This article appears in the May 2020 issue of Potato Grower.

Foliar fungicides continue to be the primary means for the management of early blight caused by Alternaria solani in potatoes. Potato growers in North Dakota and Minnesota have been concerned that standard fungicides applied alone do not necessarily guarantee adequate protection against early blight epidemics in growing seasons conducive to disease development. High disease pressure based on yearly environmental fluctuations is one reason that growers have incorporated more expensive single-site specialty fungicides into foliar disease management programs.

The goal of a recent North Dakota State University study was to compare early blight control resulting from fungicide programs using solely standard protectant fungicides when compared to programs which incorporated specialty fungicides.

What We Did

Twelve field trials were conducted in North Dakota and Minnesota from 2003 to 2014. Early blight severity was visually estimated on a weekly basis for 11 weeks beginning five weeks after planting. Specialty fungicides were separated into two groups based on mode of action. QoI (quinone outside-inhibitor) and SDHI (succinate dehydrogenase inhibiting) fungicides were applied in rotation with standard protectant fungicides. We compared early blight severity at two crop growth stages, tuber initiation to early bulking (weeks 7 to 9) and late bulking to tuber maturation (weeks 10 to 11). We also compared total yield in response to standard and specialty fungicides using network meta-analytic models.

What We Learned

In this study, there was a significant difference in overall efficacy and yield between fungicide programs that incorporated specialty fungicides and a program that included only standard fungicides to manage early blight in potato. Early blight severity averaged across 11-week assessments in non-treated plots was 74 percent across the 12-year study period. Mean early blight severity across the same time period was 45 percent for plants treated with a specialty fungicide program compared to 54 percent early blight for a standard protectant fungicide program.

Specialty-based fungicide programs also displayed superior performance in combating early blight compared to a standard fungicide program when we looked separately at tuber initiation to early bulking and late bulking to tuber maturation. At tuber initiation to early bulking, the control of early blight increased by more than 30 percent for specialty-based fungicides compared to the standard protectant fungicides. During the late bulking and tuber maturation stage, the application of specialty-based fungicides significantly reduced early blight disease severity by 47 to 49 percent compared to the standard fungicide program.

Tuber yield ranged from 443 to 480 hundredweight per acre across all treatments and trials. Predicted yield loss for each 1 percent early blight increase at early bulking was 1.8 hundredweight per acre; at late bulking stage it was 1.1 hundredweight per acre. Over the 12-year study period, potato tuber yields increased by 10 to 11 perent when specialty-based fungicide programs were utilized over standard fungicide programs.

What It Means to Growers

Despite reports of reduced efficacy due to development of pathogen resistance and the high costs of single-site-specific fungicides, it is common for growers to incorporate specialty fungicides in rotation with multi-site protectant fungicides for early blight management. Results from the current studies clearly demonstrate that programs incorporating specialty fungicides used here can contribute to significant yield increases when compared with standard fungicides.

Specialty-based fungicide programs significantly outperformed standard fungicides at both the tuber initiation/early bulking and late bulking/tuber maturation growth stages, indicating that they continue to be effective until the end of the crop growing season. However, higher yield loss potential from early blight at early bulking crop stage when compared to late bulking growth stage indicate that timing fungicide applications at early bulking are critical for minimizing tuber yield loss. Tuber yield significantly increased when specialty fungicides were incorporated in the program. Improved yields are likely the direct result of fungicide application minimizing the loss of green area of the leaf and thereby improving plants’ ability to photosynthesize and store tuber constituent compounds. The higher cost of specialty fungicides may be justified due to increased efficacy and resultant increased yield when compared to less expensive standard protectant fungicides.

In conclusion, fungicide programs incorporating specialty products were most effective in managing early blight and improving tuber yields in North Dakota and Minnesota. Under high early blight pressure, potato growers should not rely on standard fungicide applications alone, as they are less effective when compared with foliar fungicide programs where specialty fungicides are incorporated. However, it is critical to safeguard fungicides because the current intensive rate of application may exert selection pressure on A. solani, reducing fungicide efficacy. This would prove to be a significant loss to the potato growers. Results of our studies demonstrate the advantage of fungicide rotational programs incorporating specialty and standard fungicides for early blight management.

Treatments evaluated in a network meta-analysis of 12 field trials conducted in North Dakota and Minnesota from 2003 to 2014

Positive sign (+) indicates alternation with fungicide with different active ingredient. A non-treated treatment was included for all trials. Overall, 10 fungicide applications were performed during each potato-growing season. Each specialty group fungicide regime included a total of three applications of a specialty single-site mode-of-action fungicide in rotation with seven standard protectant fungicide applications.

Each of the authors is a researcher at North Dakota State University’s North Dakota Agricultural Experiment Station. For more information about research being done there, visit www.ag.ndsu.edu/research.