Frogeye leaf spot and fungicide resistance

Symptoms and where to look

Lesions start as small dark water-soaked spots on the upper leaf surface and expand over about two weeks into circular to slightly angular spots with a gray to tan centre and a distinct red-brown to purple margin, the "frog's eye". Turn the leaf over: in humid weather the centre of a mature lesion carries a faint gray fuzz of spores on the underside. Young, expanding leaves are the most susceptible, so new lesions keep appearing in the top of the canopy through pod fill while older leaves lower down may have none. Severe infection blights and drops leaves early. Later in the season look for long, narrow dark stem lesions with flattened centres, circular to elongated sunken red-brown pod lesions, and gray to brown blotches on the seed coat.

Symptoms are usually seen from R1 through R7. Because the lesion takes 7 to 14 days to appear after infection, what you see today reflects the weather two weeks ago.

Look-alikes and how to tell them apart

The two that matter most are bacterial blight, because a fungicide is wasted on it, and brown spot, because it is nearly always present in the lower canopy and rarely costs yield unless it climbs into the upper third.

Conditions that favour it

The fungus overwinters in infested soybean residue for at least two years and can be seed-borne, although seed transmission is rarely seen in the field. Spores spread by wind and splashing rain. Infection and lesion development are fastest at 77 to 86°F (25 to 30°C) with relative humidity above 90 percent, frequent rain or overhead irrigation, and extended overcast periods. Risk is highest in continuous soybean, short rotations, reduced tillage, overhead-irrigated fields and susceptible varieties, and in any field with a frogeye history.

Scouting and thresholds

Start at R1 and scout weekly through R5, concentrating on the upper canopy. There is no formal numeric threshold; extension guidance is to treat when lesions are present and increasing in the upper canopy, the variety is susceptible, the forecast is warm and humid, and the crop is at R3 to R5. Disease that starts after R5.5 or stays at low severity causes little loss. Where severe disease begins early or just after flowering under favourable weather, losses of up to 30 to 35 percent have been recorded, and Nebraska estimates up to 20 percent on highly susceptible varieties.

After any fungicide application, go back in two weeks. New lesions on leaves that were protected mean either the timing missed or the product did not work, and in a FRAC 11-only program the second explanation is now the more likely one.

Fungicide resistance: what is confirmed and what it means

QoI (strobilurin, FRAC 11) resistance in C. sojina is caused by a single point mutation, most commonly G143A in the fungal cytochrome b gene. The mutation changes the target site that every FRAC 11 active binds to, so switching from pyraclostrobin to azoxystrobin or picoxystrobin changes nothing, which is why the Crop Protection Network rates all three as poor.

The record, as of October 2026:

• First confirmed in Tennessee in 2010, with detections in several more states, including Iowa's neighbours, over the following years.
• Iowa: first confirmed from 2017 samples; a 2019 survey of 73 fields in 51 counties found resistant isolates in nearly every county, with a single field in Adair County the exception.
• Nebraska: confirmed in 2019 in all 10 counties sampled, and described as widespread in follow-up surveys.
• The Crop Protection Network's efficacy table now rates pyraclostrobin, azoxystrobin and picoxystrobin alone as poor against frogeye, with a footnote that they may perform better only where resistant isolates are absent.

For a grower in the Corn Belt the practical implication is to assume resistance is present. Iowa State's plant pathologists put it plainly: using QoIs as the primary control of frogeye leaf spot is no longer a solution. FRAC 11 actives still have value in a premix for other diseases, but the frogeye activity has to come from the partner.

Management

Variety. The Rcs3 gene has been effective against all races of C. sojina known in North America. Ask the seed rep for the frogeye rating and whether the variety carries Rcs3 or partial resistance; in a field with history this is the cheapest control available.

Rotation and residue. Rotate to non-hosts (corn, small grains, grain sorghum, alfalfa) for two or more years after a bad year, since the fungus survives in residue at least that long. Where tillage fits the system, burying residue reduces inoculum. Plant high-quality certified seed.

Fungicides. Choose by FRAC group and by the CPN frogeye rating, and use at least two effective modes of action:

Timing is R3 (beginning pod) for a single application, with the useful window running to R5. Rates, intervals and pre-harvest intervals are on the label registered in your state or province. Thiophanate-methyl (FRAC 1, an MBC) is effective but belongs to a group with its own single-site resistance risk, so do not lean on it alone either.

Ontario growers: products and registered uses differ in Canada. Check the Health Canada PMRA label and PCP number for the frogeye claim; a US registration does not apply.

When a spray pays and when it does not

A spray is likely to pay on a susceptible variety with frogeye lesions building in the upper canopy at R3, in a warm, humid forecast, in a field with history or continuous soybeans, using a product whose frogeye activity comes from a FRAC 3 or 7 active. Under those conditions the disease can take a quarter or more of the yield.

A spray is unlikely to pay on a resistant variety, when lesions are few and confined to older leaves, when the disease shows up after R5.5, or when the only spots in the field are brown spot in the bottom third. And a FRAC 11-only application for frogeye in the Corn Belt is money spent on a product the fungus no longer notices. Let what is on the leaves drive the decision, not the calendar, and let the FRAC group drive the product choice.