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What To Know About White Mold In Soybeans

Sep 18, 2019

White Mold In Soybeans
Harvest is often the time when white mold is noticed the most. It can stick out like a sore thumb and decrease yield. According to UW Madison, white mold of soybean, also called Sclerotinia stem rot, was discovered in Central Illinois in 1948. Although white mold became a chronic problem in Michigan, Minnesota, and Wisconsin by the 1970’s, the remainder of the North Central states experienced no problems with the disease. Even in the Great Lakes states, outbreaks of white mold were generally localized and occurred where soybeans were grown in rotation with other susceptible crops.

Soybean producers are adapting management practices to maximize yield, and white mold is a disease of high yield potential soybeans. This situation is unfortunate because white mold penalizes the progressive grower with a yield hit.

The growth and activity of the white mold fungus are controlled by the environment in the crop canopy. This fungus is particularly favored by dense soybean canopies created by planting in narrow row widths, high seeding densities, early planting, high soil fertility and other factors that promote good plant health.

The Pathogen, Sclerotinia Sclerotiorum
White mold is caused by the fungus Sclerotinia sclerotiorum. This fungus is easily recognized by the presence of fluffy white mycelium, or the vegetative body of the fungus that is the source of the name white mold.  Each year, the occurrence of white mold is mostly dependent on weather conditions during soybean flowering and early pod development. Rain, cool temperatures (less than 85° F), high relative humidity and moist soil favor the growth of the fungus.
 
The white mold pathogen infects many broadleaf plants. Highly susceptible crops are: soybeans, dry beans, snap beans, lima beans, sunflowers, canola, carrots, and cabbage. Peas, potatoes, alfalfa, and red clover are hosts, but are examples of crops that are much less susceptible to the white mold pathogen. Examples of non-host crops are corn, small grains, and all forage grasses. Currently all forms of the pathogen are believed to be equally pathogenic to all hosts and varieties of soybeans.

The Disease Cycle
The life cycle of the white mold fungus begins with the germination of sclerotia on the soil surface into a mushroom looking structure called apothecia. Apothecias are tan, have a sponge like texture and are 1/4 to 1/2-inch-tall at maturity. They are found on the soil surface and form from sclerotia when the soil is moist and dim light is filtered through the leaf canopy.

The number of apothecia is related to the number of sclerotia near the soil surface, soil moisture and temperature and the timing and density of crop canopy closure. Apothecia will readily form under the canopy of a non-host.

Under the cap of the apothecia, microscopic spores are produced and forcibly ejected. The disease cycle of white mold begins when ascospores germinate and colonize senescing flower petals that stick to emerging pods. Infection eventually progresses from pods to nodes and stems, resulting in a premature death of stems.  If adjacent plants come into contact with an infected plant, they may also become infected, but plant to plant spread of the pathogen is minimal and not as important as the infection of blossoms.

Sclerotia are formed from the white mold fungus growing on and inside of stems and pods.  Sclerotia that are formed on stems and pods eventually fall to the soil surface.  Those formed inside stems and pods are released when plants pass through the combine at harvest and are deposited on the soil surface.
Sclerotia will also be buried within the soil profile depending on the degree and type or lack of tillage.
Sclerotia are also removed from the field with the grain. Fortunately, they are not toxic to livestock, and are believed to be killed by the roasting process.

Signs and Symptoms
Leaves wilt, and the tissues between major veins develop a gray-green cast while vein tissues remain green. Leaves yellow, eventually die and turn completely brown, but often remain attached to the stem past plant maturity. These symptoms can be seen at pod setting in August and September.  Foliar symptoms of white mold can easily be mistaken for that of brown stem rot, Phytophthora root rot, sudden death syndrome, or stem canker.
 
White Mold Risk Assessment
Each year, the occurrence of white mold is heavily dependent on weather conditions during soybean flowering and early pod development. Rain, cool temperatures, high relative humidity, and moist soil favor the growth of the fungus if it is present. However, pathogen biology and crop management decisions interact strongly with weather to determine the risk of yield loss.
 
Management of White Mold
Reducing the risk of white mold and maximizing soybean yield in the presence of white mold can be achieved on a field-by-field basis by choosing management practices that:
  • Prevent or slow the introduction of the white mold pathogen into a field.
  • Steadily work towards reducing the population of the fungus in the soil.
  • Provide a feasible compromise between lower white mold potential and maximum yield potential.
 
If you are seeing white mold in your fields, work with your local Centra Sota Crop Advisor to put together a plan to deal with it for future seasons.
 


 


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