No. 14 Article 1/July 8, 2011

White Mold Q&A

White mold, also known as Sclerotinia stem rot and caused by the fungus Sclerotinia sclerotiorum, is a disease of soybean and other broadleaf crops. Historically, white mold on soybean has been observed sporadically in central Illinois, more frequently in northern Illinois, and rarely in southern Illinois. In 2009, white mold was prevalent in central and northern Illinois, causing yield losses in many fields. Cool (temperatures below 85°F) and wet weather, especially when soybean plants are blooming, favor the development of white mold. These conditions occurred in central and northern Illinois in 2009, which was the primary reason for the high levels of the disease.

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


Soybean stem affected by white mold, also called Sclerotinia stem rot.

I have received many questions about white mold in the past few weeks, probably because many of the fields seriously affected in 2009 have returned to soybean this year. These "frequently asked questions" and my answers follow.

Q. White mold was a severe problem in my soybean fields in 2009. Does that mean there is a greater risk of white mold this season when these fields have returned to soybean?

A. Yes, the risk has increased because more sclerotia went back into the field after the 2009 season. However, the risk of white mold is driven primarily by weather conditions. Wet, and especially cool (less than 85°F), weather is needed for spores of the white mold fungus to infect soybean plants and for disease to develop. So even though more sclerotia mean more spores present in a year favorable for the white mold fungus, disease will not develop unless favorable weather conditions occur.

Q. I have observed small mushrooms in the soil of my corn and soybean fields. Are these apothecia of the white mold fungus?

A. The only evidence of the white mold fungus that can be observed in the soil is the small mushrooms known as apothecia. I received some reports last week that apothecia had been spotted in some Illinois fields. The apothecia are small and cup-shaped--they look a bit like small golf tees.


Mushroom-like structures known as apothecia growing from a sclerotium. (Photo courtesy J. Venette, North Dakota State University.)

Q. Which foliar fungicides registered for use on soybean include white mold (Sclerotinia stem rot) on the label?

A. Domark, Endura, Proline, and Topsin M are registered for use on soybean and include white mold on the label. (The active ingredient in Topsin M is thiophanate-methyl, which is off--patent-other products containing thiophanate-methyl also may have white mold on the label.)

Q. Do any herbicides besides Cobra suppress white mold?

A. Cobra and Phoenix (both containing the active ingredient lactofen) are the only two herbicides that currently have "white mold suppression" on their label.

Q. Has the University of Illinois done any research evaluating products for control of white mold?

A. Yes, trials have been conducted at the U of I research farm near DeKalb. See Tables 1 and 2 for 2009 and 2010 results.

Table 1. Effect of registered products on white mold incidence and soybean yield at the University of Illinois research farm near DeKalb in 2009.

Treatment*

Incidence (%)
8-11-09

Incidence (%)
9-14-09

Yield (bu/A)

Nontreated

75

95

24

Topsin 4.5 FL at 20 fl oz

43

96

24

Proline at 3 fl oz

38

95

24

Domark at 5 fl oz

68

98

23

Cobra at 12.5 fl oz

15

51

42

Endura at 8 oz (2x)

38

86

39

LSD 0.05**

33

15

8

*All products were sprayed at R1 (July 20, 2009). Endura was sprayed again 9 days later.
**Least significant difference value.

 

Table 2. Effect of registered products on white mold incidence and soybean yield at the University of Illinois research farm near DeKalb in 2010.

Treatment*

Incidence (%)
8-26-10

Incidence (%)
9-21-10

Yield (bu/A)

Nontreated

18

95

62

Topsin 4.5 FL at 20 fl oz

9

83

61

Proline at 3 fl oz

11

93

64

Domark at 5 fl oz

7

76

63

Cobra at 6 fl oz

6

86

56

Endura at 8 oz

4

79

69

LSD 0.05**

11

NS

8

*All products were sprayed at R1 (July 10).
**Least significant difference value. "NS" indicates that treatments did not significantly differ from each other for that column.

--Carl A. Bradley

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