Biological products available for late season white mold treatment

Beneficial fungus is another tool to help growers combat a common fungal disease.

Sclerotium of Sclerotinia sclerotiorum colonized by the bacterium Coniothyrium minitans. Photo credit: A.J. Peltier
Sclerotium of Sclerotinia sclerotiorum colonized by the bacterium Coniothyrium minitans. Photo credit: A.J. Peltier

The cool, wet weather that seemed impossible to shake during the 2014 summer created ideal conditions for the development of white mold, Sclerotinia sclerotiorum, in susceptible crops such as soybeans, dry edible beans and potatoes. White mold is a soil borne fungus that overwinters in the form of resting bodies known as sclerotia. If environmental conditions are appropriate during the growing season, sclerotia in the top 2 inches of soil will germinate and produce small mushrooms (apothecia) on the soil surface. Spores released from apothecia then infect crop plants through their flowers during the early reproductive stages of growth. Infected plants can be easily identified by the presence of white, cottony mycelium (mold) on lower tissue and the production of new sclerotia, which resemble rat droppings on or within plant stems.

A field history of white mold and rotations including susceptible crops are critical factors determining disease pressure. Crop conditions, including narrow row widths, high plant populations and high yield potential can also create a favorable microclimate for disease development. For this reason, white mold is most effectively managed using cultural practices such as crop and variety selection, reduced plant populations and increased row spacing. When the potential for disease is great, foliar fungicides applied during early flowering can provide some additional control (0-60 percent). However, chemical controls for the disease are preventative more than curative and the decision to apply a fungicide must be made mid-summer before disease pressure is evident. As a result, many acres of susceptible crops go untreated.

In a year like this when conditions did favor disease development, nothing can be done to resurrect an infected crop. However, new biological control products paired with strategic tillage and crop rotation are expanding the set of tools available to growers interested in addressing severe white mold infestations and protecting future plantings of susceptible crops.

Coniothyrium minitans is a fungal parasite of white mold that occurs naturally in many soils. It attacks and degrades white mold sclerotia within the top 2 inches of the soil, reducing the formation of apothecia and ability of the disease to infect crop plants. C. minitans was isolated from white mold sclerotia way back in 1947, but has only recently been used to formulate a number of biological control agents for white mold including the products Contans WG and KONI. The advantage of these biological control products is that they can be applied in the fall following crop harvest, or the next spring, to fields with known white mold pressure.

Contans, the most commonly used biological agent for white mold, is applied as a spray to the soil surface or crop residue. The product label dictates an application rate between 2 and 5 pounds per acre. As it contains a sensitive living organism, Contans cannot be tank-mixed with any other pesticide or fertilizer products. In order to maximize contact of the product with sclerotia in the soil, Contans is ideally tilled-in to a depth of approximately 2 inches. Deeper tillage is not recommended when incorporating the product. This is due to the increased likelihood of burying sclerotia below the effective depth of the beneficial fungus, where they will not germinate, but often remain viable for five years or more. Subsequent deep tillage operations are also of concern due to the possibility of reintroducing variable sclerotia into the top 2 inches of soil.

Efficacy of biological control products like Contans varies according to other production practices employed and field conditions. For example, rotation to a white mold host crop following a fall Contans application is not recommended as the ability of the beneficial fungus to degrade sclerotia is dependent on keeping the disease inoculum within its viable depth for germination. Still, greenhouse soybean research at Michigan State University has demonstrated a reduction in apothecia and new sclerotia by 81.2 and 50 percent, respectively following an application of Contans.

There is also some evidence that repeated applications of C. minitans can create a build-up of the organism in soil and increase long-term white mold control, given that some sclerotia are present to support the beneficial fungus. A single application of Contans is expected to cost growers between $30 and $35 per acre. The product can be stored for up to one year at temperatures 35-40 degrees Fahrenheit, or for six months without cold storage.

Michigan State University Extension continues to recommend cultural controls such as crop and variety selection, planting population, row spacing and irrigation timing as the primary means of white mold management. Data on the effectiveness of biological control products like Contans WG and KONI in Michigan cropping systems remains limited. One question of particular interest is how commercial preparations of C. minitans differ from native populations, which also attack white mold sclerotia. Yet, when it comes to late-season sclerotia management, biologicals add another valuable tool to grower’s white mold management tool box.

The information in this article is supplied with the understanding that no discrimination is intended and no endorsement by MSU Extension is implied.


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