Multiple mid-Atlantic field experiments show no economic benefit to fungicide application when fungal disease is absent in winter wheat.

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. To help evaluate this claim, available publicly sponsored tests of fungicides on soft red winter wheat in Virginia and North Carolina (n = 42) were analyzed for the period 1994 to 2010. All tests were replicated and had a randomized complete block, split-plot, or split-block design. Each test included 1 to 32 cultivars and one to five fungicides (two strobilurins, one triazole, and two strobilurin-triazole mixtures). There was a total of 311 test-cultivar-fungicide treatment comparisons, where a comparison was the reported yield difference between sprayed and unsprayed treatments of a given cultivar in a given test. Parameters used to calculate the economic benefit or loss associated with fungicide application included a grain price range of $73.49 to 257.21 Mg(-1) ($2 to 7 bu(-1)), a total fungicide application cost of $24.71 to 74.13 ha(-1) ($10 to 30 acre(-1)), and a 0.14 to 0.21 Mg ha(-1) (2.3 to 3.4 bu acre(-1)) loss in yield from driving over wheat during application (with a sprayer 27.4 or 18.3 m [90 or 60 feet] wide, respectively). 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 "no-disease" or "diseased" based on reports of 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 no-disease versus diseased experiments. From the results, it was concluded that, with routine fungicide application based solely on wheat growth stage, total fungicide application costs had to be <$24.71 ha(-1) ($10 acre(-1)) in order to average a ≥ 50% probability of breaking even or making a profit (compared with not spraying). By contrast, if fungicides were applied when fungal disease was present, total application costs of ≤ $47 ha(-1) ($19 acre(-1)) for strobilurins and ≤ $72 ha(-1) ($29 acre(-1)) for propiconazole alone were associated with a ≥ 50% probability of breaking even or making a profit at a wheat price of $184 Mg(-1). The results do not support the application of strobilurin or triazole fungicides to mid-Atlantic wheat crops for "plant health" in the absence of disease. Rather, they support basing the decision to apply fungicide on observation of disease, if an economic return for the input is desired.