White mold in soybeans: risk, timing and fungicide ROI

Symptoms and where to look

Infection happens at flowering, but you will not see it then. Stem symptoms show up from R3 to R6, three to four weeks after bloom, and most fields are first noticed in August. Look at the nodes on the lower and middle stem, typically 6 to 14 inches (15 to 36 cm) above the soil, where a dead flower sat in the leaf axil. Lesions start gray to white and water-soaked, spread up and down from the node, and girdle the stem. In humid weather the lesion is covered in white, cottony mycelium. Later the stem bleaches, turns pithy and shreds, and hard black sclerotia form on and inside the stem. Leaves above the lesion turn gray-green between the veins, wilt and die, but stay attached. Infected pods carry small, chalky seed, and sclerotia can replace seeds entirely.

Look-alikes and how to tell them apart

Several stem and wilt diseases kill soybeans in patches in August. Split the stem.

Black sclerotia, outside or inside the stem, are diagnostic.

Conditions that favour it

The fungus survives as sclerotia in the soil. Most die within three to four years between soybean crops, but buried sclerotia can persist up to seven years, and only those within the top 1 to 2 inches (2.5 to 5 cm) germinate. Germination produces apothecia, small tan cup-shaped mushrooms, when the soil under the canopy is moist, cool, about 59 to 68°F (15 to 20°C), and shaded. Apothecia release airborne spores that land on senescing flowers; infection then needs free moisture and prolonged air temperatures below about 85°F (29°C) with frequent rain or high humidity.

That makes canopy closure at flowering the single most important risk factor. Anything that closes the canopy early raises risk: narrow rows, high populations, early planting, high fertility, tall lush varieties and irrigation during bloom. Field history matters because the inoculum is already there; the fungus does not blow in from neighbours in any quantity. Other host crops (dry beans, peas, sunflower, canola and other brassicas) and broadleaf weeds such as lambsquarters, pigweed, ragweed and velvetleaf add to the sclerotia bank.

Scouting and risk tools

There is no in-season threshold for white mold because the spray decision has to be made before infection. Scouting is a risk assessment at R1 to R2: walk the field at first flower, look under the canopy for apothecia on moist soil, note how close the rows are to closing, and check the two-week forecast.

Sporecaster, the University of Wisconsin smartphone app, formalises this. You enter the date, whether flowers are present, the percent canopy row closure, and whether the field is non-irrigated, irrigated on 15-inch (38 cm) rows or irrigated on 30-inch (76 cm) rows. It combines that with weather data to predict the probability that apothecia are present and returns a spray recommendation. In validation across 60 commercial fields in Wisconsin, Michigan and Nebraska it was 81.8 percent accurate across R1 to early R4. Run it every few days from R1 through R3 and follow the app's own guidance for the spray trigger.

The CPN White Mold ROI Calculator answers the money question. It is built from multi-year, university-run fungicide trials across the United States and Ontario. You enter the treatment cost, the disease severity you expect, and your yield and price assumptions, and it returns the estimated net benefit and break-even probability by product. It is not a forecast; it tells you what a given level of disease has been worth controlling in the trial data. Include the applicator charge or your own machinery cost; depreciation is real even when you spray it yourself.

Management

Variety is the first lever. No variety is resistant, but tolerance ratings range from moderate to very susceptible, and in a field with history that range matters. Shorter, lodging-resistant varieties keep the canopy more open.

Row spacing and population come next. Purdue suggests 15-inch rows or wider and 125,000 to 150,000 plants per acre (310,000 to 370,000 per hectare) in infested fields; Nebraska points to 30-inch rows over 15-inch to slow canopy closure. Both cost some yield potential in a clean field, so apply them where history justifies it.

Rotate two to three years away from soybean and other host crops, using corn and small grains, and control broadleaf hosts in the rotation years. Tillage results are inconsistent: deep ploughing buries sclerotia but later tillage brings them back, and Iowa State research found long-term no-till carries a smaller risk than conventional tillage because undisturbed sclerotia near the surface decline. Harvest infested fields last, clean the combine, and do not keep seed from them.

Foliar fungicides, with the CPN efficacy rating for white mold:

A single pass goes on at R1 to R2; a two-pass program in a high-history field starts at late R1 with the second about 14 days later. Coverage matters more than for leaf diseases because the target is flowers low in a closing canopy, so use the high end of the label water volume. Rates, intervals and water volumes are on the label registered in your state or province.

Ontario and Manitoba growers: US registrations do not apply in Canada. Check the Health Canada PMRA label (PCP number) for the product and the white mould claim. OMAFRA's inoculated Ontario trials under heavy pressure found all registered products gave a yield benefit, from 3.8 to 17.5 bu/ac (0.26 to 1.18 t/ha), with Cotegra (boscalid + prothioconazole, FRAC 7 + 3) and Allegro (fluazinam, FRAC 29) among the products named as effective in a single R2 application.

When a spray pays and when it does not

Each 10 percent increase in plant incidence costs roughly 2 to 5 bu/ac (0.13 to 0.34 t/ha), and Iowa State considers yield risk minimal below 20 percent incidence. The question is whether this field, this year, is headed for 20 percent or more.

A spray is likely to pay where the field has had white mold before, the canopy is closing at R1, Sporecaster shows apothecia risk, and the two-week forecast is cool and wet. In Ontario's inoculated high-pressure trials every registered product gave a yield benefit, and a two-pass program has been the most consistent under those conditions.

A spray is unlikely to pay where there is no history (the inoculum is not there), where the canopy is open at flowering in 30-inch rows, where flowering coincides with hot dry weather, or where the decision is being made at R3 or later because symptoms have appeared. Fungicides do not cure infected stems. In those fields the money is better spent on next year's variety, row spacing and rotation.