Corn fungicide timing (VT–R2): a decision guide

What the decision depends on

Disease pressure at VT. This is the strongest predictor. Scout the lower canopy from V10 onward and again at VT, looking at the leaves below the ear for gray leaf spot, northern corn leaf blight, tar spot or southern rust. Iowa State's statewide trials found no yield effect from fungicide at any of six locations in 2022, and its pathologists note that corn yields are rarely higher after a fungicide when disease is absent.

Hybrid rating. Iowa State does not recommend fungicide on hybrids rated resistant or moderately resistant to the diseases present; on susceptible or intermediate hybrids the decision turns on pressure near application.

Weather. Gray leaf spot, northern leaf blight and tar spot all need extended leaf wetness and humidity. A wet July forecast raises the odds; a dry one lowers them. Irrigated fields behave like a wet year.

Yield potential and price. The response is roughly proportional, so a 220 bu/ac (13.8 t/ha) field with a 4 percent response gains more bushels than a 150 bu/ac (9.4 t/ha) field, and the break-even yield falls as corn price rises.

Cost of the pass. Product plus application. In the CPN dataset, median ground application fees were $3.00–$8.50 per acre and aerial $6.50–$12.00 per acre (2014–2015 pricing); get a current quote.

Step-by-step

1. Stage the field. Use the leaf-collar method. The target window opens when the last tassel branch is visible (VT) and closes around blister (R2). Purdue's guidance is to never apply before V10 and to wait until disease is active and the crop is at or near VT to R2.
2. Scout. Walk at least five spots away from the edge, check the ear leaf and the three leaves below it on 10 plants per spot, and note which disease is present and how high in the canopy.
3. Check the forecast. For tar spot and gray leaf spot, run the CPN Crop Disease Forecasting Tool for your field. For southern rust, check the Corn ipmPIPE map for confirmed counties to your south.
4. Do the break-even math (worked example below).
5. Pick the product. Use the current CPN fungicide efficacy table, which the Corn Disease Working Group rebuilds every year from multi-year, multi-location trials, and match it to the disease you found. In the CPN timing study a QoI + DMI premix (FRAC 11 + FRAC 3) at VT gave about twice the response of a QoI alone (7.2 vs. 3.3 bu/ac, or 0.45 vs. 0.21 t/ha), and single-active DMI products did not give a substantial response; Purdue's advice for tar spot is to choose a product with more than one mode of action, which today usually means a premix that adds a FRAC 7 SDHI. Rates, adjuvants and water volumes are on the label registered in your state or province.
6. Check the PHI and REI before you book the plane. Corn fungicide labels carry preharvest intervals for grain and for forage or silage that can differ from each other, plus a restricted-entry interval, and all of these differ by product. Read the specific label for the use you intend.
7. Leave check strips. Two untreated strips per field, wide enough for the combine's yield monitor, turn next year's decision into data.

Worked example with numbers

Suppose a VT application is quoted at $30 per acre all-in (product plus aerial application) and corn is $4.50 per bushel. These are example inputs; use your own.

• Break-even yield = cost ÷ price = $30 ÷ $4.50 = 6.7 bu/ac (0.42 t/ha).
• Low-pressure field, resistant hybrid, dry forecast: the expected response is close to the no-disease average, which in Iowa State's 2007 statewide trials was 3.3 bu/ac (0.21 t/ha) against a 6 bu/ac (0.38 t/ha) break-even, and only 27 percent of sites covered the cost. Expected outcome: a loss. Do not spray.
• Moderate pressure, intermediate hybrid, humid forecast: the CPN average of 7.2 bu/ac (0.45 t/ha) for QoI + DMI at VT sits just above break-even, with roughly a coin-flip probability of a positive return. Spray if the field has high yield potential and the disease is moving up; hold if it is not.
• Tar spot confirmed on lower leaves at VT, susceptible hybrid, cool wet forecast: Purdue's 2024 northern Indiana trial recorded 7–18 bu/ac (0.4–1.1 t/ha) of yield protection from a VT/R1 pass. Spray.

The CPN Fungicide ROI Calculator runs this calculation from the full trial database using your own cost, disease severity and price.

Common mistakes

• Spraying on the calendar. A fungicide applied at V8 to save a trip protects leaves that will be gone by grain fill. Most products hold for about 14 to 21 days, so a V6 pass has run out before the disease period that matters; in the CPN study it added about 1 bu/ac (0.06 t/ha) over a VT pass alone when both were made.
• Waiting until R3 or later. After milk stage a fungicide can still slow a tar spot or southern rust epidemic, and CPN's pathologists accept a late pass when scouting shows disease still increasing, the weather stays favourable and the crop is still filling kernels. But a fungicide does not restore leaf area that is already dead, so the later the pass, the more of the loss has already happened. The exception is a heavy late epidemic on a crop with most of grain fill ahead: about half of kernel dry matter accumulates after R5, which is why a late tar spot application paid 20 bu/ac (1.3 t/ha) in one Michigan trial in 2019 and nothing in 2020.
• Mixing up the PHIs. The grain interval and the forage or silage interval on the same label can be different numbers. A field destined for silage needs the silage interval.
• Using a single mode of action on tar spot every year. It invites resistance and, in the CPN data, gave a smaller response than a premix.
• No check strip. Without one, a good-looking field proves nothing.

Regional differences

US Corn Belt. Gray leaf spot and northern corn leaf blight drive most VT decisions; tar spot has become the deciding disease across much of the eastern and northern Corn Belt, and southern rust adds pressure in years when it moves north early. Aerial application is the norm at VT because a standard ground rig cannot enter tasselled corn without damage; in the CPN dataset aerial application fees ran a few dollars an acre above ground rates, and the ROI calculation should use whichever you will actually hire.

Ontario and the Prairies. The same decision logic applies: the CPN timing dataset included Ontario sites, and OMAFRA's field crop pathologist co-authored the CPN tar spot guidance. Product choice is narrower. Only products with a Health Canada PMRA registration (PCP number) and the target disease on the Canadian label may be used, and a US registration does not transfer. Check the Ontario or provincial crop protection guide for the registered list, the Canadian preharvest interval and the label's directions for aerial application.