There is no doubt that there are myriad foliar pathogens that plague soybeans. One such pathogen is the fungus Fusarium virguliforme, the causal agent of sudden death syndrome [SDS]. According to a survey of soybean yield loss to diseases in the Midsouth, SDS causes significant yield loss in a majority of years. And depending on weather conditions and other factors that will favor proliferation of the causal pathogen, SDS will continue to be a major yield-limiting disease in soybeans.
In a Nov. 2015 article in Plant Health Progress [Vol. 16, p. 163-164], Singh et al. at LSU reported the first incidence of SDS in Louisiana. According to information in a publication by Drs. Faske and Kirkpatrick at the Univ. of Ark., SDS was first reported in Arkansas in 1971, and now plagues soybeans in most regions of the U.S. that grow the crop. This publication also provides details about the SDS infection process in soybeans, and the symptoms of its occurrence. The authors state that foliar fungicides are not effective at suppressing SDS, and that there are no highly resistant varieties that are available to producers who anticipate a problem with this disease in their soybean fields.
According to information in an article titled “Evaluation of soybean germplasm for resistance to Fusarium virguliforme, the major pathogen causing sudden death syndrome in the United States” by Herman et al. that was published in Dec. 2022 in the journal Crop Science, it is probable that soybean varieties that are resistant to the SDS-causing pathogen will be forthcoming. This is based on the conduct of and results from the study that was reported in the above article. Major points from that article follow.
• According to the authors, host resistance is a major sustainable tool to reduce SDS severity, and screening soybean germplasm for sources of resistance is a key part of discovering host resistance.
• In the study, 10,144 soybean lines [PI’s] in MG’s 000 to X from the USDA Soybean Germplasm Collection were evaluated for resistance to the SDS causal organism.
• Progressive tiers of testing were conducted to identify and eliminate the most susceptible PI’s based on SDS foliar symptoms.
• The reaction of PI’s–foliar infection severity, shoot and root dry weights–was rated/measured at 28 days after planting and compared to that of non-inoculated plants and the partially resistant check, PI 567374.
• Overall, decreased SDS foliar severity was noted as testing advanced, and foliar SDS symptoms of 81 PI’s did not significantly differ from or were significantly lower than those of PI 567374. The authors concluded that these 81 accessions are potential sources of SDS resistance. These identified PI’s also contain a range of traits that include previously reported resistance to a variety of soybean pathogens.
• In the end, 16 PI’s from a range of MG’s had foliar severity ratings that were not different from those of PI 567374, and 5 of these 16 PI’s had significantly greater relative weights compared to the other entries. Testing of these PI’s in fields with a history of SDS occurrence will verify how they perform against F. virguliforme and other SDS-causing species of Fusarium in those fields.
• The resources used in this study came from those contained in the USDA Soybean Germplasm Collection as noted above, and demonstrate the value of this collection for such studies that are conducted to support genetic improvement in soybean.
A list of major diseases of soybeans and management options for their control can be found in the White Paper on this website. Note that the stem canker disease of soybeans also has no fungicide control option, but the development of varieties that are resistant to the highly virulent causal pathogen of this disease has reduced its adverse effect on soybeans to a minimal level as shown in the linked survey results above. Thus, it is reasonable to assume that genetic resistance to SDS can be just as effective. In fact, in the absence of fungicide control measures, it is the only effective option to manage SDS just as it is for stem canker.
Arguably, genetic or host plant resistance to diseases that plague soybeans is likely the most effective and sustainable long-term solution to their control. This is certainly a point worth considering since resistance to fungicides is becoming common in pathogens that cause diseases associated with yield loss in this crop. Therefore, funding entities and agencies should look to supporting research and studies that seek to find genetic solutions to soybean disease problems rather than providing funds solely to explore the best way to control diseases with fungicides that may eventually become ineffective. After all, when a fungicide that is commonly used to control a particular disease or diseases loses its effectiveness against those diseases, there may not be a suitable alternative to fill the void. Genetic resistance will likely last longer and can be coupled with more judicious use of available fungicides to control the myriad diseases that plague soybeans over the long term.
Composed by Larry G. Heatherly, Mar. 2023, email@example.com