Few studies have been conducted to evaluate the degree of variation of bacterial canker caused by Clavibacter michiganensis subsp. michiganensis. Strains of C. michiganensis subsp. michganensis were highly genetically similar (>80%) based on repetitive-sequence-based PCR (rep-PCR) fingerprinting (Louws etal. 1998). Nonetheless, the pathogen could be grouped into four distinct genetic types designated A through D. Studies on mapping for resistance have used strains that were either highly aggressive or belonged to one of the predominant genetic groups. Sources of resistance to C. michiganensis subsp. michiganensis have been identified in several wild tomatoes. Durability of resistance from LA407 against several predominant strains in the USA has been proven (Francis et al. 2001). However, progress on breeding for resistance to the disease has been limited. This may be due to the complicated genetic control of resistance traits. Resistance loci from LA2157 (S. peruvianum) and LA407 (S. hirsutum) have been the most studied in terms of molecular genetic mapping; results are summarized in Table 12. At least two or three QTLs were identified with additive gene action and various gene interactions. Rcm2.0 and Rcm5.1 have been located in short chromosome intervals of 4.4 and 2.2 cM, respectively. Molecular markers within the intervals could be used for MAS.
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