Options available for late-season treatment of damaging white mold

The cool, wet weather that seemed impossible to shake this summer created ideal conditions for the development of white mold, Sclerotinia sclerotiorum, in susceptible crops such as soybean, dry edible beans and potato. 

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 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 mouse droppings on or within plant stems. 

A field history of white mold and rotations including susceptible crops are critical factors in 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 commonly controlled with rotation to non-host crops and foliar fungicides applied during early flowering (R1-R3). However, chemical controls for the disease are preventative more than curative and the decision to apply a fungicide must be made in mid-summer before disease pressure is evident. For this reason, many fields 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 offer options for growers looking to protect future plantings of susceptible crops.

Coniothyrium minitans is a fungal parasite of white mold which occurs naturally in many soils. It attacks and degrades white mold sclerotia, 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 following 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 two and five 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 two 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 often remain viable for five years or more.

Efficacy of biological control products like Contans varies according to other production practices employed and field conditions. For example, no-till systems commonly see less of a positive effect from Contans and other white mold biologicals due to the inability to incorporate these products. However, research at Michigan State University has demonstrated a significant decrease in disease severity in soybean following a fall 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. 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 F, or for six months without cold storage.

Rotation to non-host crops and the application of foliar fungicides are currently the most effective and economical methods of controlling white mold. In cases where crop rotation options are limited, or fungicide application is not viable, biological control products offer growers another option for white mold control. 

Michigan State University Extension educator James DeDecker’s  Northern Farm and Field regional crop report appears biweekly on Wednesdays on the Business page. He can be emailed at dedecke5@msu.edu. MSU is an affirmative-action, equal opportunity employer. Michigan State University Extension programs and materials are open to all without regard to race, color, national origin, gender, gender identity, religion, age, height, weight, disability, political beliefs, sexual orientation, marital status, family status or veteran status.