Race-specific PCRs for Verticillium dahliae reveal a high frequency of race 2 strains in spinach seed
D. P. G. SHORT (1), S. Gurung (2), K. Maruthachalam (2), K. V. Subbarao (2). (1) University of California, Salinas, CA, U.S.A.; (2) University of California-Davis, Salinas, CA, U.S.A.
<i>Verticillium dahliae</i> causes an economically important wilt disease on a range of crops including lettuce. Two pathogenic races of <i>V. dahliae</i> have been described. Sources of resistance to race 1 exist in a few crops, due to a plant immune receptor <i>Ve1</i>, which recognizes the race 1-specific fungal effector <i>Ave1</i>. However, only partial resistance to race 2 exists in lettuce. PCR assays are available to identify race 1, but no complementary test exists to positively identify race 2. Here, we compared the genome sequences of race 1 and 2 <i>V. dahliae</i> to identify sites useful as markers to distinguish the two races. We designed primers based on polymorphisms in a hypothetical protein, which we hypothesize represent fixed nucleotide substitutions between these two races. PCRs with these primers produced an amplicon of 255 bp with DNA from race 2 isolates but failed to amplify with race 1 DNA. We screened over 400 <i>V. dahliae</i> isolates from spinach seed that previously did not amplify with a race 1-specific PCR. All of them amplified with primers designed from race 2 sequence. To validate the PCR result, 50 of these isolates from spinach seed were used to inoculate two differential lines of lettuce, Salinas (<i>ve1</i>) and La Brillante (<i>Ve1</i>). The disease reactions strongly supported the race 2-specific PCR data. These race 2 primers will complement existing race 1 PCR assays and are useful for the purposes of resistance breeding and disease surveillance.
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