High genotype diversity and lack of isolation by distance in the Alternaria solani populations from China

A genomic library was used to develop seven SSR markers for studying the population genetics of Alternaria solani, a pathogenic fungus causing early blight disease of potato and tomato worldwide. Population genetic analysis of 268 isolates of A. solani sampled from four locations, each representing one of four potato production systems in China, indicates that these SSR markers are moderately diverse, selectively neutral and possibly unlinked. Population genetic analysis also indicated that genetic variation of A. solani in China is high. About 2/3 of 123 genotypes were detected only once and genotype diversity measured by the standardized Shannon index ranged between 0·82 and 0·92 in the populations. Although clones were detected in multiple populations separated by thousands of kilometres, random association among SSR loci was found in half of the populations assayed. On average, nearly six copies of genetic material were exchanged among these populations each generation and no isolation by distance was detected. It is hypothesized that the joint effects of cryptic sexual reproduction and human‐mediated gene flow may account for the observed population genetic structure of A. solani in China.