THE INFLUENCE OF SOIL-APPLIED HERBICIDE AND PATHOGEN INTERACTION ON UPREGULATION OF SYSTEMIC ACQUIRED RESISTANCE IN SOYBEAN

Exposure of crop plants to stress or injury, such as soybean injury by PPO-inhibitor herbicide, may stimulate the upregulation of Systemic Acquired Resistance (SAR) and reduce plant susceptibility to other stressors, such as disease-causing pathogens. Field and laboratory studies were initiated to evaluate the upregulation of SAR, examining the effects of PPO-inhibiting herbicide treatment on Sudden Death Syndrome incidence and severity in soybean and the relationship of disease incidence and severity related to stand count and yield with various population densities. A two-year field study was established in Shawneetown, IL to evaluate grain yield and disease potential of soybean cultivars which are either sensitive or tolerant to protoporphyrinogen oxidase (PPO)-inhibitor herbicides, with seed either treated with insecticide, thiamethoxam and fungicides, fludioxonil and mefanoxam (Upshot) and biological fungicide Bacillus amyloliquefaciens strain D747 (Avonni) (biological fungicide) or non-treated. The seeds were planted at six different seeding rates: 197,684; 247,105; 296,526; 345,947; 395,368; 444,789; with the controls planted at a density of 345,947 seeds ha-1 in a 2x2x7 factorial study design. Field experiments were planted on April 25, 2016 and May 6, 2017 in 76 cm, 4-row plots measuring 3m by 7m, and herbicide was applied to treated plots over the center 2 rows. Data collection included crop injury at 14, 28 and 56 days after treatment (DAT), stand count at 14 and 28 (DAT), plant height and node count at end-of-season (EOS), and disease incidence and severity ratings beginning at the onset of symptomology. Yield data was collected from the center two treated rows. All plots, except the non-treated controls, received an application of sulfentrazone + cloransulam-methyl (316 g ai ha-1). In 2016 the greatest crop injury, categorized by stunting, at 14 DAT occurred in the PPO-tolerant seed variety without a fungicide and insecticide seed treatment at 4.2% planted at 444,789 seeds/ha. At 28 DAT with means pooled over seed treatment and seed variety, we observed the 197,684 seeds/ha plots having greatest crop injury at 5.25%, and lastly at 56 DAT, the 197,684 and 247,105 seeds/ha plots containing untreated, PPO-sensitive seed were the most injured at 12% crop injury. In 2017, 14 DAT was excluded from the analysis, as there was no injury at the time of rating. At 28 DAT, the PPO-sensitive seed variety, pooled over seed treatment, at 197,684 seeds/ha resulted in greater crop injury at 8.6%, similar to 2016. At 56 DAT, similar results were observed as in 2016, at 12% crop injury in the PPO-sensitive seed variety without a seed treatment planted at 197,684 seeds/ha. There were differences in stand count by seeding rate at 14 and 28 DAT, but no interactive effects between the factors in 2016; seed treatment and seed variety were not significant. However, in 2017, there were differences in stand count by seed variety and seed treatment at 14 and 28 DAT, but again, no interactive effects between factors. Relationships between stand count and seeding rate indicated a threshold at which the environment cannot sustain higher planting densities. Environmental conditions were more favorable for crop growth in 2016 than 2017. Rainfall 10 days following planting was recorded at 67 mm and 290 mm in 2016 and 2017, respectively. Soybean node counts in 2016 were greater in the PPO-tolerant variety were seed was treated with a fungicide and insecticide seed treatment. In 2017, node counts were not influenced by seed treatment or seed variety; however, the greatest number of nodes were in the 444,789 seeds/ha planting population. Disease was more prominent in the high-density plots than in the low-density plots, as would be expected because of the effects of competitive stress on plant susceptibility to pathogens as well as more plants to be infected by the pathogen. Sudden Death Syndrome disease incidence (scale of 0 to 100%) in 2016 ranged from 1.2 to 25.5 across rating dates, while severity (scale of 0 to 9 based on leaf symptomology) ranged from 0.3 to 2.2 across rating dates. In 2017 disease incidence ranged from 0 to 25.0 across all rating dates, and disease severity ranged from 0 to 1.6 across all rating dates. Yield in 2016 ranged from 3,449.8 kg/ha to 4,060.3 kg/ha with the highest yield in the PPO-tolerant variety and the lowest in the -sensitive variety. However, in 2017, yield was lowest in the 197,684 plants/ha treatments at 1,509.1 kg/ha and highest in the 444,789 plants/ha treatments at 4,053.9 kg/ha. Significant varietal and seed treatment differences were also noted in 2017