Factors to Consider When Deciding Whether to Treat Stripe Rust in Wheat

Factors to Consider When Deciding Whether to Treat Stripe Rust in Wheat May 22, 2015

The first appearance of stripe rust pustules in a field indicates that spores arrived in the field and initiated infections at least seven days earlier. Apply a fungicide if stripe rust is seen in a field and the following conditions or factors exist:
  1. Wet, cool weather.
  2. The flag leaf has emerged (or earlier depending on the situation).
  3. It's a susceptible variety.
  4. You would see a positive net return on investment based on the yield potential and the price of wheat.

Not all four factors have to exist to warrant treatment. Base your decision on the combination of factor No. 1 with any or all of the other three factors.

Table 1 illustrates the potential net profit from foliar fungicide treatment of wheat based on a $5 or $8 per bushel selling price at the elevator. Research conducted by the author in 2007 in Nebraska (wet growing season with foliar diseases — rust and leaf spots) provided estimates of how much yield increase or net profit to expect from spraying at the beginning of stem elongation (growth stage Feekes 6) compared to spraying at flag leaf (growth stage Feekes 9). Averaged across five fungicides (Quilt, Headline, Tilt, Quadris, and Stratego) and four locations (Mead, Clay Center, North Platte, and Sidney), spraying at Feekes 6 resulted in a net yield increase of 19 bu/ac and a net profit of $67/ac. Spraying at flag leaf resulted in a net yield increase of 22 bu/ac and a net profit of $85/ac. In a dry growing season with little disease (2006), yield increase was the same (6 bu/ac) from spraying at Feekes 6 or Feekes 9 and net profit was also about the same (about $5/ac) from spraying at Feekes 6 or Feekes 9.

The fungicide spray should be timed to protect the flag leaf. The concept of a threshold level of stripe rust above which a fungicide should be applied is not very helpful because in general, plant diseases are best controlled preventively. In addition, because rust spores are numerous and microscopic and it takes 7 to 10 days from infection to appearance of pustules (the incubation period), waiting until a certain threshold is reached based on appearance of pustules gives the pathogen more time to infect and produce more spores which spread and cause new infections.

In fields in which stripe rust appears at the heading growth stage, or when heading is starting, it is better to apply a fungicide at full heading that will control stripe rust as well as Fusarium head blight (scab).  One of the conditions favoring stripe rust (wetness) also favors Fusarium head blight. The fungicides Prosaro and Caramba have good efficacy on Fusarium head blight. A list of fungicides and their efficacies on wheat diseases is provided in a table developed by the North Central Regional Committee on Management of Small Grain Diseases (NCERA-184).

Table 1. An illustration of the potential net profit from foliar fungicide treatment of wheat based on a $5.00 or $8.00 per bushel selling price at the elevator.
Potential yield of the field in bushels per acreExpected yield increase due to treatment (in %)Expected yield increase due to treatment (in bu/a)$ increase based on a wheat price of $5$ increase based on a wheat price of $8Net profit/loss ($) at a $20 treatment cost based on a wheat price of $5Net profit/loss ($) at a $20 treatment cost based on a wheat price of $8Net profit/loss ($) at a $25 treatment cost based on a wheat price of $5Net profit/loss ($) at a $25 treatment cost based on a wheat price of $8
30 5 1.50 7.50 12.00 -12.50 -8.00 -17.50 -13.00
30 10 3.00 15.00 24.00 -5.00 +4.00 -10.00 -1.00
30 20 6.00 30.00 48.00 +10.00 +28.00 +5.00 +23.00
30 30 9.00 45.00 72.00 +25.00 +52.00 +20.00 +47.00
45 5 2.25 11.25 18.00 -8.75 -2.00 -13.75 -7.00
45 10 4.50 22.50 36.00 +2.50 +16.00 -2.50 +11.00
45 20 9.00 45.00 72.00 +25.00 +52.00 +20.00 +47.00
45 30 13.50 67.50 108.00 +47.50 +88.00 +42.50 +83.00
60 5 3.00 15.00 24.00 -5.00 +4.00 -10.00 -1.00
60 10 6.00 30.00 48.00 +10.00 +28.00 +5.00 +23.00
60 20 12.00 60.00 96.00 +40.00 +76.00 +35.00 +71.00
60 30 18.00 90.00 144.00 +70.00 +124.00 +65.00 +119.00
75 5 3.75 18.75 30.00 48.00 +10.00 +28.00 +5.00
75 10 7.50 37.50 60.00 +17.50 +40.00 +12.50 +35.00
75 20 15.00 75.00 120.00 +55.00 +100.00 +50.00 +95.00
75 30 22.50 112.50 180.00 +92.50 +160.00 +87.50 +155.00
90 5 4.50 22.50 36.00 +2.50 +16.00 -2.50 +11.00
90 10 9.00 45.00 72.00 +25.00 +52.00 +20.00 +47.00
90 20 18.00 90.00 144.00 +70.00 +124.00 +65.00 +119.00
90 30 27.00 135.00 216.00 +115.00 +196.00 +110.00 +191.00
105 5 5.25 26.25 42.00 6.25 +22.00 +1.25 +17.00
105 10 10.50 52.50 84.00 32.50 +64.00 +27.50 +59.00
105 20 21.00 105.00 168.00 85.00 +148.00 +80.00 +143.00
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