First Description of the Causal Agent of Taproot Decline of Soybean, an Emerging Disease in the Southern United States

Abstract

Over the past decade, a soybean root issue of unknown etiology has been observed across a widespread geography in the southern United States. Recently, pathologists began referring to the problem as taproot decline of soybean, based on the appearance of root symptoms. Taproot decline has been observed to cause foliar symptoms in vegetative and reproductive soybean plants ranging in maturity from V6 to R6. Symptom expression can appear similar to other notable root-associated diseases except that taproot decline exhibits a progression of symptom expression from subtle to severe interveinal chlorosis during the season. However, one distinct symptom associated with taproot decline is observed as darkened, black stroma on the taproot and, in some cases, the lateral roots of affected plants. Pathogenicity was confirmed by isolating the suspected fungus from naturally-infected soybean roots in multiple states and completing Koch’s postulates. The causal agent was identified, based on morphological characters and multilocus phylogenetic inference, as a member of the Xylaria arbuscula aggregate. At present, research projects are underway to address the role of the newly described disease and extent of the pathogen in the southern soybean production region in addition to developing integrated strategies for managing the disease.

Metagenomic Analyses of the Soybean Root Mycobiome and Microbiome Reveal Signatures of the Healthy and Diseased Plants Affected by Taproot Decline

“…Diseased plants snap at the soil line, providing a key diagnostic feature in the field. Another diagnostic feature of TRD is the presence of stromata (deadman’s fingers) colonizing soybean or other crop debris remaining from the previous growing season [ 2 ]. The foliar symptoms of TRD are similar to those of other root diseases of soybean and include interveinal chlorosis and necrosis.…”

“…The disease pattern is consistent with a soilborne pathogen, as conidia or ascospore production has not been observed in situ [ 1 ]. Since initial observations of TRD, disease occurrence has increased within the United States and is now widespread in soybean production throughout the southern United States [ 2 , 3 ]. Significant yield loss associated with TRD has been reported in Arkansas, Louisiana, and Tennessee [ 1 , 4 , 5 ].…”

Draft genome sequence of Xylaria sp., the causal agent of taproot decline of soybean in the southern United States

“…Plant-pathogenic species of Xylaria include Xylaria mali , the causal agent of root rot of apple in the southern and midwestern United States [4] , and Xylaria arbuscula associated with quick decline of Macadamia trees [5] . Recently, a taxonomically unresolved member of Xylaria was reported as the causal organism for taproot decline of soybean in the southern U.S., especially along the Mississippi River Valley [6] . Initial phylogenetic analyses of the isolate with four nuclear loci placed the isolate within the Xylaria arbuscula species aggregate [6] .…”

“…Recently, a taxonomically unresolved member of Xylaria was reported as the causal organism for taproot decline of soybean in the southern U.S., especially along the Mississippi River Valley [6] . Initial phylogenetic analyses of the isolate with four nuclear loci placed the isolate within the Xylaria arbuscula species aggregate [6] . Soybean plants affected by taproot decline often exhibit interveinal chlorosis in the leaves of the lower canopy during vegetative growth stages, which increases in severity as plants mature through reproductive stages of growth and can be observed in the upper canopy during advanced reproductive growth stages (>R6).…”

“…Taproots and lateral roots display a blackened appearance, with stroma of the fungus frequently visible upon examination. The disease causes premature plant death in severe cases [6] .…”