Spray Early and Often….Is That Really True?

While the first part of that title may have been true when trying to control powdery and downy mildew with Bordeaux mixture in wine grapes, it is no longer the case when controlling small grain diseases with the current systemic fungicides.

When does adding a fungicide to your herbicide program in small grains make sense?

The simplest answer is “Only when you have found disease in your field or have insufferable early-season disease risk”.  Adding fungicides in the absence of a disease does not buy you anything other than risking crop injury, as certain fungicide formulations may ‘heat up’ specific herbicides. It is therefore important to read the label restrictions on the herbicides and fungicides you plan to use.

What about yield?

This past winter, Dr. Andrew Friskop, NDSU Extension Plant Pathologist, summarized 16 years of fungicide timing trials across North Dakota (Figure 1). One of the questions he wanted to answer was when it made sense to make an early-season fungicide application.  To answer this question, he first divided the data into four risk scenarios – very low risk, low risk, moderate risk, and high risk.  The very low and low risk scenarios included varieties that were moderately resistant to the leaf spotting diseases (tan spot and/or Septoria spp.), seeded after a broadleaf crop, in years with below average rainfall, and either infrequent or sporadic dews.  The moderate and high-risk scenarios included varieties susceptible to leaf spotting leaf diseases (tan spot and/or Septoria spp.), following wheat, in years with average or above rainfall and frequent dews.  The final disease severities at the end of the grain fill period averaged less than 1%, between 1 and 10%, between 10 and 20%, and more than 20% for very low, low, moderate, and high-risk scenarios, respectively. The corresponding yield responses averaged 0.0%, 1.7%, 4.6%, and 4.3% for the very low, low, moderate, and high-risk scenarios. Disease could only be found at the time of herbicide application in the high-risk scenario.

What has changed?

Tan spot epidemics were frequent throughout the mid-90’s in early 2000s. Recent disease survey data have shown a sharp decrease in tan spot prevalence over the past 10 years in both MN and ND. Aside from Mother Nature influencing epidemics, host resistance has gotten better; crop diversity has increased; longer rotations away from wheat are common; and fungicide applications have become a routine crop input. The impact of crop rotation on tan spot cannot be understated, as that single management tool can provide up to 63-70% suppression in no-till environments.

To reiterate (and maybe slightly more nuanced): The yield response of adding a fungicide to your herbicide program is highest if you are following wheat, grow a variety that is rated (moderately) susceptible to tan spot, and disease is already present in the field.

Figure 1 - Yield response (%) and estimated bushel response for fungicides rated very good
to excellent on tan spot across four disease risk environments in hard red spring wheat.