Strains of race 0 of Pseudomonas syringae pv. tomato causing bacterial speck are predominant in the world. This makes the single semi-dominant resistance gene, Pto highly durable. The original source of Pto is S. pimpinellifolium, possibly PI 370093 (Pitblado and Kerr 1980; Kerr and Cook 1983; Pitblado etal. 1984; Martin et al. 1991); the gene was named by Pitblado and MacNeill (1983). Due to its semi-dominant nature, disease is occasionally observed on commercial hybrids carrying only one copy of Pto (Buonaurio et al. 1996). Pto is tightly linked to Fen, a gene controlling sensitivity to fenthion. Thus, sensitivity to fenthion can be used to select for Pto. To confer resistance to race 0 of P. syringae pv. tomato, both Pto and Prf genes are required. Both genes have been cloned (Loh and Martin 1995; Salmeron et al. 1996), and thus, gene-based markers can be designed and used in MAS (Table 12).
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