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Among the grain legumes, dry pea (Pisum sativum subsp. sativum) ranks first in production in Europe and second in the world (FAOSTAT data, 2005). Pea powdery mildew, caused by Erysiphe pisi, is an air-borne disease with a worldwide distribution, being particularly important under climatic conditions characterized by warm dry days and cool nights (Smith et al. 1996). The disease can cause 25–50% yield losses (Munjal et al. 1963, Warkentin et al. 1996), reducing the total biomass yield, number of pods per plant, number of seeds per pod, plant height and number of nodes (Gritton and Ebert 1975). The use of resistant varieties is the most efficient, economical and ecologically sound strategy for disease control. Although the cultivated pea germplasm has been extensively screened for resistance to powdery mildew, only two resistance genes, er1 and er2, have been identified so far (Heringa et al. 1969). Generally, single gene-controlled resistances are ephemeral due to the rapid evolution of pathogen virulence. Particularly, pathogens such as the powdery mildew fungi show a high risk of resistance erosion due to the coexistence of sexual and asexual stages and their high dispersal capability (McDonald and Linde 2002). Although races of E. pisi with specific virulence to the resistance genes of pea have not been described, breakdown of the resistance conferred by the genes er1 and er2 has been reported (Schroeder and Providenti 1965, Tiwary et al. 1997a, 1997b). Therefore, new genes for resistance to E. pisi in pea should be identified for the control of this important disease. In the absence of new genes for resistance to powdery mildew in cultivated pea, searches for resistance were expanded to include wild accessions belonging to the pea primary gene pool (P. sativum and all subspecies) and to the secondary pool (P. fulvum). The screening of a wide collection of wild Pisum species enabled to identify moderate levels of resistance to E. pisi in the P. sativum subspecies and a high level of resistance in P. fulvum (Fondevilla et al. 2007). P. fulvum originates in the eastern Mediterranean and Near East areas, where pea domestication took place (Clement et al. 2002). This species is more resistant than P. sativum to several pea diseases and insect pests such a powdery mildew (Erysiphe pisi) (Fondevilla et al. 2007) broomrape (Orobanche crenata) (Rubiales et al. 2005), ascochyta blight (Mycosphaerella pinodes) (Fondevilla et al. 2005) and pea weevil (Bruchus pisorum) (Clement et al. 2002). However, crosses between P. sativum and P. fulvum cannot be easily made. The cross-incompatibility has been attributed to chromosome structural differences (Ben-Ze’ev and Zohary 1973, Errico et al. 1991) and pollen-stigma incompatibility (Ochatt et al. 2004). Nevertheless, using P. fulvum as a male parent, we successfully obtained hybrid progenies between resistant P. fulvum accessions and cultivated pea, and currently this species is included in our breeding programs for resistance to broomrape and ascochyta blight. Similarly, other researchers have succeeded in crossing these two species using P. fulvum as pollen parent (Muehlbauer et al. 1994, Worth 1998, Ochatt et al. 2004). In the present investigation, the mode of inheritance of the P. fulvum resistance to E. pisi was studied by analysing a cross between a fertile resistant plant derived from an interspecific cross between P. fulvum and P. sativum, and the susceptible pea cv. ‘Messire’.

Identification of a New Gene for Resistance to Powdery Mildew in Pisum fulvum, a Wild Relative of Pea