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Issue No. 12, Article 4/June 25, 2010

Questions and Answers About White Mold of Soybean

White mold (aka Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease of soybean and other broadleaf crops. Historically, white mold has been observed on soybean sporadically in central Illinois, more frequently in northern Illinois, and rarely in southern Illinois. In 2009 it was prevalent in central and northern Illinois, causing yield losses in many fields. Temperatures below 85°F and wet conditions, especially when soybean plants are blooming, are favorable for the development of white mold. These conditions were the primary reason for last year's high levels of the disease in central and northern Illinois.

White mold gets its name from the fuzzy white growth on affected soybean plants, which is the mycelia of the fungus that causes the disease. Symptoms of white mold include wilting leaves, stems that appear to be "bleached," and shredding of the stem tissue. Small black structures known as sclerotia can be found on and inside plants that have been affected by white mold.


Soybean stem affected by white mold (Sclerotinia stem rot).

I have gotten many questions about white mold in recent weeks and decided to outline and answer the most common ones here.

White mold was a severe problem in my soybean fields in 2009. Does that mean there is a greater risk of a repeat in 2010?

Not necessarily. It is true that more sclerotia (survival structures of the white mold fungus) would have been added back into your affected fields last year, but the risk of white mold will not change drastically, for a couple of reasons. First, it is likely that you rotated to corn in those fields for the current season; second, the risk of white mold is driven primarily by weather conditions, and we can probably assume that most fields in northern Illinois and several in central Illinois already have a certain level of sclerotia in the field. These "normal" levels of sclerotia were enough to cause disease in the 2009 season--even in fields where white mold had not been observed for several years. So even though more sclerotia in a field will mean more spores present in a year favorable for the fungus, disease will not occur unless the weather conditions are conducive.

So far this season has been wet, wet, wet. Will I have a white mold problem again in 2010?

Wet weather is only half of the conditions required for white mold to occur. Temperatures below 85°F also are needed for infection and disease development. So far this year we are experiencing a much warmer season than last; if the trend continues, I don't expect to see the prevalence of white mold that occurred in 2009.

I have observed some small mushrooms in the soil of my corn and soybean fields. Are these apothecia of the white mold fungus? Related question: I have observed a fungal white-colored mold growing around the residue on the soil of my corn and soybean fields. Is this the white mold fungus that causes disease on soybean?

The only evidence of the white mold fungus that can be observed in soil is the small mushrooms known as apothecia. Small and cup-shaped, they look a bit like golf tees. Sclerotia may also be found in soybean residue on the soil surface. The "white mold" that is observed growing around residue on the soil is most likely not Sclerotinia sclerotiorum (the causal fungus of white mold of soybean).


Apothecia (mushroomlike structures) growing from a sclerotium. (Image courtesy of J. Venette, North Dakota State University.)

Which foliar fungicides are registered for use on soybean that have white mold on the label?

Several foliar fungicides are registered for use on soybean, but only Domark, Endura, Proline, and Topsin M have white mold on the label. Note that the addition of white mold to the Proline label occurred earlier this year; it is through a Section 2(ee) recommendation for Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.

In your white mold research trial last year in northern Illinois, I noticed that Cobra herbicide reduced white mold. Are there other herbicides that can reduce it as well?

Cobra and Phoenix herbicides, which contain the active ingredient lactofen, are the only herbicides that currently have "white mold suppression" on the label. I have not tested any other herbicides for their effect on white mold. (The results of the 2009 white mold trial were published in the December 4, 2009, issue of the Bulletin, which is still available online.)

I have made the decision to apply a product to control white mold and want to apply it at R1 (first flower). My early-planted soybean plants are small, but they have already begun to flower. Should I go ahead and spray even though the plants are small and the field is several days away from row canopy closure?

If the rows have not begun to close the canopy, and if sunlight can still reach the soil between the rows, then it is unlikely that many apothecia have begun to emerge from the soil. Apothecia are most likely to emerge after rows have completed canopy closure, with very little sunlight reaching the soil between the rows. If you have decided to apply a product for white mold control, it might be best to wait until the row canopy draws nearer to closing. For more information on soybean plants flowering earlier than normal, see Vince Davis's article in issue 10 of the Bulletin (dated June 10).--Carl A. Bradley

Author:
Carl A. Bradley

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