New concept for determination and denomination of pathotypes and races of cucurbit powdery mildew 1

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1 New concept for determination and denomination of pathotypes and races of cucurbit powdery mildew 1 A. Lebeda 1*, E. Křístková 1, B. Sedláková 1, J.D. McCreight 2, and M.D. Coffey 3 1 Palacký University, Faculty of Science, Department of Botany, Šlechtitelů 11, Olomouc- Holice, Czech Republic 2 U.S. Department of Agriculture, Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905, USA 3 University of California, Department of Plant Pathology and Microbiology, 3206 Webber Hall, Riverside, CA 92521, USA * Corresponding author ales.lebeda@upol.cz Keywords: Golovinomyces cichoracearum, Podosphaera xanthii, Citrullus, Cucumis, Cucurbita, pathogenic variability, sextet, septet Abstract Cucurbit powdery mildew (CPM), which is caused most frequently by two obligate erysiphaceous ectoparasites (Golovinomyces cichoracearum, Podosphaera xanthii), occurs on field and glasshouse cucurbit crops worldwide. We review the current state of understanding of variation for pathogenicity of these two pathogens at the pathotype and race levels, and propose for use by the international CPM research, breeding, seed and production communities: 1) two sets of differential cucurbit genotypes for the identification of CPM pathotypes and races, and 2) an objective, efficient, uniform, comprehensive coded system for meaningful, concise designation of CPM pathotypes (sextet code) and races (septet code). The proposed set of pathotype differentials includes six genotypes, three genera (Cucumis, Cucurbita, Citrullus), and five species. The proposed the set of CPM race differentials includes 21 genotypes of a single species, Cucumis melo L. The race set can determine the race identity of either pathogen species when isolated from the cucurbit genera included in the pathotype differential set. INTRODUCTION Powdery mildew is a serious disease of field and greenhouse cucurbit crops worldwide (Sitterly 1978; McGrath and Thomas 1996). Three species are known to cause CPM (Braun 1995), but Golovinomyces cichoracearum (Gc) (DC.) V.P. Gelyuta (Vakalounakis and Klironomou 2001), and Podosphaera xanthii (Px) (Castag.) U.Braun & N. Shish. (Shishkoff 2000) are considered more important than Leveillula taurica (Lev.) Arnaud (Braun 1995; Pitrat et al. 1998). Podosphaera xanthii (Px) is common in subtropical and tropical areas and greenhouse crops, while G. cichoracearum (Gc) occurs more frequently in temperate and colder areas under field conditions (Křístková et al. 2007). Both species occur singly or mixed on cucurbits in Central Europe (Lebeda 1983; Křístková et al. 2007; Lebeda et al. 2007b). Cucurbit isolates of Gc and Px cross-infect all cucurbit species and genera 1 Cucurbitaceae 2008, Proceedings of the IX th EUCARPIA meeting on genetics and breeding of Cucurbitaceae (Pitrat M, ed), INRA, Avignon (France), May th,

2 (Lebeda et al. 2007a). Populations of Gc and Px vary spatially and temporally (Křístková et al. 2007). We have limited genetic knowledge of the cucurbit-powdery mildew pathosystem (Jahn et al. 2002). Breeding of cucurbits for resistance to CPM is hindered by the lack of clear and uniform descriptions of the genetic variation in the pathogenicity of the CPM pathogens (Bardin et al. 1997; Bardin et al. 1999; Lebeda and Sedláková 2006; McCreight 2006; Lebeda et al. 2007a) and genetics of resistance in the cucurbits (Pitrat et al. 1998; McCreight 2006). For melon, this is due in part to the diverse genetic pool from which many sources of genetic resistance have been identified (Pitrat et al. 1998; McCreight 2006). We have no knowledge about the genetic basis for any trait of the CPM pathogen species, nor do we have any molecular basis for distinguishing physiological races of the pathogens (Montoro et al. 2004) The relatively recent and widespread pathogenicity of Px on watermelon (Citrullus lanatus) worldwide (McGrath and Thomas 1996; McGrath 2001; Davis et al. 2002), increased number of races of Px on melon (McCreight 2006) and Gc on Cucurbita spp. (Lebeda and Sedláková 2006) highlighted the need for an internationally developed, agreeable and workable framework to facilitate the future development of genetic knowledge of the CPM pathosystem and elite CPM-resistant germplasm adapted to the diverse cucurbit production systems worldwide. Toward this end, we put forth here: 1) an initial proposal for an improved, uniform, and internationally standardized description of Gc and Px pathotypes and races using standard sets of differential cucurbit germplasm, and 2) a system for designating CPM pathotypes and races. FRAMEWORK FOR CPM The pathogenic specialization in cucurbit powdery mildew is well known (Sitterly 1978; Thomas 1988). In host plant-powdery mildew interactions, there is often a very clear expression of compatibility or incompatibility (Lebeda and Sedláková 2008), which allows classification of pathotypes and races based on the pattern of compatible and incompatible reactions on the differential hosts (species or genotypes). Important issues when investigating host-parasite interactions are (Lebeda and Widrlechner 2003): 1) General knowledge about host-pathogen variability and specificity of the interaction; 2) Knowledge about host-pathogen genetics; and 3) Availability of a theoretical model explaining the genetics of hostpathogen interaction. The need for expanded, cooperative, international CPM research has been previously noted (McCreight and Pitrat 1993; Pitrat et al. 1998; Lebeda and Sedláková 2006; Lebeda et al. 2007a). Of primary importance is the establishment of 1) a characterized set(s) of differential host genotypes and 2) a collection of characterized strains of Gc and Px. The differential set(s) and the pathogen strains must, subject to applicable phytosanitary restrictions, be readily available to the CPM research and breeding community. The details about the most important host genera (Cucumis, Cucurbita and Citrullus), including their genetic variation were summarized recently (Lebeda et al. 2007b). Current knowledge about the genetics of CPM host-pathogen interactions is limited and unclear (Jahn et al. 2002) with the exception of C. melo and Px 126

3 (McCreight 2006). The CPM pathosystem is likely to be similar to other hostpowdery mildew interactions, which are based on the principles of gene-for-gene interactions (Brown 2002). The most recent review of sources of resistance and their genetic control to CPM was published by Jahn et al. (2002). Gene lists for Cucumis, Cucurbita and Citrullus including the known genes for resistance to Gc and Px are maintained and periodically updated under the auspices of the Cucurbit Genetics Cooperative (USA): cucumber (Xie and Wehner 2001), melon (Pitrat 2006), watermelon (Guner and Wehner 2003), and Cucurbita spp. (Paris and Brown 2004). PATHOGENICITY DIFFERENTIATION Powdery mildew pathogenicity, including cucurbit powdery mildew, is most frequently expressed as virulence using a binary system (compatible versus incompatible) (Bardin et al. 1999; Brown 2002; McCreight 2006; Lebeda and Sedláková 2008). Interactions between most cucurbits and Gc or Px are pathotypeand / or race-specific (Bardin et al. 1999; Křístková and Lebeda 1999; Jahn et al. 2002; Lebeda et al. 2007a). Clear expression of virulence in the CPM pathosytem (Lebeda and Sedláková 2008) permits distinction, identification and denomination of Gc and Px pathotypes and races based on the patterns of virulence reactions of differential hosts (Pitrat et al. 1998; Bardin et al. 1999; Lebeda and Sedláková 2006; Lebeda et al. 2007a). Extremely broad variation at the level of pathotypes and races within both CPM species has been reported, but a unified system for the determination, description and classification of pathotypes and races has yet to be adopted by the cucurbit research community (Lebeda and Sedláková 2006; McCreight 2006; Lebeda et al. 2007a). Robust, comprehensive systems for designation of pathotypes and races require two critical components: 1) a set of differential host genotypes (open-pollinated cultivars, breeding lines), and 2) a system of identification and denomination of pathotypes and races. Such unified systems have been rare for powdery mildew pathogens (Brown 2002). Guidelines for more complete denominations of pathotypes and races are available in the phytopathological literature, e.g. Limpert et al. (1994). Descriptive knowledge of the CPM pathosystem began with a simple number system and three differentials in the 1920s in response to the appearance of Px race 2 (Jagger et al. 1938; McCreight 2004). A new melon differential and an accompanying numbered race (race 3) were described 50-years later (Thomas 1978). The six races known on melon in 1998 had simple designations that reflected their order of discovery, or as in the case of race 0 their unique reaction on the known melon differentials (Pitrat et al. 1998). Genetic variation in CPM races on melon reported from the 1980s through the present has been designated with alpha (Křístková and Lebeda 1999; Lebeda and Sedláková 2006), or alphanumeric (McCreight et al. 1987; Alvarez et al. 2000; Hosoya et al. 2000) denominations. The most recent report of new CPM races on melon clearly demonstrated that the current system of race identification and denomination is unwieldy (McCreight 2006). Recently developed principles for identification and denomination of CPM pathotypes and races (Lebeda and Sedláková 2008) will be briefly reviewed. 127

4 PATHOTYPE AND RACE CONCEPTS A new concept for the identification and designation of CPM pathotypes and races is proposed, based on the two critical components given earlier. Pathotypes Identification of CPM pathotypes is based on intergeneric and interspecific differences in host-cpm interactions. The currently used set CPM pathotype differentials (Bertrand 1991; Bertrand et al. 1992; Křístková and Lebeda 1999) includes one each of the agriculturally most important cucurbit genera and species (Tab. 1). Table 1. Commonly used CPM pathotype differentials and binary reactions (+, compatible; -, incompatible) of six Px isolates (Lebeda et al. 2007a and unpublished). Kveta is proposed to replace Diamant F 1 in the standard set of CPM pathotype differentials. No. Taxon / Isolates and reactions differential 55/03 71/03 58/03 99/04 76/03 115/04 1 C. sativus / Marketer C. melo / Védrantais 3 C. melo / PMR C. pepo / Diamant F 1 5 C. maxima / Goliáš 6 C. lanatus / Sugar Baby Isolate source z Cp Cp Cp Cm Cs Cx z Cp, Cucurbita pepo; Cm, Cucumis melo; Cs, Cucumis sativus; Cx, Cucurbita maxima. Races Races of Gc and Px have, to date, been observed only on melon; two races of Gc (Pitrat et al. 1998), and about 30 races of Px (McCreight et al. 2005; Coffey et al. 2006; McCreight 2006). We propose, therefore, a CPM race system based on the intraspecific genetic variability of melon (race-specific resistance). The most frequently cited set of melon differentials includes 11 genotypes (Bertrand 1991; Bertrand et al. 1992; Lebeda and Sedláková 2006; McCreight 2006; Lebeda and Sedláková 2008) that can differentiate CPM races originating from melon (McCreight 2006) and other cucurbits, e.g., cucumber, Cucurbita spp. and watermelon (Lebeda and Sedláková 2006; Lebeda et al. 2007a). A new set of 20 differential melon genotypes is proposed for differentiation of Gc and Px races (Tab. 3), an additional genotype has to be discussed and added. This enlarged set is based on the set of melon CPM differentials developed by French 128

5 investigators (Bertrand 1991; Bertrand et al. 1992), and is supplemented with ten additional melon genotypes (Tab. 3) that revealed new Px and Gc races (Hosoya et al. 2000; Bertrand 2002; Lebeda and Sedláková 2006; McCreight 2006; Lebeda et al. 2007a). Table 2. Unique CPM pathotype (Px) sextet codes based on compatability scores summed across pathotype differentials (see Tab. 1). Differential number Differential value Sextet Isolate code Px 55/ Px 71/ Px 58/ Px 99/ Px 76/ Px 115/ Table 3. Proposed CPM race differentials by group and number. All of them are C. melo. Differential Other Original Country Group.No. Designation designation(s) source of origin 1.1 Iran H Iran 1.2 Védrantais France 1.3 PI Teti India 1.4 PI Sweet Melon S. Africa 1.5 ARHBJ AR Hale s Best Jumbo USDA USA 1.6 PMR 45 USDA USA 1.7 PMR 6 USDA USA 2.1 WMR 29 USDA USA 2.2 Edisto 47 Ames 8578 Koelz 2563 USA 2.3 PI LJ PI India 2.4 PMR 5 USDA USA 2.5 PI Koelz 2564 India 2.6 MR-1 PI USA 2.7 PI India 3.1 PI VIR 5682 India 3.2 Noy Yizre el Israel 3.3 PI England 3.4 Negro Spain 3.5 Amarillo Spain 3.6 Nantais Oblong France 3.7 Open for discussion and addition 129

6 PROPOSED SYSTEM FOR PATHOTYPE AND RACE IDENTIFICATION We propose a numerical system for designation of pathotypes and races that is derived from a previously developed sextet based system for plant pathogens (Limpert et al. 1994; Limpert and Müller 1994). The binary results of CPM assays are translated into unique codes that serve as an identifier for each pathotype or race of Gc and Px. Pathotypes The proposed sextet for pathotype differentiation is represented by six components (Tab. 1) comprised of three genera (Cucumis, Cucurbita and Citrullus), five species (C. sativus, C. melo, C. pepo, C. maxima, C. lanatus), and, ultimately, six unique genotypes. Each genotype is arbitrarily assigned a permanent differential order (1-6; Tab. 1) and value (1, 2, 4, 8, 16, or 32; Tab. 2) for a compatible, i.e., + or susceptible, CPM interaction. In a given CPM assay with a particular Gc or Px strain, the interactions are thus scored and then summed to yield a unique sextet code for that strain. An example of pathotype identification is presented in Table 2. Races The CPM race differentials are arbitrarily divided into three groups (Tab. 3) due to their large number. The race differentials are assigned an arbitrary permanent order with a group prefix (1-3; Tab. 3) and value (1, 2, 4, 8, 16, 32 or 64; Tab. 4) within each group. The binary results of any CPM assay are thus translated into a triple-part, septet code, one part for each group of seven differentials. The three sums are then presented as an unique code in the format: sum of group 1.sum of group 2.sum of group 3, which serves as a unique identifier for each race. This unified denomination of triplet septet codes will thus standardize the communication on international level. An hypothetical example of four isolates of Px on the 21 race differentials is presented in Table 4. CONCLUSIONS AND FUTURE PROSPECTS The increased international trade of cucurbits (fruit and seed) warrants a coordinated, standardized, uniform system for further development and of exchange of information on CPM pathotypes and races to facilitate optimal and strategic control of CPM. Our increased knowledge of CPM pathotypes and races makes this effort more imperative. The proposed sets of pathotype and race differentials are the first step toward this end. Adoption of an objective, efficient, uniform, comprehensive system for designation of pathotypes yields meaningful, concise, clear descriptions of CPM pathotypes and races. Such a system creates a basis for the application of population biology and genetic studies (Lebeda 1982) of CPM. Such studies have been very rare (Lebeda et al. 2007a), but are considered important from both the theoretical genetic and the applied resistance breeding viewpoints (Lebeda et al. 2006). 130

7 Table 4. Septet codes for four hypothetical Px isolates on CPM race differentials Triplet septet Isolate code

8 There remains a need for additional discussion among the international community of CPM researchers and cucurbit breeders about these proposals. In particular, the differentials should be open-pollinated to ensure their continued availability as discussed earlier. Moreover, the focus of this paper has been influenced or restricted by the authors knowledge or focus of the cucurbit production systems in Europe and the U.S. With this in mind, should the CPM pathotype differential set represent a broader scope of cucurbits, e.g., Lagenaria spp.? In addition, it is our intent to address in the future the many aspects of CPM assay protocols (e.g., whole plant versus leaf disc), international standardization, cooperation and coordination of activities related to CPM pathogenicity variation. ACKNOWLEDGEMENTS This research was supported by the following grants: 1) MSM (Czech Ministry of Education); 2) QH71229 (NAZV, National Agency for Agricultural Research of the Czech Republic). Literature cited Alvarez JM, Gómez-Guillamón ML, Torés NA, Cánovas I, Floris E (2000) Virulence differences between two Spanish isolates of Sphaerotheca fuliginea race 2 on melon. Acta Hort 510: Bardin M, Nicot PC, Normand P, Lemaire JM (1997) Virulence variation and DNA polymorphism in Sphaerotheca fuliginea, casual agent of powdery mildew of cucurbits. European J Plant Pathol 103: Bardin M, Carlier J, Nicot PC (1999) Genetic differentiation in the French population of Erysiphe cichoracearum, a causal agent of powdery mildew of cucurbits. Plant Pathol 48: Bertrand F (1991) Les oïdiums des Cucurbitacées: Maintien en culture pure, étude de leur variabilité et de la sensibilité chez le melon. PhD thesis (in French). Univ. Paris XI, Orsay (FR) Bertrand F, Pitrat M, Glandard A, Lemaire JM (1992) Diversité et variabilité des champignons responsables de l oïdium des cucurbitacées. Phytoma La Défense des Végétaux 438: Bertrand F (2002) AR Hale's Best Jumbo, a new differential melon variety for Sphaerotheca fuliginea races in leaf disk test. In Cucurbitaceae 2002, (Maynard DN, ed), Ed. ASHS Press, Alexandria (VA, USA) pp Braun U (1995) The powdery mildews (Erysiphales) of Europe. Gustav Fischer Verlag, New York Brown JKM (2002) Comparative genetics of avirulence and fungicide resistance in the powdery mildew fungi. In The powdery mildews: a comprehensive treatise, (Bélanger RR, Bushnell WR, Dik AJ, Carver TLW, eds), Ed. APS Press, St. Paul (MN, USA) pp Coffey MD, McCreight JD, Miller T (2006) New races of the cucurbit powdery mildew Podosphaera xanthii present in California. Phytopathology 96: S25 (Abstr.) Davis AR, Thomas CE, Levi A, Bruton BD, Pair SD (2002) Watermelon resistance to powdery mildew race 1. In Proceedings of Cucurbitaceae 2002, (Maynard DN, ed), Ed. ASHS Press, Alexandria (VA, USA) pp Guner N, Wehner TC (2003) Gene list for watermelon. Cucurbit Genet Coop Rpt 26: Hosoya K, Kuzuya M, Murakami T, Kato K, Narisawa K, Ezura H (2000) Impact of resistant melon cultivars on Sphaerotheca fuliginea. Plant Breeding 119: Jagger IC, Whitaker TW, Porter DR (1938) A new biologic form of powdery mildew on muskmelons in the Imperial Valley of California. Plant Dis Rpt 22:

9 Jahn M, Munger HM, McCreight JD (2002) Breeding cucurbit crops for powdery mildew resistance. In The powdery mildews: a comprehensive treatise, (Bélanger RR, Bushnell WR, Dik AJ, Carver TLW, eds), Ed. APS Press, St. Paul (MN, USAA) pp Křístková E, Lebeda A (1999) Powdery mildew of cucurbits in the Czech Republic species, pathotype and race spectra. In The First International Powdery Mildew Conference, Programme and Abstracts, Avignon (FR) pp Křístková E, Lebeda A, Sedláková B (2007) Temporal and spatial dynamics of powdery mildew species on cucurbits in the Czech Republic. Acta Hort 731: Lebeda A (1982) Population genetics aspects in the study of phytopathogenic fungi. Acta Phytopath Acad Sci Hung 17: Lebeda A (1983) The genera and species spectrum of cucumber powdery mildew in Czechoslovakia. Phytopath Z 108: Lebeda A, Widrlechner MP (2003) A set of Cucurbitaceae taxa for differentiation of Pseudoperonospora cubensis. J Plant Dis Protect 110: Lebeda A, Sedláková B (2006) Identification and survey of cucurbit powdery mildew races in Czech populations. In Proceedings of Cucurbitaceae 2006, (Holmes GJ, ed), Ed. Universal Press, Raleigh (NC, USA) pp Lebeda A, Widrlechner MP, Urban J (2006) Individual and population aspects of interactions between cucurbits and Pseudoperonospora cubensis: pathotypes and races. In Proceedings of Cucurbitaceae 2006, (Holmes GJ, ed), Ed. Universal Press, Raleigh (NC, USA) pp Lebeda A, Sedláková B, Křístková E (2007a) Temporal changes in pathogenicity structure of cucurbit powdery mildew populations. Acta Hort 731: Lebeda A, Widrlechner MP, Staub J, Ezura H, Zalapa J, Křístková E (2007b) Cucurbits (Cucurbitaceae; Cucumis spp., Cucurbita spp., Citrullus spp.). In Genetic Resources, Chromosome Engineering, and Crop Improvement Series, Volume 3 Vegetable Crops, (Singh R, ed), Ed. CRC Press, Boca Raton (FL, USA) pp Lebeda A, Sedláková B (2008) Screening for resistance to cucurbit powdery mildew (Golovinomyces cichoracearum, Podosphaera xanthii). In Disease Resistance. Mass Screening Techniques for Banana and Other Crops, (Miranda M, Lebeda A, eds), Ed. Springer, Dordrecht (NL), (submitted) Limpert E, Clifford B, Dreiseitl A, Johnson R, Müller K, Roelfs A, Wellings C (1994) Systems of designation of pathotypes of plant pathogens. J Phytopathol 140: Limpert E, Müller K (1994) Designation of pathotypes of plant pathogens. J Phytopathol 140: McCreight JD, Pitrat M, Thomas CE, Kishaba AN, Bohn GW (1987) Powdery mildew resistance genes in muskmelon. J Amer Soc Hort Sci 112: McCreight JD, Pitrat M (1993) Club mildew: working group on resistance of melon to powdery mildew. Cucurbit Genet Coop Rpt 16: 39 McCreight JD (2004) Notes on the change of the causal species of cucurbit powdery mildew in the U.S. Cucurbit Genet Coop Rpt 27: 8-23 McCreight J, D., Coffey MD, Turini TA, Matheron ME (2005) Field evidence for a new race of powdery mildew on melon. HortScience 40: 888 (Abstr.) McCreight JD (2006) Melon powdery mildew interactions reveal variation in melon cultigens and Podosphaera xanthii races 1 and 2. J Amer Soc Hort Sci 131: McGrath M, Thomas C (1996) Powdery mildew. In Compendium of Cucurbit Diseases, (Zitter T, Hopkins D, Thomas C, eds), Ed. APS Press, St. Paul (MN, USA) pp McGrath MT (2001) Distribution of cucurbit powdery mildew races 1 and 2 on watermelon and muskmelon. Phytopathology 91: S197 (Abstr.) Montoro T, Salinas M, Capel J, Gomez-Guillamon M, Lozano R (2004) Genetic variability in Sphaerotheca fusca as determined by AFLPs: the case of race 2 as a causal agent of 133

10 powdery mildew in melon. In Progress in cucurbit genetics and breeding research. Proceedings of Cucurbitaceae 2004, the 8th EUCARPIA Meeting on Cucurbit Genetics and Breeding (Lebeda A, Paris HS, eds), Ed. Palacky University, Olomouc (CZ) pp p Moravec J, Lebeda A, Křístková E (2004) History of growing and breeding of cucurbitaceous vegetables in Czech Lands. In Progress in cucurbit genetics and breeding research. Proceedings of Cucurbitaceae 2004, the 8th EUCARPIA Meeting on Cucurbit Genetics and Breeding (Lebeda A, Paris HS, eds), Ed. Palacky University, Olomouc (CZ) pp p Paris HS, Brown RN (2004) Gene list for Cucurbita species Cucurbit Genet Coop Rpt 27: Pitrat M, Dogimont C, Bardin M (1998) Resistance to fungal diseases of foliage in melon. In Cucurbitaceae '98: Evaluation and enhancement of cucurbit germplasm, (McCreight JD, ed), Ed. ASHS Press, Alexandria (VA, USA) pp Pitrat M (2006) 2006 Gene list for melon. Cucurbit Genet Coop Rpt 29: Shishkoff N (2000) The name of the cucurbit powdery mildew: Podosphaera (sect. Sphaerotheca) xanthii (Castag.) U. Braun & N. Shish. comb. nov. Phytopathology 90: S133 Sitterly WR (1978) Powdery mildews of cucurbits. In The powdery mildews, (Spencer DM, ed), Ed. Academic Press, New York (NY, USA) pp Thomas CE (1988) Physiological specialization in downy and powdery mildews of cucurbits. In EUCARPIA Meeting 'Cucurbitaceae 88', (Risser G, Pitrat M, eds), Ed INRA, Avignon- Montfavet (FR) pp Thomas EC (1978) A new biological race of powdery mildew of cantaloups. Plant Dis Rpt 62: 223 Vakalounakis DJ, Klironomou E (2001) Taxonomy of Golovinomyces on cucurbits. Mycotaxon 80: Xie J, Wehner TC (2001) Gene List 2001 for Cucumber. Cucurbit Genet Coop Rep 24:

Powdery mildew (Podosphaera xanthii) resistance in melon is categorized into two types based on inhibition of the infection processes

Journal of Experimental Botany, Vol. 57, No. 9, pp. 2093 2100, 2006 doi:10.1093/jxb/erj166 Advance Access publication 11 May, 2006 RESEARCH PAPER Powdery mildew (Podosphaera xanthii) resistance in melon