The cool, wet weather that seemed impossible to shake across much of the U.S. 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.
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. 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. However, chemical controls are preventative more than curative, and the decision to apply a fungicide must be made mid-summer before disease pressure is evident.
When conditions 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 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. 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 about 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 longer. 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 buildup of the organism in soil and increase long-term white mold control, given that some sclerotia are present to support the beneficial fungus.
Cultural controls such as crop and variety selection, planting population, row spacing and irrigation timing continue to be recommended as the primary means of white mold management. Data on the effectiveness of biological control products 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 toolbox.