Indirect evidence for sexual reproduction in Cercospora beticola populations from sugar beet

Cercospora beticola is the main causal agent of cercospora leaf spot on sugar beet and has a large negative impact on the yield and quality of sugar beet production worldwide. Previous studies have shown that both mating type idiomorphs of C. beticola are present in natural populations, suggesting that C. beticola is heterothallic and may be reproducing sexually. Cercospora beticola isolates are diverse in the morphology of their conidia, onset of disease symptoms and fungicide resistance. To find the source of this diversity and to determine if sexual reproduction occurs in this fungus, C. beticola populations were collected from Western Europe, Iran and New Zealand. The mating types of these isolates were determined and AFLP analyses were used to study the genetic diversity in these populations. The mating type ratios did not deviate significantly from a 1:1 ratio in most of the populations and AFLP analyses showed high levels of genetic variation within and between the populations, with 86.4% of the isolates having unique genotypes. All populations were in significant linkage disequilibrium but levels of disequilibrium were low, and loci from only one primer pair were in significant gametic equilibrium in populations from the Netherlands and Italy. From these results there is the possibility that C. beticola reproduces sexually. High levels of gene flow among the samples from Europe demonstrated a single panmictic European population. This study confirms C. beticola to be a genetically highly diverse species, supporting the assumption that some populations are reproducing sexually.