Doctor of Philosophy (PhD)


Plant Pathology & Crop Physiology

Document Type



Taproot decline (TRD) is an emerging disease of soybean [Glycine max (L) Merr.] recently found in the southern United States. Symptoms include interveinal chlorosis followed by necrosis on leaves and roots. The causal pathogen was recently identified as a member of the Xylaria arbuscula species complex, but questions regarding the novelty of this species, its emergence on soybean, evolutionary potential, and etiology remained unanswered. TRD-associated isolates and saprophytic contemporary and historical specimens collected within and outside the known range of the disease clustered together in a monophyletic clade based on multilocus phylogenetic analyses. No previously described species clustered within this clade, indicating that the TRD pathogen represents a novel species, X. necrophora. Historical specimens suggest X. necrophora has been present in the region and shifted its lifestyle from saprophytic in the forest to pathogenic on soybean. Whole-genome sequences of 160 pathogenic isolates were used to determine the population structure, diversity, and evolutionary potential of X. necrophora. Two partially-clonal lineages of X. necrophora were found to be distributed throughout the region, but no evidence of geographical population structure was observed. These results suggest X. necrophora was likely endemic to forests in the southern US and shifted to soybean with its introduction into North America. The presence of mostly clonal populations with signatures of recombination indicates the species has moderate evolutionary potential if sexual reproduction is occurring in soybean populations. To better understand the etiology of TRD, cell-free culture filtrates (CFCFs) were used to (i) determine if secondary metabolites (SMs) produced by X. necrophora in the roots are responsible for foliar symptoms on leaves, (ii) assess the levels of resistance to SMs among potentially resistant cultivars, and (iii) test the host specificity of these SMs. Results suggested host-specific, phytotoxic SMs produced by X. necrophora are responsible for the symptoms observed on the soybean foliage, and cultivar resistance observed in the field is unrelated to SM resistance. The overall results of this study provide insights into the pathogen life history and context for further studies of this disease, which will be useful in the development of effective, long-lasting management strategies for TRD.



Committee Chair

Doyle, Vinson P.



Available for download on Saturday, October 19, 2024