The impact of fungicide application method on soybean canopy coverage, disease, yield, seed quality, and seed fill duration.

Viggers, Joshua
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As input costs continue to rise and profits fluctuate, soybean farmers have maintained interest in new fungicide applications methods. Customarily, fungicides are applied directly over the top of the soybean canopy as are herbicides and insecticides. However, fungicide application methods that could improve fungicide coverage within the canopy, such as undercover applications, has gained considerable interest. Undercover applications spray fungicides between the soybean rows, using multidirectional nozzles, and over the canopy with nozzles placed above the canopy. During 2017 and 2018, field experiments located in Iowa were used to investigate the effect of fungicide application methods on coverage, disease severity, yield, seed quality, and duration of seed fill. The objective of this research was to: (1) investigate how traditional and undercover application methods impact canopy coverage; (2) examine the effect of application method on disease control, seed quality, and yield; and (3) to determine if a fungicide application increases the rate or duration of seed fill. Studies were conducted in two small plot field experiments in 2017, six small plot field experiments in 2018, and two on-farm strip trials in 2018. Small plot and on-farm strip trials were analyzed separately.

Fungicide coverage was detected in the upper, middle, and lower canopy in two different ways: (1) water sensitive spray cards; and (2) tracer dye. Results for spray card detection in small plot experiments showed one significant difference between traditional and undercover application technologies in the upper canopy (P=0.017). The middle and lower canopies coverage values were not significant between the traditional and undercover and no canopy zone was significantly different between traditional and undercover for on-farm strip trials when using spray cards as a detection method. Conversely, the amount of tracer dye detected in the upper and lower canopy in small plot experiments differed between the traditional and undercover (P=0.023 and P=0.034), but there were no significant differences between application method at the on-farm strip trials. For both years, the primary diseases present were frogeye leaf spot and Septoria brown spot. Greater disease severity for both diseases was observed in 2018 than in 2017. Regardless of the year, location in the canopy there was no significant difference between the treatments for foliar disease control. As a result, the inability to control disease led to there being no significant differences among seed quality and yield. Results from these trials show adequate coverage, but the inability to control disease suggests that resistance of QoI fungicide in pathogen population maybe an issue in Iowa.

The last objective of this thesis is to investigate if a QoI fungicide affects the duration of seed fill. Four small plot field experiments were conducted in 2017 and 2018. There was no significant difference in both the seed growth rate and duration of the seed filling period, but at one location there was a difference in yield. These results support other studies that suggest the probability of affecting soybean yield beyond disease control is less likely.

Soybean farmers should be aware that QoI fungicide resistance for the pathogens that cause frogeye leaf spot and Septoria brown spot have been identified in Iowa. Resistance to these two pathogens made it difficult to determine which fungicide application method was more effective. Future work comparing the traditional versus undercover applications may be necessary with more effective fungicide. Regardless of the application type it is important to only apply fungicides when necessary for disease management and to rotate mode of actions to mitigate the development of fungal resistance.

Fungicide Application Equipment, Fungicide Application Technology, Fungicide Coverage, Plant Pathology, Soybean Diseases, Undercover Sprayer