Economics of early blight/brown spot management of potato in the semi-arid environment of southern Idaho

Early blight and brown spot are common foliar diseases in southern Idaho and can cause significant foliar damage if not managed. Due to the semi-arid environment of southern Idaho, late blight occurrence is rare. Because of this, growers in southern Idaho may only apply foliar fungicides four times to manage foliar disease. The combined cost of product and application for common fungicide programs can cost close to $150 per acre. Results of ten research trials conducted over eight years which utilized the same fungicide program were analyzed to evaluate the relationship between early blight/brown spot control and potato yield. Seven of the trials were conducted with Russet Burbank and three with Ranger Russet. The fungicide program was fluopyram + pyrimethanil (Luna Tranquility) + chlorothalonil (Bravo WS) at row closure, and then again two weeks later, followed by two additional applications of chlorothalonil (Bravo WS) on two-week intervals. Foliar disease severity was visually estimated and used to calculate the percentage of control. Tubers were harvested and graded to estimate total and marketable yield. Disease severity in the untreated check ranged from 18% to 41% with percent control ranging from 47% to 89%. No correlation was observed between percent control and total yield. The average total yield increase was 4% with the foliar fungicide program. A similar increase in yield was observed in a two-year study using the same fungicide program on Clearwater Russet which is more resistant to early blight than Burbank or Ranger. Using a mock processing contract, this translated to an average increase of $275 per acre, or a net increase of $128 per acre on average. Even under the relatively lower early blight pressure in southern Idaho, fungicide programs are cost-effective and return to the grower more money than the cost of the program.