Sclerotinia Stem Rot (White Mold)

Berlin D. Nelson
Professor
Department of Plant Pathology

Sclerotinia stem rot or white mold of soybeans is a disease caused by the fungus Sclerotinia sclerotiorum. This disease is common in the northern soybean growing areas of the state. The disease can cause major seed yield reductions or even completely destroy a crop when soybeans are planted in infested soil and there is a dense plant canopy with prolonged periods of wet weather. Yield losses usually occur when incidence of disease is 15 % or greater. Yield losses can range from 1.3 to 3.7 bu/A for every 10 % increase in disease incidence. In most years throughout North Dakota, Sclerotinia stem rot is only a minor to moderate problem for growers, and the disease is rarely observed during drought years. Wet weather is a major factor in disease development. It has been a persistent problem in irrigated soybeans. Besides seed yield reductions, the disease also results in reduced seed quality and seed contaminated with the black sclerotia of the fungus. Seed contamination can be a serious problem for exported seed since it may result in rejection of the seed lot at foreign ports of entry. Furthermore, sclerotia returned to the soil can affect other crops in the rotation. Sclerotinia stem rot can be managed by growers through an understanding of the pathogen and disease cycle. Additional information can be found at the North Central Soybean Research Program White Mold Coalition Web Site.

Evaluation of cultivars for reaction to Sclerotinia: TEST RESULTS

Symptoms
Symptoms usually are not observed until the crop canopy between rows has closed, creating a humid microclimate. Wilting and withering of leaves followed by death of plants are usually the first symptoms observed. A close inspection under the plant canopy will reveal a cottony, white mycelial (fungus threads) growth on stems, leaves or pods . Lesions develop on main stems and side branches. Eventually, lesions girdle the stems and the plant parts above die. Stems appear bleached and sometimes shredded from advanced decay. Large, black sclerotia of varying shapes and sizes will form from the white mycelium growing on plant tissue. Sclerotia also form in the stem pith and have a characteristic cylindrical shape. Seeds in diseased pods are usually shriveled and may be infected by the fungus, or replaced by black sclerotia. The seed is usually contaminated with sclerotia when infected plants are harvested.

The Pathogen
Sclerotinia sclerotiorum, the fungal pathogen, is common in soils throughout eastern North Dakota, but is also found in western counties. The fungus has an extensive host range of over 370 plant species and causes diseases on a wide variety of crops such as sunflower, dry bean, canola (rapeseed), potato, alfalfa, buckwheat, lupine, mustard, Jerusalem artichoke, safflower, lentil, flax, field peas and many vegetables. In North Dakota it is rare to find this pathogen causing serious damage on some of these crops such as flax and potato. There are also many common broadleaf weed hosts such as marsh elder, lambsquarter, pigweed, Canada thistle and wild mustard. The fungus that causes white mold on soybean is the same one that causes white mold or Sclerotinia disease of sunflower, dry beans, canola, and other crops.

Sclerotinia sclerotiorum overwinters principally as sclerotia in soil. The sclerotia germinate to form small tan to brown mushrooms called apothecia (about one-eighth to one-fourth inch in diameter) . These produce spores termed ascospores which initiate the disease on soybean and other susceptible crops.

Disease Cycle
Moisture and flowering are critical factors in disease development. Disease usually does not occur before the closing of the crop canopy, because a dense canopy promotes cool temperatures and a humid microclimate around the stems and maintains high soil moisture following rain or irrigation. Initiation of disease is also closely associated with flowering. Following seven to 14 days of high soil moisture, sclerotia within the upper several inches of soil will germinate to form the mushroom-like apothecia. A single sclerotium can produce several apothecia. The apothecia will forcibly eject their ascospores into the air where they will be carried by air currents to soybean plants. The most important source of ascospores is from apothecia produced within the field, but ascospores can blow in from adjacent or nearby fields. One apothecium can produce enormous quantities of ascospores over several days. Ascospores survive for short periods on plant tissue but do not overwinter.

Ascospores require a film of water and a food base such as dead or senescing flower tissues to germinate and grow before they infect the plant. Flower tissue is the most important food base for initiating infections. Often infections will start in stem axils where senescing flower tissues have fallen and lodged. Infections may also occur through wounds caused by hail or other injury. A film of water on the plant surface promotes development of lesions and increases the amount of tissue damage. Initial disease development generally requires greater than 40 hours of continuous plant surface wetness, but once disease is started, shorter periods of wetness will allow lesion development. This is the reason disease is associated with lengthy periods of cloudy, humid, rainy weather. As plant surfaces dry, the progress of disease is slowed. Cool temperatures between 59 and 75 degrees Fahrenheit favor disease development. The greater the density of the plant canopy the more favorable the environmental conditions are for disease. Solid seeded soybeans and lodged crops are therefore more prone to disease development.

Sclerotia will be formed as the mycelium grows in and on the plant tissues. These sclerotia will not germinate to form more apothecia during the season, but rather will be returned to the soil during harvest and tillage operations and will over-winter to become inoculum (source of infective fungus) for a future susceptible crop. Sclerotia are highly resistant structures and survive for long periods in soil. The relatively dry soil conditions in North Dakota and the long cold winters favor survival of this pathogen.

Management
The most important controls for Sclerotinia stem rot of soybean are variety selection, the use of cultural practices that lessen disease severity, and avoid planting on highly infested land. Do not use seed from a Sclerotinia infected crop. Seed quality could be low and sclerotia may be introduced into the field along with the seed. Also, maintain good control of broadleaf weeds since they can be hosts of Sclerotinia. Refer to the North Central White Mold site for some additional information on management.

Variety Selection
Although most common soybean cultivars adapted for this region are susceptible to Sclerotinia stem rot, some cultivars are less susceptible than others. Research is ongoing to identify soybeans with some resistance to Sclerotinia. When planting on soils with a history of this disease, choose a less susceptible cultivar or a cultivar that has shown high yield under disease pressure.. Information on cultivar susceptibility is available from the NDSU Extension Service or from an NDSU Extension Circular titled North Dakota Soybean Performance Testing. This link will provide some information on variety testing in North Dakota. TEST RESULTS

Cultural Practices

Canopy management - Under low to moderate disease pressure, white mold increases as the row spacing is narrowed . Disease risk is reduced on 15 inch row spacing compared to 7 inch spacing. Under high disease pressure, row spacing may have no effect on disease severity. Narrow row production (solid seeded) soybeans almost always have higher disease levels than wider rows, but narrower rows favor higher yields. Maintaining open rows so air movement through the crop reduces plant wetness can reduce disease. Orienting the rows toward the prevailing wind, for example, will help dry the crop following precipitation. Under very prolonged rainy periods or in protected areas such as along shelter-belts where humidity is higher, disease many develop even in an open canopy. When growing soybeans under irrigation, avoid practices that favor a dense canopy or high moisture following flowering, because that will create ideal conditions for disease development.

Standard rotations used in soybean production in this area generally will not have a profound affect on incidence of disease. Crop rotation to non-host crops (such as corn and small grains) will reduce levels of sclerotia in soil, but the amount of reduction in a one to three year rotation often is not sufficient to affect disease levels. Research suggests that only a few sclerotia per square meter of soil is sufficient to cause substancial disease. Four to five year rotations, however, may substantially reduce sclerotia levels.

Tillage operations used in North Dakota generally are not useful in manageing this disease. In some other states, white mold has been found more frequently in conventional till fields compared to no-till fields.

Application of Cobra herbicide during early flowering has been demonstrated to suppress white mold in studies in other states, but we have no data on its effectiveness in North Dakota. Further information on Cobra can be found at the North Central White Mold site.

Monitor for Disease
Soybean fields should be monitored for disease incidence. Often it is necessary to walk the field and look under the canopy for evidence of disease. Also, check the seed hopper at harvest for the presence of sclerotia. As disease begins to increase in a field, the rotation time to non-susceptible crops such as small grains and corn should be increased. Crop rotation will reduce populations of sclerotia in soil but will not entirely eliminate the pathogen. Do not plant highly susceptible crops such as drybeans and sunflowers during the rotation. If you rent land, find out the disease and cropping history before making planting decisions.

Chemical and Biological Control

One fungicide, Topsin, is registered for control of white mold on soybean. There are two formulations of this thiophanate-methyl fungicide: Topsin M 70W and Topsin M WSB. See label instructions for correct application times and rates. The first application should be at early bloom (R1 to R2). We do not have research data on the effectiveness or economics of chemical control of this disease in North Dakota.

For seed treatments see the following links:

A biological control compound called Intercept WG was recently (2001) registered for control of Sclerotinia diseases. The product consists of a granular formulation of spores of a fungal parasite (Coniothyrium minitans) of the sclerotia of S. sclerotiorum. It is applied to the soil to reduce the population of sclerotia and thus reduce disease. We have no data as of 2002 on the effectiveness of this product for control of Sclerotinia on soybean in North Dakota.

EVALUATION OF CULTIVARS FOR REACTION TO SCLEROTINIA

Soybean cultivars adapted to North Dakota are tested for susceptibility/resistance to Sclerotinia sclerotiorum using both field and greenhouse testing methods. Because weather has a marked effect on development of disease in the field, in some years we do not have conditions for disease development and thus no field data for the year. Generally, only a rating for the amount of disease on a cultivar is provided, but in 2002, yield data was also obtained. Each of the following sections will explain the type of testing that was used for that year. It is important to note that all cultivars have a certain level of susceptibility, but some are less susceptible than others and some yield well even under moderate levels of disease. The conditions of each test are explained at the end of each of the sections.

YEAR 2002

Growing conditions during 2002 were excellent at the Fargo Sclerotinia test plot. Besides white mold development there was a substantial amount of root rot caused by Phytophthora and other root pathogens. Some of the yields were reduced by root rot. The following test results are presented as round up ready then conventional cultiavrs. There were no significant differences among cultivars for % Disease, but yields were significantly different.

ROUND UP READY CULTIVAR ...................% DISEASE....... YIELD BU/A

Thunder Seed 2404RR ............................................... 20.7................. 65.6
Quality Seed Genetics QSG-0818RR.......................... 34.2 ................ 65.5
Syngenta S06-L6........................................................ 25.5................. 59.5
Monsanto DKB06-51................................................. 15.3................. 54.5
Thunder Seeds 2303RR ..............................................29.7 .................50.4
Proseed RR0079 ........................................................30.6.................. 49.2
Nu Tech NT-0606RR ................................................18.3.................. 48.7
Mustang Seed M-033RR ............................................30.6 .................48.5
Mallard Seed RR0512...................................................9.8 ..................48.4
Dairyland Seed DSR-040/RR ......................................24.6................. 48.3
Nu Tech NT-0777RR .................................................28.4.................. 46.5
Syngenta Seeds S02-G2...............................................18.9................. 46.3
PSI 0201RR ...............................................................23.4 ..................46.0
Thompson Seed T-0555RR ........................................13.7.................. 45.9
Mustang Seed M-053RR ............................................12.3 ..................45.9
Interstate Seed GARST 070RR ....................................9.6 ..................45.5
Proseed RR0069 ........................................................10.9.................. 45.0
Peterson Farms Seed 0305EX ....................................29.8 ..................44.8
Pioneer Hi-Bred P.90B74 ...........................................24.5 ..................44.3
Sabre Initiatives 044RR............................................... 23.4.................. 44.1
Quality Seed Genetics QSG-0303RR ..........................35.7 ..................44.1
Interstate Seed AGRIPRO 0527RR............................ 28.6.................. 43.9
Proseed EXP5 ............................................................35.2.................. 43.4
Hyland Seeds HX274RR ............................................15.0.................. 43.2
NorthStar Genetics NS 0057RR .................................23.2.................. 43.0
Stine Seed S0090-4.................................................... 24.2 .................42.8
Dairyland Seed DSR-009/RR ......................................20.1................. 42.7
Mustang Seed M-023RR ............................................21.6.................. 42.3
PSI 0301RR .................................................................8.2.................. 42.2
BioGene 009............................................................... 24.0...................41.5
Hyland Seeds RR RUGGED .......................................28.5.................. 41.4
Dairlyland Seed DSR-007/RR ....................................27.6................... 41.4
Sabre Initiatives 021RR ..............................................25.6................... 40.6
AgSource 9023 ..........................................................39.8................... 39.6
AgSource 9053 ..........................................................17.4................... 39.6
Interstate Seed GARST 0601RR ................................25.1................... 39.5
Thompson Seed T-0444RR ........................................38.3................... 39.3
Prairie Brand Seed PB-0532RR ..................................29.1.................. 39.3
Peterson Farm Seed 0304EX ......................................25.2.................. 39.1
Seeds 2000 2021RR ...................................................10.8.................. 38.6
Stine Seed S0093-4 ....................................................21.2 ..................38.6
Monsanto AG0501 .....................................................35.6 ..................38.5
Mallard Seed X0513 ..................................................35.6 ...................38.4
Gold Country Seed 3205RR .......................................18.0 ...................38.1
Gold Country Seed 1300RR .......................................22.2................... 38.1
Mallard Seed RRX0211 .............................................25.5 ...................37.9
Peterson Farm Seed 0202RR .....................................33.0................... 37.7
Wensman Seed W 2025RR ........................................25.0 ...................37.2
Excel Brand 8007RR ..................................................17.0................... 36.6
Atlas Mycogen B076RR .............................................35.7 ...................36.5
Stine Seed S0096-4 ....................................................29.3.................. 35.7
Pioneer Hi-Bred 90B51 ..............................................34.2 ...................35.6
RoughRider Genetics RG200RR .................................17.9 ...................35.4
Quality Seed Genetics QSG-0333RR .........................30.9 ...................35.1
NorthStar Genetics NS 0427RR................................. 11.0 ...................35.1
BioGene BG0.5 ..........................................................24.0....................34.9
Wensman Seed W 2054RR........................................ 46.8 ...................34.7
UAP Dynagro 3009 ....................................................29.8 ...................34.7
Hyland Seeds HX262RR .............................................28.5 ..................34.4
AgSource 9009 ...........................................................46.2 ..................34.4
NorthStar Genetics NS 0027RR ..................................19.1 ..................34.4
NDSU BARNES(RR) .................................................53.8 ..................34.1
Wensman Seed W 2033RR .........................................35.3 ..................34.0
Excel Brand 8046RR ...................................................26.0 ..................33.3
Prairie Brand Seed PB-0352RR ..................................32.0 ...................31.5
Thunder Seed 2200RR................................................ 20.1................... 31.2
Kaltenberg Seed Farms KB051RR ..............................37.0................... 30.9
Prairie Brand Seed PB-0799RR.................................. 39.3.................... 30.0
BioGene BG0.2 ...........................................................24.0 ...................29.5
UAP Dynagro 3010 .....................................................33.0................... 29.1
Seeds 2000 0051RR ...................................................36.4 ....................28.3
Monsanto AG0801 ......................................................24.9 ....................27.1
Thompson Seed T-0636RR .........................................34.3 ....................25.6
Gold Country Seeds 3202RR ......................................34.4 .....................23.8
PSI 0081RR ...............................................................27.8 .....................23.2
Wolf River Valley 23005RR ........................................32.5 .....................22.3
Kaltenberg Seed Farms KB073RR.............................. 26.6 ....................16.2
Nu Tech Seed NT-0707RR .........................................26.1 ....................12.7

LSD 0.05.......................................................................NS.....................16.7

MEAN..........................................................................26.6.....................39.1

% Disease = % of plants with a girdling lesion on main stem.
50% of the plants in a row were inoculated on the petioles on 24 July and plot was misted continuously until Sept. 3. Plot planted May 22, and harvested Oct. 16. Yield given at 13% moisture.

CONVENTIONAL CULTIVAR........................ %DISEASE.......... YIELD BU/A

Dairyland Seeds DST 0810 ............................................9.3.................... 64.2
Thunder Seeds 2009 ....................................................27.4.................... 64.1
Pioneer Hi-Bred 90B43 ...............................................21.4.....................47.5
U of MN MN0302...................................................... 13.8.................... 46.6
Dairyland Seeds DSR-061........................................... 19.8.................... 46.4
NDSU Traill................................................................... 6.6.................... 46.0
Hyland Emerson........................................................... 17.9.................... 41.9
Gold Country 3103...................................................... 35.9.................... 37.4
Richland Organics MK0953 .........................................26.9.................... 35.5
Thunder Seeds 0598 .....................................................41.1.................... 34.1
NDSU Walsh............................................................... 24.2 ....................33.9
Gold Country Bygland ..................................................24.9 ....................33.0
Richland Organics EX 396HPN.................................... 27.3................... 26.2
NDSU Jim.................................................................... 18.4................... 26.1
NDSU Nannonatto....................................................... 22.3 ...................25.9
NK98-1185 ...................................................................1.1................... 25.5
Gold Country X3203 ....................................................25.7 ...................24.9
NDSU Barnes.............................................................. 10.9................... 23.2
NDSU Nornatto........................................................... 30.6................... 21.7
Circle C Seeds 0502NSP............................................. 45.3................... 21.7
Thunder Seeds 2030 ....................................................29.3.................... 21.1
Richland Organics MK0649N...................................... 21.5.................... 16.2
Peterson Farm Seed KORADA................................... 16.5 ....................15.6
Peterson Farm Seed Lena ............................................10.5.................... 12.8
U of MN MN0201 ......................................................24.3 ....................12.5
Circle C Seeds 0702NSP............................................ 18.7 ....................11.9
Peterson Farm Seed VISION...................................... 13.7.....................11.9

LSD (P=0.05)..............................................................NS.......................18.6

MEAN .......................................................................21.7......................30.7

% Disease = % of plants with a girdling lesion on main stem
50% of the plants in a row were inoculated on the petioles on 24 July and
plot was misted continuously until Sept. 3. Plot planted May 22, and harvested Oct. 16.