Progress on Research of Taproot Decline (TRD) in Soybean
On Thursday, Aug. 14, 2014, I traveled with Drs. Billy Moore, Tom Allen, and Trent Irby, and Brian Ward of Southern Ag., to a large soybean field near Chatham, Miss. that was affected by a heretofore unidentified disease (called “mystery disease” at the time) that was having a visible detrimental effect on the potentially high-yielding soybeans in that irrigated field. The most obvious effects were leaf chlorosis and a dead root system on affected plants. Affected plants also easily broke at the soil surface when they were pulled. They had shriveled pods with BB-size seeds compared to unaffected plants in the field that had seeds of normal size. The affected plants were determined to be in an amount that would reduce yield potential of soybeans in this field. Drs. Moore and Allen told me that this malady was widespread throughout Miss. soybean fields in 2014. David Wansley (MSPB member) indicated in an email to me that this was also the case in his area of the Delta. This new and unidentified disease needed immediate attention.
At the behest of the MSPB, a request for proposals to research this unidentified malady was forwarded to state pathologists so that the MSPB could offer immediate support for the identification of the causal organism as a first step toward addressing this problem. It was determined that this had to happen before further steps could be taken. It was also determined that this initial effort would involve the collection and assay of both soil and plant samples from affected fields so the causal organism could be definitively identified.
The proposal that was chosen for funding with a start date of Oct. 15, 2014 was “Identification and characterization of ‘mystery disease’ pathogen that is prevalent in Mississippi soybean fields in 2014". The listed investigators were Drs. Maria Tomaso-Peterson, Tom Allen, and Trent Irby. The objectives of this funded research were to 1) identify the fungal pathogen(s) associated with taproot decline of soybean, 2) assess pathogenicity of the recovered fungi through completion of Koch’s Postulates, and 3) determine the potential yield loss associated with the taproot decline malady. With slight modifications to the title, this research was continued through early 2020 with total funding from MSPB of over $260,000.
This investment by the MSPB, along with resources provided by the Louisiana Soybean and Feed Grains Research Promotion Board (both acknowledged in the below article), have resulted in the identification of the TRD pathogen as a novel species described as Xylaria necrophora. The background and procedures that led to the identification of this TRD pathogen are described in an article titled “Xylaria necrophora, sp. nov., is an emerging root-associated pathogen responsible for taproot decline of soybean in the southern United States” that was published online on Feb. 8, 2021 in the journal Mycologia (https://doi.org/10.1080/00275514.2020.1846965). The list of authors includes scientists/specialists from Louisiana State Univ., Miss. State Univ., the Univ. of Arkansas, Auburn Univ., and the Univ. of Tenn. Thus, this significant accomplishment reported in the above-cited article involved a concerted regional effort to address this new malady in soybean.
A first step in managing this newly-identified soybean pathogen is determining if current soybean varieties exhibit tolerance to it. In 2020, Dr. Paul Price and colleagues at the LSU AgCenter assigned ratings to selected soybean varieties in MG’s 3.0-6.0 for their response to both inoculated and natural infestations of this pathogen. Those ratings can be accessed here. This first year of ratings indicate that varietal differences in response to the TRD pathogen do exist. This result can and should lead to breeding efforts to ascertain the genetic nature of this natural tolerance/resistance, and the development of future varieties that may contain genetic resistance to the TRD pathogen.
It is assumed that this identification of the TRD pathogen will also be followed by research to determine how it can be controlled/managed so that it does not cause yield reductions in Midsouth soybean fields. Options for its control will likely include both cultural and chemical (fungicide) remedies. Potential fungicide remedies will involve the evaluation of efficacy of current and new fungicide products against this pathogen.
The above-cited results and findings are an excellent example of how grower-supplied funds for soybean research can result in important findings and information.
Composed by Larry G. Heatherly, Mar. 2021, larryheatherly@bellsouth.net