Sclerotinia Stem Rot of Canola
Biology and Management

PP-1201 (Revised), April 2003

Arthur Lamey, Professor Emeritus, North Dakota State University
Carl A. Bradley, Extension Plant Pathologist, North Dakota State University

Sclerotinia stem rot has been the most serious disease of canola in North Dakota and Minnesota in recent years, with average incidence (percent infected plants) as high as 19 percent in North Dakota in 1993 and 19 percent in Minnesota in 1997. Estimated state-wide losses from Sclerotinia were as high as 13 percent in North Dakota (1993) and 13 percent in Minnesota (1997). In severely infected fields losses were estimated as high as 50 percent.

Although Sclerotinia has always been a threat to canola production, it has become more serious as canola production has increased and wet weather has favored disease development in recent years.

Symptoms

Sclerotinia stem rot develops late in the season, with the first visual symptoms appearing by the end of flowering. Dead and lodging plants occur singly or in patches in infected fields (Figure 1). Infections of individual plants usually develop around cast petals. The infections may produce a target pattern of light brown, mushy tissues. Infections may spread from infected leaf petioles or branches to larger stems. Infected areas eventually become bleached or white and the tissues become shredded (Figure 2). If the main stem is infected, plants may die early, reducing seed production, and plants may lodge. Hard black bodies which resemble rat droppings may be produced in infected stems. These are known as sclerotia (Figure 3). They are helpful in identifying Sclerotinia, but may not be present in every infected stem.

Blackleg is another common disease that also may cause lodging. Blackleg produces black lesions on the stem. The internal root tissues of blackleg-infected plants turn dark gray to black or have dark gray streaks in them. Blackleg infections near the soil surface may result in stem breakage at or near the soil surface (Figure 4). This contrasts to Sclerotinia, which produces shredded white stem tissues with stem bending or breakage at a height of 6 to 18 inches above the soil but no symptoms in the roots (Figure 5). For more information about blackleg, see Extension Circular PP-1024.

Figure 1. Sclerotinia stem rot: Note dead plants. (28KB color photo)

Figure 2. Sclerotinia stem rot: Note bleached stem. (15KB color photo)

Figure 3. Sclerotinia stem rot: Note shredded stem with sclerotia (arrows). (11KB color photo)

Figure 4. Blackleg: Note dark gray stem breaking at the soil surface and tiny black fruiting bodies. (13KB color photo)

Figure 5. Sclerotinia stem rot: Note bleached and shredded tissues resulting in lodging some distance above the soil line. (37KB color photo)

Biology

Disease Cycle. The Sclerotinia fungus, Sclerotinia sclerotiorum, produces sclerotia in the stems. Although there may not be many sclerotia produced per stem, the total production of sclerotia per acre may be quite high, up to 40 or 50 pounds (Schatz, unpublished data). Sclerotia fall to the soil at harvest and survive on or in the soil for several years.

If the soil is at or above field capacity for 10 to 14 days, the sclerotia may germinate to produce tiny mushroom-like bodies that resemble golf tees. These fruiting bodies, called apothecia, produce millions of airborne spores (Figure 6). The spores can be produced not only in canola fields, but also in fields of other crops, including small grains. The spores escape from the canopy and may be wind borne to nearby fields. The spores can survive long enough to be blown from field to field.

The spores do not infect healthy green plant tissue but need a dead or dying food source. As canola petals die and fall onto lower portions of the plant, any spores on the dead petals may germinate and begin to grow if the canopy stays wet for long periods of time. Once growth is established on the cast petals the fungus invades the surrounding tissues. Infections in canola may continue to spread as long as the canopy remains wet. Sclerotinia development may cease in hot or dry weather, but it can resume once cooler, wet weather returns. Sclerotia are produced in the infected stems and later drop to the soil (Figure 7).

Figure 6. Apothecia of Sclerotinia sclerotiorum emerging from a sclerotium. Apothecia are very tiny, as indicated by the standard-sized office paper clip. (Photo by J.R. Venette, NDSU) (14KB color photo)

Figure 7. Disease cycle of Sclerotinia stem rot. (9KB color illustration)

Environment. Wet weather preceding flowering and at flowering favors disease development. At least 1 to 2 inches of rain are required in the 10 to 14 days before flowering to saturate the surface soil and stimulate formation of apothecia. The foliage in the canopy must be wet for most of two days for the petals to be colonized and infection to occur. If wet weather continues, the infection will continue to spread. Dry weather will stop further spread. Temperatures in the 70s are more favorable than higher temperatures. Infection and development does not occur at temperatures of 86 degrees Fahrenheit or higher.

Survival. The pathogen survives as sclerotia in or on the soil. Some sclerotia may survive as long as four to six years. Each year some sclerotia die. They may die due to freezing and thawing or wetting and drying, especially if they are near the soil surface. Various soil microorganisms may colonize the sclerotia and kill them.

Hosts. Many broadleaved plants are hosts of Sclerotinia. Sunflower, dry bean, canola, crambe, and soybean are some of the best hosts and support the greatest buildup of sclerotia in the soil. Chickpea and lentil are also quite susceptible but support less buildup of sclerotia in the soil. Field pea is less susceptible and flax is much less susceptible. Many broad-leaved weeds are also susceptible, including lambsquarters, Canada thistle, ragweed and marsh elder. Members of the grass family including small grains, corn and grassy weeds are immune.

Management

Crop Rotation. Crop rotation is important, but the sclerotia survive for long periods in the soil, and the spores may blow into canola fields from nearby fields. Large concentrations of susceptible crops and several years with wet weather will contribute to a buildup of Sclerotinia in an area. Try to avoid more than one highly susceptible crop in a rotation, including canola, crambe, sunflower, dry bean or soybean. Semi-leafless pea supports less buildup of Sclerotinia than does the vining types and may be acceptable in a rotation with canola. Flax and buckwheat are less susceptible. In irrigated trials at Carrington, N.D. (Schatz, unpublished) no sclerotia were produced in flax; therefore flax appears to be a relatively safe crop in rotation with canola.

Tillage. Some studies indicate that sclerotia near the soil surface break down faster than those buried deeper; other studies indicate that infection is greatest when sclerotia are left on the soil surface. In any case, sclerotia that are within an inch of the soil surface are capable of producing apothecia, which liberate spores that may start new infections. Deep tillage may be used to bury sclerotia after a susceptible crop has been heavily infected; this practice will help to reduce Sclerotinia spore showers in subsequent years if the sclerotia remain deeply buried, but not if deep tillage is used the following year. Burial of sclerotia may or may not result in longer survival of sclerotia. If deep tillage is used to bury sclerotia, shallow tillage should be used for the next two or three years to assure that sclerotia are not returned to the soil surface where they can produce apothecia and liberate spores. Deep tillage is of limited value if used on only a few fields in an area that has many infested fields; it may be more effective if practiced area-wide.

Biological Control. Intercept is a biological control product registered for control of Sclerotinia. It contains Coniothyrium minitans, a fungus which attacks the sclerotia in the soil and kills them. Several months are required for the fungus to kill the sclerotia. Since this product is new, very little field data is available from North Dakota and Minnesota on how well it controls Sclerotinia.

Varieties. Although all varieties are susceptible some are less susceptible than others, and will perform better under moderate to severe disease pressure. Research has been initiated to test for varieties that are less susceptible. Some varieties are available that are apetalous, that is they have no petals. This removes a major food source for Sclerotinia, and apetalous varieties escape a severe infection incidence. Apetalous varieties may be useful if they yield as well as conventional varieties.

Fungicides. Quadris was registered for suppression of Sclerotinia on canola in March of 1999, Ronilan was registered in June of 2000 and Topsin M was registered in June of 2002. Other fungicides may be registered within the next several years. Information on fungicides currently registered is available in the most current edition of the North Dakota Field Crop Fungicide Guide (PP-622), from county extension offices, NDSU and University of Minnesota Research Extension Centers, the Northern Canola Growers Association and the Minnesota Canola Council.

Timing. Effective suppression of Sclerotinia requires timely application of a fungicide. Quadris should be applied at 10-25 percent bloom, or three to seven days after initiation of bloom. There will be 10 to 18 flowers on the main stem of Argentine canola when it is at 10-25 percent bloom. Quadris should be applied before or as the first petals begin to fall. Late application of Quadris is less effective than timely application.

Ronilan and Topsin M should be applied at 20-50 percent bloom, or four to eight days after initiation of bloom. There will be 14 to 16 flowers on the main stem at 20 percent bloom and 20 or more (includes any flowers that have dropped off) at 30 percent bloom . The 50 percent bloom stage is the time of maximum color development in the crop. At that stage there will be pods on the lower one third of the main stem. Once the crop is beyond 50 percent bloom, Ronilan and Topsin M are less effective for Sclerotinia control. Fungicide trials in 2001 and 2002 indicated that both Ronilan and Topsin M were more effective when applied at 35-50 percent bloom than when applied earlier.

Rates. Quadris should be applied at 9.6 to 13.8 fl oz/A. The 9.6 fl oz rate has been supported by the registrant, Syngenta, and may provide adequate control if applied before any petals begin to fall. Results with Quadris have been variable, however.

Ronilan should be applied at 10.6 to 16 oz/A. Extensive data from North Dakota, Minnesota and Canada indicates that the 12 oz rate provides excellent and consistent Sclerotinia suppression under even severe disease pressure. The 10.6 oz rate is slightly less effective, but may be adequate for moderate disease pressure.

Topsin M should be applied once at 1-2 lb/A at 20-50 percent bloom or twice at 1 lb/A for each application, with the first application at 20-30 percent bloom and the second at 40-50 percent bloom. One application of the 1 lb rate has performed very well at 35-50 percent flowering in most trials in Minnesota and North Dakota.

Spray decisions. Fungicides for suppression of Sclerotinia are expensive and the decision to spray should be made only when: 1) yield potential is above normal (at least 40 bushels or 2,000 lb/A) if canola prices are minimal, 2) weather leading to early bloom has been wet (at least 1-2 inches of rain in the two weeks prior to early bloom), 3) more rain or high humidity is expected, and 4) Sclerotinia has been a problem in recent years in fields currently planted to canola or in fields nearby. A fungicide is more likely to be needed if canola is on tight rotations (three years or less) or if other susceptible crops were in the rotation.

A Sclerotinia risk map, similar to that used in Canada, was initiated in 2001 for North Dakota and northwest Minnesota. The risk map is posted on the Northern Canola Growers Association web site:

http://www.northerncanola.com/

and the NDSU web site:

http://www.ag.ndsu.nodak.edu/aginfo/sclerotinia/sclerotinia.htm

The risk map site contains three maps: a map showing the growth stage of canola, a map showing soil moisture and where the soil is at field capacity, and a map showing the risk from Sclerotinia.

A risk assessment checklist follows. This checklist was duplicated, with permission, from the Web site of the Canola Council of Canada.

Acknowledgements

We thank the Northern Canola Growers Association for support of publication costs and Deb Tanner for publication layout. �

Sclerotinia Stem Rot Checklist

Reproduced from the Canola Council of Canada Website, by permission

When to complete the checklist:
Fill out the checklist and assess the crop shortly after first flower. First flower occurs when 75 percent of the canola plants have three open flowers on the main stem. Usually this occurs during the last week of June or the first week of July.

How to complete the checklist:
Read each question and circle the point value assigned to the answer you choose. Count up the points for each question and enter the total for each section. Answer all the questions in this section.

If you scored 60 or more in this section you probably had sclerotinia stem rot in your canola crops.�
Proceed to section two with a 60 or more score.

Section Two
8. When you walk through the crop during the morning at the beginning of flowering are your boots and pant legs wet when you come out?	Yes - 20� No - 10
9. Have you had wet weather in the immediate area within 2 to 3 weeks prior to flowering that allowed the soil to remain moist for extended periods?	Yes - 20� No - 10
10. Were apothecia found in the field, around the field, or in any neighboring cereal or canola fields where canola was grown in the previous 1 to 3 years?	Yes - 20� No - 10
11. Do you feel it will be dry throughout the flowering stage of the crop?�	Highly likely - 0 Moderately likely - 10 Not likely - 20
Total points for section two = __________

If you had a high score in section one and more than 50 for section two, you should consider�
applying a fungicide to protect your crop again sclerotinia stem rot.

Section Three
12. What is the condition of your stand of canola in terms of height, vigor and uniformity?�	Excellent - 20 Good - 10 Fair - 5 Poor - 0
13. When you walk through your crop, how dense is the canopy?�	Light - 0 Moderate - 10 Very Dense - 20
14. What is the yield potential of the stand?�	10 - 20 bu/ac - 0 20 - 30 bu/ac - 10 Greater than 30 bu/ac - 20
15. In previous years, when your yield potential was 30+ bu/ac, what were the actual yields?�	Greater than 30 bu/ac - 0 20 - 30 bu/ac - 20
Total points for section three = __________

If you have scored 50 or higher in section three, along with high scores from the first and second sections�
(60 and 50 plus respectively), it may be worthwhile to protect your crop against sclerotinia stem rot.�
If you scored less than 50 in the last section it is not likely worth applying a foliar fungicide.

Permission by the Canola Council of Canada to reproduce this checklist is gratefully acknowledged.

In Memory of Dr. Richard A. Meronuck. This publication was first published in May 1999 as Extension Report 54 by Arthur Lamey and Richard A. Meronuck, University of Minnesota. This revision is dedicated to Dr. Meronuck's memory.

PP-1201 (Revised), April 2003