ABSTRACT

Taproot decline (TRD) is a disease of soybean that has been reported recently from the southern United States (U.S.). Symptoms of TRD include foliar interveinal chlorosis followed by necrosis. Darkened, charcoal-colored areas of thin stromatic tissue are evident on the taproot and lateral roots along with areas of necrosis within the root and white mycelia within the pith. Upright stromata typical of Xylaria can be observed on crop debris and emerging from infested roots in fields where taproot decline is present, but these have not been determined to contain fertile perithecia. Symptomatic plant material was collected across the known range of the disease in the southern U.S., and the causal agent was isolated from roots. Four loci, ⍺-actin (ACT), β-tubulin (TUB2), the nuclear rDNA internal transcribed spacers (nrITS), and the RNA polymerase subunit II (RPB2), were sequenced from representative isolates. Both maximum likelihood and Bayesian phylogenetic analyses showed consistent clustering of representative TRD isolates in a highly supported clade within the Xylaria arbuscula species complex in the “HY” clade of the family Xylariaceae, distinct from any previously described taxa. In order to understand the origin of this pathogen, we sequenced herbarium specimens previously determined to be “Xylaria arbuscula” based on morphology and xylariaceous endophytes collected in the southern U.S. Some historical specimens from U.S. herbaria collected in the southern region as saprophytes as well as a single specimen from Martinique clustered within the “TRD” clade in phylogenetic analyses, suggesting a possible shift in lifestyle. The remaining specimens that clustered within the family Xylariaceae, but outside of the “TRD” clade, are reported. Both morphological evidence and molecular evidence indicate that the TRD pathogen is a novel species, which is described as Xylaria necrophora.

 抽象的

主根衰退（TRD）是最近在美国南部报道的一种大豆病害。 TRD 的症状包括叶脉间失绿，随后坏死。主根和侧根上有明显的黑色、炭色的薄基质组织区域，以及根内的坏死区域和髓内的白色菌丝体。木霉属典型的直立子座可以在作物残骸上观察到，并且在存在主根衰退的田地中从受侵染的根中出现，但尚未确定这些子囊含有可育的子囊壳。在美国南部已知的疾病范围内收集了有症状的植物材料，并从根部分离出致病因子。对代表性分离株的四个基因座，⍺-肌动蛋白 ( ACT )、β-微管蛋白 ( TUB2 )、核 rDNA 内转录间隔区 (nrITS) 和 RNA 聚合酶亚基 II ( RPB2 ) 进行了测序。最大似然分析和贝叶斯系统发育分析均表明，木聚糖科“HY”进化枝中木木霉物种复合体中高度支持的进化枝中代表性 TRD 分离株的聚类一致，与之前描述的任何分类单元不同。为了了解这种病原体的起源，我们对先前根据在美国南部收集的形态和木聚糖内生菌确定为“木聚糖”的植物标本进行了测序。在系统发育分析中，来自马提尼克岛的单个样本聚集在“TRD”分支中，表明生活方式可能发生转变。 据报道，其余的样本聚集在 Xylariaceae 科内，但在“TRD”分支之外。形态学证据和分子证据均表明 TRD 病原体是一种新物种，被描述为Xylaria necrophora 。