Abstract
Corynespora cassiicola is a destructive plant-pathogenic fungus causing widespread target spot epidemics, including outbreaks on cotton, soybean, and tomato in the southeastern United States. Previous studies revealed that populations from the three hosts are genetically distinct and host specialized. Although variation in aggressiveness to cotton and tomato were observed, no genetic diversity was detected within populations sampled from each of these hosts. We aimed to gain a better understanding of the emerging target spot epidemics by developing microsatellite markers for C. cassiicola to assess genetic variation, population structure, and to infer modes of reproduction and mechanisms of dispersal. Two hundred sixty-five isolates from cotton, soybean, tomato, and other host plants were genotyped with 13 microsatellite markers. Genotypic diversity revealed genetic variation within each of the populations collected from different hosts, with the population from cotton dominated by clonal genotypes and showing the least genetic diversity. In addition, C. cassiicola populations on different host species were genetically distinct and structured based on host species. No association between genetic and geographic distances was identified in the tomato populations, and the association in cotton populations was low. However, significant regional geographic structure was detected in the soybean populations of C. cassiicola. These results further support previous findings of introduced host specialized isolates or the evolution of more aggressive strains on each host. The lack of geographic structure suggests that the clones on cotton and tomato spread rapidly, or similar founder populations were established by human-mediated dispersal, and that dispersal is not limited. However, regional geographic structure of populations on soybean suggests limited dispersal among more established populations of C. cassiicola, or genetic differences in founder populations that colonized different geographic areas.
Highlights
Asexual reproduction is common among many fungal and oomycete plant pathogens, resulting in clonal populations with parents and offspring that are genetically identical [1]
A few of these are classic examples of plant pathogens that substantially impacted agriculture or forest ecosystems. These include: Phytophthora infestans, which causes late blight of tomato and potato [6], where epidemics in Europe and United States were identified as introductions of clonal lineages carrying either A1 or A2 mating types [7]; Phytophthora ramorum, which causes the widespread outbreaks of sudden oak death in the Pacific Northwest forests, where studies on the genetic diversity and reproductive mode revealed that clonal lineage NA1 dominates in the region and no evidence of sexual reproduction has been detected [8]; and Cryphonectria parasitica, which causes chestnut blight, where studies have provided evidence for clonal expansion in Europe [9]
Phylogenetic analyses of C. cassiicola from cotton, tomato, and soybean in the southeastern U.S showed three genetically distinct populations that clustered based on the host species of origin, with isolates from cotton and soybean belonging to PL1 and isolates from tomato belonging to PL4 [21]
Summary
Asexual reproduction is common among many fungal and oomycete plant pathogens, resulting in clonal populations with parents and offspring that are genetically identical [1]. A few of these are classic examples of plant pathogens that substantially impacted agriculture or forest ecosystems These include: Phytophthora infestans, which causes late blight of tomato and potato [6], where epidemics in Europe and United States were identified as introductions of clonal lineages carrying either A1 or A2 mating types [7]; Phytophthora ramorum, which causes the widespread outbreaks of sudden oak death in the Pacific Northwest forests, where studies on the genetic diversity and reproductive mode revealed that clonal lineage NA1 dominates in the region and no evidence of sexual reproduction has been detected [8]; and Cryphonectria parasitica, which causes chestnut blight, where studies have provided evidence for clonal expansion in Europe [9]. Phylogenetic analyses of C. cassiicola from cotton, tomato, and soybean in the southeastern U.S showed three genetically distinct populations that clustered based on the host species of origin, with isolates from cotton and soybean belonging to PL1 and isolates from tomato belonging to PL4 [21]. Larger sample sizes and more variable markers are needed to detect variation within the populations, to determine if the populations are clonal, and to investigate geographic structure within populations
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