Managing White Mold of Soybeans

by Steve Butzen, Agronomy Information Specialist

 

Summary

·         White mold has spread rapidly in the last five years to become the second most damaging soybean disease in many northern and near-northern states.

·         Wet, cool conditions favor development. Many yield-enhancing production practices result in early, dense canopy formation, increasing disease incidence.

·         Current options to help control white mold include avoidance, variety selection, rotation, chemical  applications, and weed control. In the future, more resistant varieties are expected to increase control.

A decade ago, white mold – or Sclerotinia – was considered only a localized problem in soybean production, even in the northern states.  But that has changed rapidly in the ‘90’s.  Beginning with extensive spread and unprecedented damage in the cool, wet season of 1992, white mold has now become a serious problem plaguing soybean grower in nine of the ten top-producing states.

White mold is now a serious threat to most soybean fields in the northern states of Minnesota, Wisconsin, Michigan and New York.  In addition, it may cause significant losses to fields in the northern growing areas of Iowa, Illinois, Indiana, Ohio and Pennsylvania.  In most of these states, white mold is now second only to Phytophthora in damage caused to soybeans.

The spread of white mold is due to ideal conditions for disease development in many areas during recent years, but is also due to changes in soybean culture during that time. Earlier planting and increases in no-till production, drilled or narrow rows and plant population have all contributed to higher incidence of white mold.  Ironically, these practices, along with improved genetics, have also been responsible for most of the yield/profitability gains over the last decade.  For growers in affected areas, maintaining those gains now depends largely  on their success in managing white mold.  This Crop Insights will describe white mold disease of soybeans and current and future options for control.

Disease Description and Life Cycle

White mold persists in soybean fields over time by production of survival structures called sclerotia.  Sclerotia are dark, irregu-larly shaped bodies ¼ to ¾ inches long formed within the white, cottony growth on the outside of the stem and inside the stem during the fall.  These compact masses of hardened my-celia contain food reserves much like seeds, allowing them to survive for years in the soil and eventually germinate.

In the most common form of germination, a sclerotium produces one or more dark germ tubes or stipes that grow upward from a depth of two inches or less in the soil.  When it reaches the soil surface, the germ tube is triggered by light to produce a small, flesh-colored structure much like a mushroom, called an apothe-cium.  One sclerotium can produce numerous apothecia simultaneously or sequen-tially throughout the growing season.  Each apothecium produces millions of spores beneath the plant canopy, which are periodically re-leased and spread to the plants.

Spores are not able to invade plants directly, but rather, must colonize dead plant tissue before moving into the plant.  Senescing flowers provide a ready source of dead tissue for preliminary colonization.  From these senescing flowers in the branch axils or stuck to developing pods,  the fungus spreads to healthy tissue.  The first symptom of white mold infection appears as a water-soaked stem lesion originating from a node.  If the lesion remains wet, it becomes overgrown with white mold. The disease can then spread directly from plant to plant by contact with this moldy tissue.  Sclerotia are formed within the moldy growth and inside the stem to complete the disease cycle.

Plant damage is incurred as tissue rot and formation of sclerotia inside the stem result in rapid wilting and death of the upper part of the plant. As the disease progresses, premature death of the entire plant can occur.

Wet, Cool Conditions Favor Development

Wet, cool conditions are required throughout the white mold disease cycle, including germination of the sclerotia in the soil, spore release, infection of soybean flowers by spores and spread of white mold from plant to plant.

·         Sclerotia in the soil require 7 to 14 days of high soil moisture to germinate and produce apothecia (fruiting bodies).  Temperatures near 55-60 degrees F. are optimum for this process. 

·         Spores are forcibly ejected from the fruiting bodies during wet weather conditions.

·         After spores are released, a wet surface on senescing flowers or other dead or dying  tissue is required for spore germination.  Specifically, 2-3 days of continuous wetness, or more than 12 hours of daily wetness for 3-5 days is required.

·         White mycelial growth develops on stem lesions that remain wet, and spreads by contact to neighboring plants.  Temperatures from 68 to 78 degrees F. are ideal for disease spread.

Soybean Management Practices Affect White Mold Development

Early establishment of a dense soybean canopy increases the likelihood that the cool, high-humidity conditions required for white mold development will occur.  Many common soybean management practices lead to early, dense canopy formation, including early planting, drilled or narrow rows and higher plant populations.  In fact, researchers and agron- mists often suggest that achieving canopy closure before flowering begins is a key to maximizing yields. Consequently, early canopy closure  is a goal for many soybean producers, especially in northern locations and where available sunlight is limited.  Unfortunately, this practice also encourages white mold development.

 

 

Whether growers should abandon their yield-enhancing practices to help control white mold is debatable.  In areas with lower white mold levels or drier climate, production practices which increase yield but also increase white mold levels may still be highest yielding.  However, in areas with higher white mold levels and a cool, wet climate, some change in production practices may be necessary.  Recent research conducted at the University of Wisconsin addresses the effect of production practices on white mold levels and overall yield.  Results of these studies are reported below.

Row Width

Years of research studies throughout the Midwest have shown an advantage for growing soybeans in row widths less than 30 inches.  The advantage for drilled soybeans over 30-inch rows ranges from about 5% in western states to up to 20% in northern and eastern states.  Research studies conducted in Wisconsin measured yield and white mold incidence in drilled vs. 30-inch rows.

 

Table 1.  Effect of row width on the incidence of Sclerotinia stem rot and yield of soybean in Wisconsin.

		% Sclerotinia		Yield (Bu/acre)
Year	Location	30-in.	7-in.	30-in.	7-in.
1993	Galesville	34	37	37	38
	Sharon	18	35	52	56
1994	Galesville	69	74	31	43
	Sharon	56	58	52	55
Mean		44	51	43	48
Values are means of 18 varieties tested in 1993 and 1994.

These results, though limited, show that % Sclerotinia increased in narrow rows, but yields were still superior to wide rows.  This suggests that narrow-row planting systems should not be abandoned simply to help control white mold.  Another consideration in less affected areas is that narrow-row systems generally increase yields each year, and white mold may not develop every year.

 

Plant Population

Soybean yields generally increase with increased plant population within a range.  Studies have shown that optimum plant populations range for narrow row soybeans is about 200,000 to 250,000 plants per acre.  A study was conducted in Wisconsin to measure % Sclerotinia and yield in narrow-row soybeans planted at different populations.  Results are shown in table 2.

Table 2.  Effect of plant population in narrow rows on the incidence of Sclerotinia and yield of soybean.

In spite of higher incidence of Sclerotinia as stand increased, yields were not reduced.  However, the expected increase from higher seeding rate may have been partially offset by losses from Sclerotinia.  Nevertheless, reducing white mold by decreasing seeding rate may not result in higher yields or profits.

 

Planting Date

 

Later planted soybeans are generally shorter and less branched, and therefore later to canopy closure.   Some planting date studies show that later planting results in less incidence of white mold.  However, yields are generally reduced when planting is delayed past mid-May in northern states.  The tradeoff between less yield reduction due to white mold, but more yield reduction due to late planting may not be favorable, especially in years of low disease pressure.

 

Tillage

 

Sclerotia germinate from the top two inches of soil.  Below that depth, they can remain dormant for up to 10 years.  Because of its longevity in the soil, it is difficult to devise a strategy to control white mold with tillage. Deep tillage buries sclerotia from the soil surface but may also bring prior sclerotia into their zone of germination.  If the disease is new to a field and a severe outbreak has occurred,  a deep tillage followed by no-till or shallow tillage for many years may be beneficial.  However, conclusive research to determine the effect of tillage on white mold incidence and yield has not been reported.

 

Control of White Mold

White mold is a disease of high yield potential soybeans.  Oftentimes, the better the establishment and growth of the crop, the more likely that it will be damaged by white mold. However, a strategy to control the disease by

abandoning high-yield management practices is counter-productive.  Instead, growers should employ other practices to reduce white mold damage.

No one practice alone will be effective in controlling white mold , but several options are available to help reduce disease pressure. Current options include disease avoidance, variety selection, rotation with non-host crops, chemical application, and weed control.  More options are expected in the future, as Pioneer  plant breeders and other researchers continue to devote sizable resources to solve this disease problem.

 

Disease Avoidance

 

White mold spreads either by movement of spores or sclerotia from field to field.  Spores are airborne and may originate from any field that has had white mold in the past.  Spores are thought to move only short distances -- about 150 feet according to some studies – so infection by spores would progress from field edges.   There is little known about stopping the spread of spores.

Sclerotia move from field to field in harvest equipment or in contaminated seed.  Har-vest equipment should be thoroughly cleaned when moving from infected to non-infected fields.  Harvesting infected fields last provides additional safety.

Because sclerotia are roughly the size of soybean seed, they can’t be easily separated by the combine.  Soybeans harvested from infected fields are most likely loaded with sclerotia. Planting these soybeans would place them at the ideal depth for germination and infection of that crop and field.  Growers should absolutely not save seed from infected fields.

Pioneer avoids growing seed beans in fields with a history of white mold.  In addition, seed is thoroughly cleaned and inspected to insure that it is disease-free.  Seed cleaning with a gravity table or centrifugal tower is essential to remove sclerotia.

 

Variety Selection

 

At this time, there is no know genetic resistance to white mold, but there are clear differences in degree of tolerance.  Growers should select varieties that rank high in yield and low in white mold incidence across environments where white mold is a problem.  Several states, including Iowa, Ohio and Wisconsin conduct yield trials in areas with historic white mold infection.

PioneerÒ brand 9132, 9163 and 9305 soybean varieties show a degree of white mold tolerance and have performed well in university white mold yield trials, as shown in table 3 below.

 

Rotation

 

Rotation with a non-host crop is an effective means of reducing disease pressure in a field. Non-host crops include corn, sorghum, and small grains.  Susceptible crops to avoid in a rotation include alfalfa, clover, sunflower, canola, edible beans, potato and others. Because sclerotia survive for up to ten years in the soil, rotation is only a partial solution

 

Weed Control

 

White mold has over 400 plant hosts, including many broadleaf weeds.  Host weeds that are also common weed species  throughout soybean growing areas  include lambsquarter, ragweed, pigweed and velvetleaf.  In  addition to acting as host to the disease, weeds can also increase canopy density, which favors disease spread.

 

Chemical Applications

 

Two fungicides, Benylate and TopsinM are registered as foliar fungicides for soybeans and are effective in reducing infection by white mold.  However, proper timing of application and penetration of the fungicide through the soybean canopy are critical for success.  Pathologists recommend to apply when soybeans are producing flowers on the lower half of the plant.  Drop nozzles may be helpful to ensure spray coverage of those flowers.  Pathologists suggest that fungicide application may be more practical for seed beans than for commercial fields.

Researchers have begun to investigate the effects of post-emergence herbicide applications to control white mold.  Preliminary studies have shown reduced white mold incidence and increased soybean  yield by one application of Cobra at the V4 or R1 growth stage.  These investigations are continuing.

 

Future Solutions

 

Future solutions to white mold control will likely come from development of varieties with higher tolerance and resistance.  Better identification of resistant germplasm is now possible as Pioneer researchers apply new screening techniques in the field and greenhouse.  Varieties with higher resistance are expected as Pioneer’s white mold testing efforts in five affected states and Ontario reach fruition.  In addition, resistance in other crops such as canola or sunflower could potentially be transferred to soybean via biotechnology.  Pioneer is investigating this and other uses of biotechnology to address the white mold problem.

Acknowledgments:  Fig. 1 and 3 - Purdue U.  Fig 2 - U. of Wisconsin. Fig 4 and 5 - X.B. Yang, Iowa State U.