Target spot of soybean (caused by the fungus Corynespora cassiicola) is an emerging threat to soybean grown in the southern U.S. Until recently, target spot generally was considered a minor threat to soybean, but severe outbreaks of target spot have been observed in some soybean varieties grown in southern states over the last few years (Koenning et al. 2007). Some of the factors that have contributed towards increased severity of target spot are the release of susceptible varieties by seed companies, the development of fungicide-resistant strains of the target spot pathogen, and the lack of crop rotation in some areas. Although it is not known how or if any certain cotton production practices have influenced target spot in soybean, cotton also is a host to the pathogen.
Little research has been conducted on the applied management of target spot of soybean. To date, most of the field research has focused on screening varieties for resistance to the disease and evaluating fungicides for disease management. Large differences in target spot susceptibility within varieties grown in the southern U.S. exist, and fungicide products vary greatly in their ability to manage the disease. From preliminary fungicide trials conducted by the PIs and other soybean pathologists in the southern U.S., results indicate that quinone outside inhibitor (QoI; also known as strobilurin) fungicides generally have not been very effective in managing target spot. This may be due to the presence of some strains of the target spot pathogen that are resistant to QoI fungicides (Nunes Rondon and Lawrence, 2019). Results from these trials also indicate that some products that contain succinate dehydrogenase inhibitor (SDHI) fungicides have good efficacy for controlling target spot.
Initial symptoms of target spot generally are observed on leaves in the lower to mid-canopy, and depending on the weather, the disease may spread into the upper canopy, where yield losses are more likely to occur. Because the target spot pathogen appears to affect the most dense soybean foliage first, it is possible that row spacing could influence the soybean canopy architecture, thus influencing initial infection by the pathogen and potentially influencing disease spread within the plant canopy. To our knowledge, no previous research on the effect of soybean row spacing on target spot has been conducted.