Influence of application technology on foliar fungicide efficacy on <i>Cercospora sojina</i> infected soybean

Due to the constant concern with off-target contamination and application technology requirements associated with future herbicide-tolerant crops, the use of drift-reduction nozzle technology (DRT) may increase. The primary objective of this research was to evaluate the effects of coarse droplets generated by drift-reduction nozzles on foliar fungicide efficacy and residual in soybean infected by frogeye leaf spot caused by Cercospora sojina. No differences in disease control, soybean yield, spray retention, and residual when applying Quadris Top SB, a premix of azoxystrobin and difenoconazole, using nozzles that produce either a medium or ultra coarse droplet spectrum were determined.

Due to the challenge of timing fungicide applications targeting frogeye leaf spot infections, growers often make applications preventatively, prior to visual infection. Therefore, the second objective was to determine the EC₅₀ of C. sojina isolates collected from trial locations to further investigate the residual control window of azoxystrobin. The effective concentration in which 50% of mycelial growth of resistant isolates tested were inhibited was determined to be 7.44638 μg mL^-1, while the concentration for sensitive isolates was found to be 0.04789 μg mL^-1.

The third objective was to determine the effect of droplet size on plant coverage and canopy penetration when making fungicide applications in a commercial setting with increased potential for offtarget movement. These studies showed no differences between nozzles producing either medium or ultra coarse droplet spectra when considering frogeye leaf spot control or soybean yield with an application of Quadris Top SB. Coverage analyses determined that applications made with ground self-propelled sprayers deposited 43% more solution in the upper canopy than the lower canopy, but was not effected by droplet spectra.

These results indicate that DRT nozzle technology will not have a negative impact on frogeye leaf spot disease control or soybean yield when applying Quadris Top SB. Further studies should be conducted to determine the impact of DRT on other diseases of soybean or alternate crops.