Research Highlights

Biology and control of Sclerotinia stem rot (white mold) of soybean

By Julie Meyer, Lead PI:  Mehdi Kabbage, University of Wisconsin, Madison

The overreaching goal of this work is to provide farmers and farm advisors with best management practices for sclerotinia stem rot (SSR, or white mold) of soybean in the north-central region. It also provides tools that breeders can use to incorporate white mold resistance into commercial varieties.

Fungicide evaluations

Farmers continue to be challenged with filtering through marketing information on new or repurposed fungicides for management of Sclerotinia stem rot. Therefore, a big part of this study was to evaluate the efficacy of standard fungicides, and the effect of weather conditions and application timing on fungicide efficacy.

The multi-state fungicide evaluations included lactofen, boscalid, picoxystrobin, prothioconazole +trifloxystrobin, boscalid + fluxapyroxad/pyraclostrobin, fluazinam, fluoxastrobin +flutriafol, prothioconazole, tetraconazole, and thiophanate-methyl.  The treatments included 5 different application timings, from R1 to R5 growth stages.

Researchers at each location collected data on weather, disease incidence, severity of disease, white mold apothecia formation, white mold spore release, and soybean yield. An economic analysis was conducted for each product used.

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Recommendations and takeaways from the fungicide evaluation

Lactofen, boscalid, picoxystrobin, prothioconazole +trifloxystrobin, and boscalid + fluxapyroxad/pyraclostrobin had the lowest level of disease incidence and were the preferred chemistries in this study.

Lactofen was among the more efficacious active ingredients in high disease pressure situations, and its relatively low cost compared to other products, resulted in high estimated value to the farmer. Economical disease management balances efficacy and cost; therefore, boscalid, a highly efficacious active ingredient, was not as profitable in this analysis because of its high cost.

Contans® WG , a commercial biological control product labeled for the control of Sclerotinia sclerotiorum in agricultural soils, has shown promise as a potential alternative for chemical fungicides to control white mold. In Wisconsin, the best and most economical times for application are during pre-planting or post-harvest on the stubble of a previously diseased crop. The time between the application of Contans® WG and the typical onset of disease should be as long as possible.

Smartphone apps to predict sclerotinia stem rot

The smart phone application SPORECASTER developed in this project is meant to be run in-season and uses site-specific weather information to predict the risk of white mold development. The app turns a few simple taps on a smartphone screen into an instant forecast of the risk of apothecia being present in a soybean field. This provides valuable information to help growers determine the best timing for fungicide treatment during the flowering period. Sporecaster is available as a free download from the Google Play Store and iPhone app store.

Farmers and advisors also have free access to SPOREBUSTER, a smart phone application that complements Sporecaster. Sporebuster is an application which calculates the return on investment, using research-based economic models to determine if a particular fungicide program for white mold control will result in a high probability of success. Users can input their costs for programs and uses their own yield and soybean pricing scenarios to get tailored recommendations.

Sporecaster received the 2018 American Society of Agronomy (ASA) Extension Education Community Educational Award in the category of digital decision aids.

Recommended cultural practices for control of sclerotinia stem rot of soybean

White mold is a disease of high yield potential soybean production. Although several factors are believed responsible for the increased occurrence of white mold, none may be more important than management practices or environmental conditions that promote rapid and complete crop canopy closure. We have found that canopy closure percentage of 40% and a soil temperature between 20 and 25°C are better indicators for the production of apothecia (the disease-causing stage of the pathogen) than soybean growth stage alone.

The best management of white mold requires an integrated plan that matches the level of resistance in a soybean cultivar to expected disease potential, together with cropping practices that influence crop canopy closure. Cropping practices that promote rapid and complete canopy closure are planting in narrow row widths, high plant populations, early planting, and high soil fertility.

The effect of row width on incidence of white mold and subsequent yield can vary by year and is strongly controlled by annual climatic conditions. Frequently, the yield advantage of narrow row widths, compared to wide widths, is expressed even though the incidence of white mold may be greater in narrow row systems.

Increasing row width from a narrow row spacing (6-8”) to a medium spacing (15”) can reduce white mold infections without compromising yields. However, lowering seeding rates in narrow row systems is preferable to increasing row widths to a wide row spacing.

Crop rotations that employ non-hosts is an effective way to reduce the incidence of white mold. Some non-hosts are better than others. A preceding crop of small grain, in contrast to corn, has a greater impact on reducing the incidence of white mold. Rotation with non-hosts such as small grains has been show to result in fewer apothecia formed under the soybean canopy.

The density of apothecia has been found to be greatest in moldboard plow systems compared to no-tillage systems. Fewer apothecia in no-tillage systems is a partial explanation why lower incidence of white mold is observed in no-till fields compared to fields receiving some degree of tillage.

Published: Dec 16, 2019