The fungus Corynespora cassiicola is primarily found in the tropics and subtropics, and is widely diverse in substrate utilization and host association. Isolate characterization within C. cassiicola was undertaken to investigate how genetic diversity correlates with host specificity, growth rate, and geographic distribution. C. cassiicola isolates were collected from 68 different plant species in American Samoa, Brazil, Malaysia, and Micronesia, and Florida, Mississippi, and Tennessee within the United States. Phylogenetic analyses using four loci were performed with 143 Corynespora spp. isolates, including outgroup taxa obtained from culture collections: C. citricola, C. melongenae, C. olivacea, C. proliferata, C. sesamum, and C. smithii. Phylogenetic trees were congruent from the ribosomal DNA internal transcribed spacer region, two random hypervariable loci (caa5 and ga4), and the actin-encoding locus act1, indicating a lack of recombination within the species and asexual propagation. Fifty isolates were tested for pathogenicity on eight known C. cassiicola crop hosts: basil, bean, cowpea, cucumber, papaya, soybean, sweet potato, and tomato. Pathogenicity profiles ranged from one to four hosts, with cucumber appearing in 14 of the 16 profiles. Bootstrap analyses and Bayesian posterior probability values identified six statistically significant phylogenetic lineages. The six phylogenetic lineages correlated with host of origin, pathogenicity, and growth rate but not with geographic location. Common fungal genotypes were widely distributed geographically, indicating long-distance and global dispersal of clonal lineages. This research reveals an abundance of previously unrecognized genetic diversity within the species and provides evidence for host specialization on papaya.

On silicon‐deficient soils, fertilization with silicon (Si) has reduced disease severity and increased yield of rice (Oryza saliva L.). Breeding rice genotypes with greater Si concentration has been proposed, and some African and Asian rices have been evaluated for variability of Si concentration on upland and flooded mineral soils. This experiment investigated the variability and relation of Si concentration in rice plant tissue with disease severity and yield components of selected U.S. rice genotypes grown on flooded organic soils. Ten rice genotypes were grown at two Florida locations on Si‐deficient organic Histosols. Plots were replicated five times in each of two Si treatments: Siamended soil with silicon added at 2 Mg ha‐1, and an unamended control. Plots were rated for disease severity at growth stage 8 to 9 (dough to mature) and sampled for determination of Si concentration in plant tissue (leaves and straw). Genotypes differed for Si concentration and disease severity at each location and for each Si treatment. Among genotypes, disease severity was negatively correlated with Si concentration in plant tissue. Increases in yield with added Si were attributable to a greater number of grains per panicle, whereas weight per 100 seed and panicles per square meter exhibited less change. Results indicated that although Si concentration varies among genotypes and is negatively correlated with disease severity, the strength of this association is mitigated by other genotypic factors which also affect response to disease.

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