Foliar Corn Diseases in South Dakota: Visual Disease Surveys and the Genetic Studies of Tar Spot and Bacterial Leaf Streak

Corn (Zea mays L.) is one of the most important crops in South Dakota and the United States. Each year, a variety of diseases impact the production of corn in South Dakota, though very little documentation about these diseases exists in the state. This survey documents the foliar diseases of corn and the distribution of each to create a baseline of knowledge. An additional genetic study was conducted to determine the organisms associated with tar spot of corn and bacterial leaf streak (BLS), two emerging diseases in South Dakota. A variety of studies have been conducted on tar spot, caused by Phyllachora maydis, some of which have indicated that genetic variations within some populations have occurred. Samples of tar spot were collected from South Dakota, Nebraska, Kansas, Minnesota, Missouri, and Michigan and used to conduct Sanger sequencing to identify single nucleotide polymorphisms (SNP). Two primer pairs, ITS5/4 and V9D/LS266, were used in this genetic analysis. Sequences of both regions did not contain any SNPs. Multiple studies have been conducted on the genetics of BLS, which is caused by the bacteria Xanthomonas vasicola pv. vasculorum (Xvv), since its initial identification in the Great Plains in 2014. The original objective for BLS, to identify biocontrol agents for Xvv, was quickly adjusted to identifying the bacteria associated with BLS-like symptoms. No isolates from 2023, the first year of the study, were identified as Xvv, but many were identified as Pantoea spp. In 2024, samples containing symptomatic leaves were collected from South Dakota, Nebraska, Kansas, and Minnesota. The bacteria isolated from these samples were often identified as Pantoea spp, however, there was one isolate that was identified as Xanthomonas spp. Due to the lack of isolation of Xvv, Kock’s postulates was conducted with Pantoea spp., which did not produce disease symptoms, though it is inconclusive if this is due to the bacteria not being pathogenic or if the inoculation environment was not conducive to disease development. To gain a better understanding, isolates from Pantoea spp., Pseudomonas spp., Kosakonia spp., and Bacillus spp. were compared within each genus. Each comparison revealed the likelihood that multiple species from each genus were isolated during this study. Further studies will be conducted to continue exploring the genetics of tar spot and BLS to be better informed about these pathogens in the Great Plains.