White mold management in Michigan dry beans

Scott Bales<balessco@msu.edu> and Martin Chilvers<chilvers@msu.edu>, Michigan State University, Department of Plant, Soil and Microbial Sciences - July 22, 2020

Each year, white mold is the most yield limiting disease in Michigan dry beans. Learn what to watch for and how to protect yield.

White mold symptoms — Figure 1. (A) White mold apothecia, (B) white mold sclerotia below the soil surface and apothecia above, (C) birds nest fungi, NOT white mold apothecia. Photos by Martin Chilvers, MSU.

What causes white mold in dry beans?

White mold in dry edible beans is caused by the pathogen Sclerotinia sclerotiorum. Sclerotinia will survive in the soil for many years in Michigan soils as sclerotia resting bodies, which are about the same size and appearance of mouse droppings. Under cool, moist conditions, these sclerotia germinate and produce a mushroom-like fruiting structure called apothecia. Apothecia are about 0.25 inches in size and can easily be confused with other mushroom structures, which do not cause white mold such as the birds nest fungus (Figure 1).

While apothecia can be formed under a wide range of soil temperatures and moisture levels, maximum development occurs under a canopy when soils have been at high levels of sustained moisture for seven to 10 days. Wet-dry cycles sharply reduce the number of apothecia that will be formed. Once apothecia are formed, they produce airborne spores that lead to infection of the bean plant. This is the primary infection point for white mold in dry beans. The most sensitive point for infection is dead and decaying tissue, such as senesced blossoms. Secondary infection can also occur from direct contact from plant-plant under high levels of relative humidity and cool temperatures. The severity of secondary infection can be greatly increased by frequent rainfall events.

Is treatment needed?

There are many factors to consider when contemplating treatment for white mold in dry beans, including field history, environmental conditions, row spacing and canopy closure, and the planted variety.

Field location and history. Dry bean fields with a history of bean or soybean production and white mold pressure are at a greater risk for severe infections. These fields likely have a large inoculum and conducive environmental conditions for infection. The location plays a role in risk as well, production areas in the far north and eastern Thumb historically have greater white mold pressure due to cooler temperatures and high relative humidity produced by Lake Huron. Individual fields that may be in lower-lying areas or have little air movement (near wood lots) are also at greater risk for infection.

Environmental conditions. Cool, wet weather favors white mold infection throughout the flowering period (late July-August). Both high temperatures and wet-dry cycles can reduce the severity of white mold infection. Rainfall frequency has a greater implication on severity than the total rainfall amount. For example, 0.1 inch of rain every other day for a week during flowering will lead to higher severity of white mold infection than one rainfall event of 0.5 inches. Irrigation is also a risk factor as this can increase canopy closure and provide moisture for apothecia development and plant infection.

Row spacing and row closure can affect the level of white mold infection by influencing the micro-climate within the field. Typically, narrow rows close rows sooner in the growing season which helps suppress weeds and increase yields. However, when dry bean rows close with dense foliage, more moisture is retained within the foliage and at the soil surface. This can lead to a greater level of sclerotia germination and a greater number of spores released into the canopy.

Variety. While there is no strong resistance to white mold in any commercial variety of dry bean, some varieties are more tolerant than others. This tolerance is often linked to plant architecture. Varieties that are more upright and compact in growth often have a natural avoidance to white mold infection. Varieties that are more decumbent, produce more foliage or are prone to lodging are often more severely affected by white mold, eventually resulting in yield loss. This yield loss can be either directly from the infection, or indirectly from harvest loss and pick at delivery.

Other tools. Sporecaster was designed to predict the probability of apothecial presence in soybean fields. However, Sporecaster should also aid in fungicide decisions and timing for dry bean fields. We are currently collecting data in dry beans to validate Sporecaster for dry bean producers.

To use the “Sporecaster” app, download it onto your phone from the Apple Store or Google Play. The app allows the user to locate and set up multiple fields and run the apothecial risk prediction model using weather data from a third-party provider (Dark Sky API).

Once opened, you can create multiple fields to determine their apothecial risk. The app will prompt the user for information, such as field name, row spacing, if the field is irrigated or not, and the field location. Then the risk of apothecial presence can be calculated. The model will only run if it is told that flowers are present and if canopy closure meets threshold (for 30-inch row spacing only). A forecast risk expressed in percentage units is then shown, with red being above the 30% action threshold for a fungicide application. It is possible to rerun the model as desired and even go back to previous years to examine the risk from years prior.

The model has been validated in commercial soybean fields and small test plots in Michigan, Wisconsin and Iowa. However, we very much want to hear about your experiences with the model. We hope the model will facilitate fungicide application decisions and lead to greater disease management.

The app is available for download on both iPhone and Android devices.

For more information on the app and video tutorials, see the following YouTube videos:

Funding for studies that contributed to the model were sponsored in part by the Michigan Soybean Promotion Committee.

If treatment is needed

Once the decision to make an application for white mold has been made, three questions need to be answered: What product? What rate? And at what application timing?

Product

There are many products to choose from for managing white mold in dry beans. The Michigan Bean Commission has performed research analyzing the efficacy of many of these products on white mold at different application timings. Fungicide results from 2019 follow a similar trend in previous years with treatments of Endura, Omega and Propulse performing well. These results can be found at Michigan Dry Bean Research Report 2019.

Rate

Always consult specific labels for labeled rates for target pests.

Timing

Optimal fungicide application timing can be variable year-to-year, which is a reflection on conditions that favor apothecia development and plant infection. Sporecaster provides a risk assessment for apothecia production which we hope may be used to optimize timing.

Application method

When applying white mold fungicides, use ground application equipment; aerial applications have proven to be unsuccessful at penetrating the dry bean canopy and covering the blossoms. Coverage is very important as these fungicides are preventative and not curative. They work by protecting the sensitive tissue on the surface and are not able to move systemically throughout the plant. A minimum spray volume of 15 gallons per acre is recommended for white mold applications; volumes greater than 15 gallons per acre will aid in the coverage of dense canopies.

Nozzle selection is also very important. Michael Wunsch at North Dakota State University has analyzed the effects of droplet size on white mold control. He has found that medium-sized droplets offer the most consistent performance, coarse droplets can offer greater control under very dense canopy conditions (higher velocity per droplet), and fine droplets never offered superior control in dry beans. His research has also found that the addition of a non-ionic surfactant can at times improve the effectiveness of white mold fungicides. Wunsch’s research results can be found at NDSU White Mold Research.