Abstract
Cercospora leaf spot (CLS), caused by Cercospora beticola, is a major disease of Beta vulgaris worldwide. No sexual stage is known for C. beticola but in its asexual form it overwinters on infected plant debris as pseudostromata, and travels short distances by rain splash-dispersed conidiospores. Cercospora beticola infects a broad range of host species and may be seedborne. The relative contribution of these inoculum sources to CLS epidemics on table beet is not well understood. Pathogen isolates collected from table beet, Swiss chard and common lambsquarters in mixed-cropping farms and monoculture fields in New York and Hawaii, USA, were genotyped (n = 600) using 12 microsatellite markers. All isolates from CLS symptoms on lambsquarters were identified as C. chenopodii. Sympatric populations of C. beticola derived from Swiss chard and table beet were not genetically differentiated. Results suggested that local (within field) inoculum sources may be responsible for the initiation of CLS epidemics in mixed-cropping farms, whereas external sources of inoculum may be contributing to CLS epidemics in the monoculture fields in New York. New multiplex PCR assays were developed for mating-type determination for C. beticola. Implications of these findings for disease management are discussed.
Highlights
Population genetic studies of plant pathogens using selectively neutral genetic markers have made significant contributions to plant disease management in the past few decades[1, 2]
If C. beticola populations in New York are capable of sexual reproduction, overwintering sexual structures may serve as primary inoculum for newly established table beet fields within several kilometres
The objectives of this study were to i) investigate the potential for sexual reproduction in C. beticola populations in New York; ii) test the hypothesis that inoculum from lambsquarters and Swiss chard may contribute to Cercospora leaf spot (CLS) epidemics on table beet; and iii) determine the distribution of genetic diversity in C. beticola populations within and among mixed-cropping and monoculture table beet production to understand the relative contributions of local or external sources of inoculum to CLS epidemics
Summary
Population genetic studies of plant pathogens using selectively neutral genetic markers have made significant contributions to plant disease management in the past few decades[1, 2]. In pathogen populations with frequent sexual reproduction, high genotypic diversity, random association of selectively neutral genetic loci and equal distribution of the opposite mating-types (for heterothallic fungi) are expected. Cicla) are often grown in close proximity along with other vegetables, on an annual basis It is, plausible that infested plant debris from previous years may be a major source of inoculum contributing to the annual CLS epidemics on susceptible hosts in these farms. Large-scale monoculture table beet fields in New York involve rotations with non-host crops in the same field for a recommended minimum duration of three years, which aims to reduce the persistence of inoculum in soil and on infested plant debris. The major sources of initial inoculum for CLS epidemics in monoculture table beet fields in New York are largely unknown. Seedborne inoculum[32] has been suggested to contribute to CLS epidemics in some European countries[40, 41] but its contribution to CLS epidemics in New York has not been investigated
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