growth on cotyledon or hypocotyl surfaces. Symptoms often started as small spots on the cotyledon surface and gradually covered the entire cotyledon and hypocotyl. Plant ratings were taken 15 d after inoculation. A 0 to 5 rating scale was used for evaluating seedlings, where 0 = no evidence of infection, 1 = trace (less than 20%) infection of cotyledons only, 2 = low infection (21% to 50%) of cotyledons or trace (less than 20%) infection of hypocotyls, 3 = moderate infection of cotyledons (51% to 70%) and low infection of hypocotyls (21% to 50%), 4 = severe infection (greater than 71%) of cotyledons and hypocotyls, slight infection of leaves (less than 20%), 5 = whole plant infected and/or dead. Percent disease indices (PDI) were calculated using a formula developed by Wheeler (1969). PDI = Sum of numerical disease ratings No: of plants observed