|Weisz, Randy -|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 2010
Publication Date: February 1, 2011
Citation: Weisz, R., Cowger, C. 2011. Multiple mid-Atlantic field experiments show no economic benefit to fungicide application if fungal disease pressure is low or absent in winter wheat. Phytopathology. 101:323-333. Interpretive Summary: There is considerable interest in whether strobilurin fungicides can enhance yields of row crops in the absence of fungal disease pressure. We analyzed data from all available publicly-sponsored field trials of fungicides on wheat in Virginia and North Carolina. The trials, which were all replicated, involved from one to five fungicides. The fungicides were two strobilurins, one triazole, and two strobilurin-triazole mixtures. Each trial used from one to 32 wheat varieties, and for each variety we compared the yield under fungicide treatment to the yield without fungicide. Each trial was designated “low-disease” or “high-disease” based on data collected by the researchers who conducted the trial. To calculate economic benefit or loss from applying a fungicide, we used an appropriate range of grain prices and fungicide application costs, and an estimated yield loss due to driving over wheat during application. We calculated the yield increase needed to pay for the fungicide application for each combination of grain price and application cost. From that information, we computed the probability of achieving a break-even yield. Our analysis showed that if fungicides are applied based strictly on wheat growth stage, without regard to disease pressure, fungicide application costs would have to be below $10 per acre in order to achieve a 50% probability or better of breaking even or making a profit, as compared to not spraying. However, if fungicides are applied when fungal disease levels are above “low,” there is a >=50% probability level of breaking even or better for all application costs up to $25 per acre. Our results support using disease thresholds to ensure that fungicide applications to wheat in the mid-Atlantic U.S. are profitable. We found no evidence that fungicides applied for “plant health,” in the absence of disease, are profitable in this region.
Technical Abstract: Strobilurin fungicides produce intensified greening and delayed senescence in plants, and have been claimed to enhance yields of field crops in the absence of disease pressure. To help evaluate this claim, available publicly sponsored tests of fungicides on soft red winter wheat in Virginia and North Carolina (n = 39) were analyzed for the period 1994 to 2009. All tests were replicated, and had a randomized complete block, split-plot, or split-block design. Each test included from one to 32 cultivars and from one to five fungicides (two strobilurins, one triazole, and two strobilurin-triazole mixtures). There was a total of 291 test*variety*fungicide-treatment comparisons, where a comparison was the reported yield difference between sprayed and unsprayed treatments of a given variety in a given test. Parameters used to calculate the economic benefit or loss associated with fungicide application included a grain price range of $2 to $7 bu-1, a total fungicide application cost of $10 to $30 acre-1, and a 2.1 or 3.2 bu-acre-1 loss in yield from driving over wheat during application (with a 90- or 60-foot-wide sprayer). The yield increase needed to pay for a fungicide application at each combination of cost and price was calculated, and the cumulative probability function for the fungicide yield-response data was modeled. The model was used to predict the probability of achieving a break-even yield, and the probabilities were graphed against each cost*price combination. Tests were categorized as “low-disease” or “high-disease” based on the levels of fungal disease reported by the researchers rating the tests. Subsets of the data were analyzed to assess the profitability of the triazole fungicide and the strobilurin-containing fungicides separately in low-disease vs. high-disease environments. From the results, it was concluded that with routine fungicide application based solely on wheat growth stage, total fungicide application costs must be below $10 acre-1 in order to average a >=50% probability of breaking even or making a profit (as compared to not spraying). By contrast, if fungicides are applied when fungal disease levels surpass “low,” total application costs of up to $25 acre-1 are associated with a probability >=50% of breaking even or making a profit at prevalent wheat prices. The results do not support the application of strobilurin or triazole fungicides to mid-Atlantic wheat for “plant health” in the absence of disease, and instead reinforce the value of using disease thresholds if an economic return for fungicide application is desired.