Spatiotemporal Spread of Cercospora Leaf Spot and <i>Cercospora beticola</i> Genotypes in Table Beet in New York

ABSTRACT Cercospora leaf spot (CLS), caused by the hemibiotrophic ascomycete Cercospora beticola , is a destructive foliar disease of table beet. Primary inoculum sources of C. beticola include infested seed, alternative hosts and crop residue. Rapid disease progress results from multiple secondary infection cycles. However, the relative contributions of inoculum originating from sources within and external to the field of interest are unknown. This study depicted the spatial spread of CLS from linear transects (inoculated with known C. beticola genotypes), which was best explained by the modified power law model, typical of wind‐dispersed pathogen propagules. The spatiotemporal patterns of genetic diversity, differentiation and structure of 324 C. beticola isolates from populations at variable distances from the inoculum and subsequent seasons were also characterised using 11 microsatellite loci. Sixty‐two multilocus genotypes (MLGs) were identified. The majority of isolates at the inoculated transects were identified as source genotypes. However, novel genotypes in the noninoculated plots and at different distances from the transects suggest the presence of some naturally occurring inoculum. The presence of shared MLGs between two consecutive seasons within fields suggests that inoculum carryover on infested crop residue was a dominant factor in the initiation of outbreaks in the second season. A high clonal fraction, and significant mating type and linkage disequilibrium in C. beticola populations, suggest limited sexual reproduction and dominance of clonal propagation. These findings suggest CLS spread was predominantly by wind‐mediated dispersal of conidia within the season. Crop rotation with nonsusceptible hosts may be the best approach for reducing inoculum carryover between seasons.