Fusarium head blight in wheat: fungicide timing at anthesis

Symptoms and where to look

FHB (scab), caused by Fusarium graminearum, infects through the flowers. The first visible sign is one or more bleached spikelets on an otherwise green head; the fungus then spreads up and down the head by infecting neighbouring spikelets in sequence, so part or all of the head turns prematurely white while the plant below stays green. Under humid conditions pink to orange spore masses appear on infected spikelets, often at the base, and small dark fruiting bodies may follow. Infected kernels are shrivelled, chalky and white to pink ("tombstones"), lightweight, and carry most of the mycotoxin. By the time symptoms show, the spray window closed weeks earlier, which is why FHB is managed on forecast, growth stage and variety rather than on scouting for symptoms.

Look-alikes

The common confusion is with take-all, which also produces white heads around Feekes 10.5. Take-all shows up as patches of uneven, prematurely ripened plants with whole heads white, and the giveaway is below ground: roots are blackened and severely infected plants have a black crown with the blackening extending up the stem. Scab bleaches individual spikelets or sections of a head on a plant with healthy roots and a green stem, and the pink spore mass at the base of a bleached spikelet is diagnostic. Any other cause of whole-tiller death leaves the same pattern as take-all: the whole plant goes, not part of the head.

Conditions that favour it

Warm, wet weather with high humidity during flowering and early grain formation. The fungus survives on infected corn and small-grain residue, so wheat after corn or wheat with residue left on the surface starts with more inoculum. Alberta notes that winter wheat often escapes because it flowers before spores are present, and that durum is the most susceptible class, followed by CPS and hard red spring wheat. Risk changes daily, so the question is always "what is the weather around the day this field flowers".

Scouting, staging and the risk tool

Start staging when heads begin to emerge from the boot. Ontario's protocol defines Day 0 as the point when 75% of main-stem heads are fully emerged and targets the spray on Day 1 to Day 4, with Day 2 the optimum; Alberta describes the same window as opening when most main-stem heads are out of the boot and closing when 50% of them are flowering, about seven days in total, shorter in hot weather. In Feekes terms the target is 10.5.1, early anthesis. The Crop Protection Network reports that applications made five to seven days after the start of flowering still suppress FHB and DON, and Ohio State uses four days as its practical limit.

Check the FHB risk tool at wheatscab.psu.edu during that window. It combines temperature, rainfall and relative humidity into a map where red is high, orange medium and yellow low risk, and the estimate applies to fields flowering that day with a susceptible variety; re-check it as later fields flower. In Ontario the comparable service is DONcast. A moderately resistant variety lowers the risk category and the payoff from spraying; a susceptible variety after corn under a red map is the clearest case for a spray.

Management

Variety first. Moderately resistant varieties are available; check the FHB rating in your state or provincial variety trial before you order seed. Resistance is the base of the program and the fungicide is a supplement to it, not a substitute. No variety is immune.

Rotation and residue. Rotate away from corn and small grains for at least one year (Alberta recommends two years away from cereals), and chop and spread straw to speed decomposition.

Fungicide choice. Triazoles (FRAC 3) are the only class the Crop Protection Network describes as giving adequate FHB and DON suppression, and the newer SDHI (FRAC 7) actives appear in the efficacy table only as premixes with a triazole. In multi-state USWBSI trials reported in CPN-3001, metconazole, prothioconazole and prothioconazole + tebuconazole reduced DON by 45–60%, tebuconazole by 20–30% and propiconazole by 12–20%.

Ratings are the committee's multi-state consensus, not a guarantee of control in your field. Rates, water volumes and pre-harvest intervals are on the label registered in your state or province; US registrations do not automatically apply in Canada, where the product must carry a PMRA registration number.

Why no strobilurins near flowering. The efficacy table's footnote states that products containing strobilurin fungicides may elevate DON in grain damaged by head scab, and CPN-3001 reports that FRAC 11 products increased DON compared with an untreated check. Strobilurin premixes used at flag leaf for rust and leaf blotch are fine; at or after heading they are not, and several are not labeled past Feekes 10.5.

Head coverage. Heads are vertical targets, and flat-fan nozzles pointed straight down miss much of each one. CPN-3001 recommends forward- and backward-facing nozzles angled 30–45 degrees down at 8–10 in (20–25 cm) above the heads, fine to medium droplets of about 300–350 microns, and 10–20 gal/ac (95–190 L/ha) by ground; below 6 mph (10 km/h) use dual forward/back nozzles, at 6 mph and faster a single forward nozzle angled 30 degrees. Ontario's guidance is a forward-and-back double nozzle or alternating forward/back flood nozzles every 20 in (51 cm), 10–12 in (25–30 cm) above the canopy, at 18–20 US gal/ac (170–190 L/ha); doubling the boom height can cut head coverage by up to 50%. A non-ionic surfactant improves coverage where the label allows it.

DON, grading and what happens at harvest

Fungicide at anthesis reduces DON; it does not remove it. In the US, FDA's advisory level is 1 ppm DON in finished wheat products for human consumption, with 10 ppm on grain for feedlot cattle and poultry and 5 ppm for swine and other animals, and elevators discount or reject on their own DON tests. In Canada, FHB is graded on Fusarium-damaged kernels: the CWRS primary grade table allows 0.3% in No. 1, 0.8% in No. 2, 1.5% in No. 3 and 4.0% in CW Feed. DON keeps developing in grain above 19% moisture, so harvest scabby fields promptly, turn the combine fan up to blow out tombstones, store those fields separately, and test before feeding or selling. Gravity tables and colour sorters can lift a grade but are not fully effective.

When a spray pays and when it does not

It pays when a susceptible or moderately susceptible variety is flowering under medium or high risk, especially after corn, and the sprayer can get there within the four-to-seven-day window. It pays less on a moderately resistant variety under a yellow map, where the triazole's leaf-disease activity may be the main return. It does not pay at boot stage, which is too early to protect the florets, or ten days after flowering, when infection has already happened. A strobilurin anywhere in that window can cost more in DON than it returns in yield.