Phytophthora infestans, an oomycetes close to fungus, causes late blight worldwide on both tomato and potato. Conditions conducive to development of the disease include cool temperatures and cloudy weather, which if ideal, can destroy the crop within a few days. Wild species are the source for high levels of resistance to late blight in both Solanaceous crops. Currently three named genes are being used in tomato breeding: Ph-1 (Bonde and Murphy 1952; Gallegly 1952), Ph-2 (Turkensteen 1973; Laterrot 1975) and Ph-3 (Black etal. 1996). Ph-1 has been of limited use as strains rapidly overcame the resistance (Conover and Walter 1953). Ph-2 shows good resistance early, but often fails late in the season (Turkensteen 1973; Laterrot 1975). Ph-3 has a high level of resistance, but has been overcome (Brusca 2003). These three Ph genes have been mapped to chromosomes 7, 10, and 9, respectively (Pierce 1971; Moreau et al. 1998; Chunwongse et al. 2002). QTLs were identified in a S. habrochaites accession (Brouwer and St. Clair 2004; Brouwer et al. 2004), but they require markers to efficiently transfer the resistance into cultivated tomato. Recently, Kole et al. (2006) have reported mapping of a gene Ph-4 on chromosome 2 conferring resistance to late blight using an RIL population derived from a cross between a susceptible cultivated tomato variety and a resistant accession of S. pimpinellifolium.
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