Intersterility, morphology and taxonomy of Ceratocystis fimbriata on sweet potato, cacao and sycamore

Abstract

Ceratocystis fimbriata is a large, diverse complex of species that cause wilt-type diseases of many economically important plants. Previous studies have shown that isolates in three monophyletic lineages within the Latin American clade of C. fimbriata are host-specialized to cacao (Theobroma cacao), sweet potato (Ipomoea batatas) and sycamore (Platanus spp.), respectively. We paired testers of opposite mating type from isolates of these lineages to find intersterility groups. Two intersterility groups corresponded to the sweet potato and sycamore lineages, respectively. The cacao lineage contained two intersterility groups, corresponding to two genetic sublineages centered in western Ecuador and Brazil/Costa Rica/Colombia. Six isolates from cacao that were not members of the cacao lineage and were not pathogenic to cacao in an earlier study also were intersterile with members of the two cacao intersterility groups. Some pairings between testers from different lineages or sublineages yielded perithecia from which a few abnormal progeny could be recovered, typical of interspecific hybrids. These progeny showed abnormal segregation of the MAT-2 gene and mycelial morphology, showing that they were indeed the result of crosses. Isolates of the sweet potato, cacao, and sycamore lineages were indistinguishable morphologically except for the presence or absence of a doliform (barrel-shaped) conidial state and minor differences in size of perithecial bases and necks and ascospores. C. fimbriata originally was described from sweet potato. We describe the cacao pathogen as a new species, Ceratocystis cacaofunesta and we raise the sycamore pathogen from a form to species Ceratocystis platani.