Effect of Septoria brown spot on soybean yield in Illinois

Brown spot caused by Septoria glycines is a prevalent foliar disease in all soybean production areas. Application of foliar fungicides after bloom reduces the disease severity, yet yield responses are not consistent among locations and years. Our research goal was to determine the effect of different levels of Septoria brown spot on yield. Different levels of disease severity were effectively obtained in the field by weekly application of chlorothalonil for three, six, and nine times after disease inoculation at V3/V4 stage. Fungicide treatments had a significant effect on vertical progress and chlorotic area with no statistically significant effect on yield. Soybean yield was negatively correlated with vertical progress of the disease (r = −0.36). The vertical progress was the best linear predictor of yield. Based on this model, when the vertical progress of brown spot at R6 increased by 10%, the yield decreased by 142.13 kg/ha (3.4%). A variance component analyses of our data showed that location was the most critical factor, illustrating the significant effect of local environmental conditions on the disease. Power analyses indicated that at least eight locations are needed to detect an effect of 269 kg/ha. Our results provide useful information to improve the experimental design for future experiments addressing the yield constrain by late season diseases of soybean.

4 45 gradually develop to the upper canopy, leading to premature defoliation and yield 46 losses [8,9]. 47 Several studies have evaluated the yield losses caused by Septoria brown spot. In a 48 field survey with 1000 soybean plant introductions, Lim (1979) [10] showed that yield 49 losses due to Septoria brown spot ranged between 1% to 27%, depending on variety, 50 location, and whether the plots were inoculated or not. Yield reductions ranging from 51 12% to 34% were reported in inoculated plots, and from 8% to 8.7% in naturally infected 52 plots [11]. A study [12] in Ohio reported that yield losses ranged from 2.5% and 9.5% in 53 naturally infected fields with different number of chlorothalonil applications.

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No source of resistance to this pathogen has been identified from a survey of 1000 55 and 626 soybean lines [10,13]. There is no data on the resistance or susceptibility of 56 newly released soybean cultivars to brown spot [12], and no pathogenic variability 57 among isolates has been found [14].   (Fig 2) and showed a trend for necrotic area at R7 (p = 0.115) (S1 Table). Three, 208 six, and nine weekly fungicide applications reduced 17% to 33% of the disease vertical 209 progress as compared to the plots that were not treated with fungicide. For chlorotic 210 area only six and nine weekly fungicide applications significantly reduced 18% and 21% 211 of the chlorotic area as compared to the non-treated controls. No differences were 212 observed between the inoculated and not-inoculated plots (Fig 2).

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For the disease traits, the largest variance was due to the location effects (Table 1), 218 which ranged between 15% to 78%. Only a maximum of 3% of the variance was due to 219 the block within location, and a maximum of 35% due to the location by treatment 220 interaction. The one exception was for chlorotic area which had no variance due to 221 location and 52% variance due to location by treatment interaction (Table 1). 234 resulted in similar yields, but the yield was lower in Monmouth (Fig 3).

Linear regression analysis 255
Stepwise regression analysis identified the final rating and AUDPC of vertical 256 progress to be significant predictor variables when included in the model independently.
257 The estimated regression slope was -14.21 (p = 0.0357) for vertical progress rating data 258 at R6, and -0.69 (p = 0.0007) for AUDPC of vertical progress. The coefficient of 259 determination (R 2 ) was 0.08 and 0.20, respectively (Fig 4). This indicated that 8% to 20% 290 Septoria brown spot. We conducted multi-location inoculated field trials to generate 291 empirical data of disease effects and model the linear relationship of the damage due to 292 this disease.

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In this study, different levels of disease severity of brown spot were obtained at 294 three locations following three, six, or nine weekly applications of chlorothalonil after 295 disease inoculation. Among the four disease components investigated (vertical progress, 296 necrotic area, chlorotic area, and defoliation rate), vertical progress was the best trait to 297 identify differences in the development of brown spot during the reproductive stages.