A new fact sheet concerning Sclerotinia stem rot (white mold) on soybean has recently been published. The fact sheet describes symptoms of the disease and how to best manage it. To obtain a PDF version of the fact sheet, visit the ‘fact sheet’ section of the UW-Madison Field Crops Pathology website or CLICK HERE TO DOWNLOAD.
Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin
As the fall is approaching and crop harvest plans are being made, it is important to continue to assess disease issues in corn and soybean. These assessments aren’t being made in order to make plans for in-field management, but to potentially improve the quality of grain that is harvested.
Some Diseases to Consider in Corn at Harvest
Figure 1. Gibberella stalk rot on corn. Severe stalk rot on the left and less severe stalk rot on the right.
Now is the best time to begin scouting corn for stalk rot issues and also fungal ear rot potential. Diseases such as Anthracnose stalk rot and Gibberella stalk rot are becoming apparent in corn. Inspect the stalks integrity on the outside. Be sure to squeeze the outside of the stalk to gauge the potential severity of the rot on the inside of the stalk. Cut a few stalks from diverse areas of the field to see how rotted stalks might be. In figure 1, the stalk on the left has a severe case of Gibberella stalk rot, while the stalk on the right is far less rotted. The more severely rotted stalks are, the more likely they will lodge. Therefore timely harvest is important. Growers should target harvest on fields with severe stalk rot before fields that have less stalk rot, in order to minimize harvest losses due to lodging.
Figure 2. Diplodia ear rot.
Ear rots can also be an issue at harvest time. Fusarium ear rot, Gibberella ear rot, and Diplodia ear rot (Fig. 2) are just a few that can damage corn in Wisconsin. Ear rots are becoming evident in some corn I have scouted in the last week or so. It will be critical to check fields in the next several weeks in order to make decisions on what fields to harvest first. Harvest priority should be placed on fields with a high level of ear rot. As corn stands late into the fall, certain ear rot fungi can continue to grow, damage ears, and cause increases in mycotoxins in grain. The quicker these fields dry and can be harvested, the more likely the losses due to ear rot and mycotoxin accumulation can be minimized.
Soybean Disease Considerations at Harvest
Figure 3. Sclerotia of the white mold fungus inside a soybean stem.
In Wisconsin, the main disease to consider when making harvest plans in soybean is white mold. White mold is present in some soybean fields in the state and has caused considerable damage in a few of those fields. Remember that the white mold fungus not only causes stem blight and damage, but also causes the formation of sclerotia (fungal survival structures that look like rat droppings) on and in soybean stems (Fig. 3). These scelrotia serve as the primary source of fungal inoculum for the next soybean crop. They also get caught in combines during harvest. These sclerotia can then be spread in combines to other fields that might not be infested with the white mold fungus. Therefore, it is important to harvest non-infested soybean fields first, followed by white mold-infested fields, to be sure the combine does not deposit any residual sclerotia in the non-infested fields. If this is not an option and you must harvest white mold infested fields before non-infested fields, be sure to clean the combine between fields.
For more information about white mold management in soybean you can click here and scroll down to “white mold” or watch a video by clicking here.
Browning of the internal stem (left) is diagnostic for BSR. The middle stem may be developing symptoms. Compare to healthy, white pith in the stem on the right.
Dr. Damon Smith, University of Wisconsin, talks about brown stem rot (BSR) of soybean. BSR can be a significant problem in years where the spring is wet and cool resulting in infection by the fungus Phialophora gregata soon after emergence. However, BSR is often not noticed until the reproductive growth stages when foliar symptoms typically develop. The discussion here includes tips on spotting BSR, determining the difference between BSR and sudden death syndrome and how to manage the disease.
To watch the video you can click here.
For more information about BSR visit the Soybean Disease webpage at /soybean_pests_diseases/ and scroll down to the “Brown Stem Rot” section.
Also visit the Wisconsin Crop Manager newsletter at http://ipcm.wisc.edu
Symptoms of soybean vein necrosis disease.
Soybean Vein Necrosis Virus (SVNV) causes a virus disease in soybean that has been observed across the North Central U.S. in the last few years. The following video describes how to accurately diagnosis SVNV and distinguish this disease from other common soybean diseases.
Symptoms caused by this virus include light green patches or mottled green and brown speckled areas associated with veins. As symptoms progress, affected leaf tissue may die, and leaves will appear scorched.
SVNV is a virus that is vectored by tiny, winged insects called thrips. Thrips do not commonly cause economic damage to soybeans in the Midwest. Although farmers may be tempted to apply an insecticide to reduce thrips populations “just in case,” at this point in time we do not recommend insecticide applications in response to detection of SVNV since we don’t know what, if any, effect disease may have on yield. For now, we will continue to keep an eye on this disease, and assess its potential impact so that we can make more informed future management recommendations.
Check out some videos on SVNV at the links below:
New Multi-State Video – http://www.youtube.com/watch?v=uMtyO8DIiEI
UW Extension Video – https://www.youtube.com/watch?v=_2diQwO0Was
Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin
Figure 1. Northern stem canker lesion on soybean main stem
While traveling to soybean fields around Wisconsin this past week, we have noticed quite a bit of stem canker showing up. The UW Disease Diagnostic Clinic has also received several samples of soybean in which the disease has been confirmed.
Stem Canker Pathogens
There are actually two different types of stem canker caused by related, but different fungi. The fungus Diaporthe phaseolorum var. caulivora causes northern stem canker, while southern stem canker is caused by Diaporthe phaseolorum var. meridionalis. These two pathogens are part of the larger Diaporthe-Phomopsis complex, which consists of Phomopsis seed decay, pod and stem blight, and stem canker. In Wisconsin, northern stem canker is the most common stem canker disease, however, southern stem canker has been found.
What Conditions Favor Northern Stem Canker?
Cool, wet conditions in the spring and early summer favor infection by the northern stem canker fungus. The symptoms of the disease become apparent later in the season. Considering the cool and rainy weather that has been prevalent over much of the state this season, it isn’t surprising that northern stem canker is prevalent.
What Does Northern Stem Canker Look Like?
Figure 2. Patches of soybean plants killed by stem canker.
Initially symptoms of northern stem canker appear as small reddish-brown lesions near nodes. As lesions expand, they can become more brown or gray, but the red border will remain. Eventually lesions of northern stem canker will get large enough to girdle the stems (Fig. 1) and may be confused with Phytophthora root and stem rot. The best way to tell these two diseases apart is to look for the location of the lesion. Generally with northern stem canker, lesions begin at nodes away from the soil line on the main stem and move upward. Phytophthora stem lesions will progress upward from the soil line. Northern stem canker can also occur in patches and damage plants in wide swaths (Fig 2). Northern stem canker can also be confused with white mold when diagnosing above the canopy. Because the lesions can girdle stems, leaf flagging and death can resemble that of white mold damage. Therefore, careful scouting and inspection of the lower canopy and stems in necessary to tell the difference between white mold and northern stem canker.
Management of Northern Stem Canker
Spores of the stem canker pathogen originate mostly from soybean debris from the previous crop. Therefore, severity of northern stem canker can be higher in fields with minimal tillage. Burying debris can help reduce the severity of the disease. Stem canker can also be more prevalent in fields with high fertility and high organic matter. Stem canker-resistant varieties are also available. Choose varieties with the highest resistance rating possible within the appropriate maturity group for your area. Soybeans rotated with alfalfa may also have a higher incidence of the disease, because alfalfa is an alternate host of Diaporthe. Fungicide application is not recommended for this disease.
Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin
Figure 1. IPM Pipe Southern Corn Rust Advisory for August 21, 2014.
I have spent the last several days rating and scouting corn and soybeans in the southern tier of Wisconsin. There are a few active diseases out there to keep track of.
Field Corn
In field corn we have found a few fields with low levels of northern corn leaf blight (NCLB). Levels of NCLB seem to be a bit higher in southwestern Wisconsin. Severity on lower leaves in field corn was in the 10 – 15% range, with no damage apparent on ear leaves. Around the Arlington, WI area, NCLB is very limited with only a few lesions evident every 100 ft. or so.
Eyespot is becoming more evident in field corn. In fields with corn debris from a previous crop, the severity levels are in the 25-30% range on lower leaves and 10-15% on ear leaves.
Low levels of common rust (less than 5%) can also be found on some field corn hybrids in Southern Wisconsin.
Southern rust has been reported as far north as east-central Nebraska. The southern rust epidemic is being monitored closely in the Midwest. No southern rust has been found or reported in Wisconsin (Fig. 1).
For more information about corn diseases in Wisconsin, see my previous article by clicking here.
Sweet Corn
Several fields with severe epidemics of NCLB on sweet corn have been reported. These were late-planted fields. Sweet corn is generally more susceptible to NCLB than field corn. Common rust and eyespot can also be found at varying levels on sweet corn in the central and southern portion of Wisconsin.
In research plots at the Arlington Agricultural Research Station, sweet corn planted on June 25th is beginning to tassel. Levels of NCLB are currently low in this field, but common rust is increasing rapidly. Some leaves have 20-25% severity. Any late-planted and/or susceptible varieties of sweet corn should be monitored closely for foliar disease and any decision to spray fungicide should be made by the tasseling/R1 growth stage.
Soybean
The most widespread disease on soybean that we have observed is Septoria brown spot. Overall levels of Septoria brown spot are low, and can mostly only be found on lower leaves, which is typical for this disease. In many cases a fungicide specifically for this disease is not warranted in Wisconsin, unless there are factors that might lead to increased levels of severity, including continuous soybean rotation, very susceptible varieties, or extremely conducive weather. Most soybean fields are past the R3 growth stage , when a fungicide application might be beneficial for control of foliar diseases. However, this disease should be monitored in fields that were planted late.
Downy mildew has also been observed on soybean in various areas from central to southern Wisconsin. Fungicide application for control of this disease has not proven beneficial in university research trials. Therefore, fungicide application is not recommended for this disease under most circumstances. In soybean fields that are irrigated, the frequency between irrigation events should be lengthened in order to reduce the levels of downy mildew. Warmer, dry weather will also further reduce the level of downy mildew.
Figure 2. Damage from white mold in a soybean field under irrigation.
Active white mold has been found in fields in the central and southern portions of Wisconsin. Severity levels vary greatly depending on the fields and level of previous infestation by the white mold fungus. We have observed levels ranging from a few plants in spotty areas of a field to widespread damage with plant mortality across the entire field. The latter case was in a field with a history of white mold and frequent overhead irrigation (Figure 2). Application of fungicide for control of white mold is not recommended after the R3 growth stage. However, fields should be scouted and damage noted to facilitate future planting and management decision in that field. Fields with white mold should be harvested after fields that do not have white mold. The black survival structures (sclerotia; resemble rat droppings) of the white mold fungus can be easily spread on combines from one field to the next. If harvesting white mold infested fields last is not feasible, care should be taken to thoroughly clean combine mechanisms where soybean trash and debris can be trapped, between fields. For more information about white mold and management of the disease, click here. To watch a short video about white mold you can click here.
Other diseases such as brown stem rot, sudden death syndrome, and stem canker have been found at extremely low levels in soybean fields in Wisconsin this season. This situation should be monitored closely as soybeans approach the R6 and R7 growth stages. These two diseases may become more apparent at that time. Again, good record keeping of where these diseases are found can facilitate future management decisions for those fields.
Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
Figure 1. Alfalfa mosaic virus symptoms on soybean leaves in the field.
Calls, photos, and plant samples have been coming in over the last week (8/1/2014) pertaining to soybeans exhibiting abnormal growth and varying degrees of leaf mosaic (interwoven green and yellow areas). These symptoms are indicative of Alfalfa mosaic virus (AMV) on soybean.
Alfalfa mosaic virus
Alfalfa mosaic virus is transmitted in low levels in soybean seed (Tolin, 1999). Aphids transmit AMV. Symptoms of AMV can vary from localized dead lesions on leaflets (Fig. 1), to large areas of yellowing (Fig. 2). Plants can also be stunted and produce few pods. In Wisconsin, research has shown that yield reductions can can occur as a result of AMV. However, in those studies, only AMV incidence levels of 30% or greater resulted in yield loss (Mueller and Grau, 2007).
Figure 2. Severe symptoms of Alfalfa mosaic virus on soybean leaflets.
When trying to scout or diagnose a field with AMV consider the incidence (number of plants exhibiting symptoms) level of the symptomatic plants. Giesler and Ziems (2006) conducted a survey of AMV, BPMV, and SMV in Nebraska in 2001 and 2002. In that survey it was possible to find an occasional field with incidence of these viruses as high as 90-100%. However, the majority of fields that tested positive for one or more of these viruses, had incidence levels of 50% or less. High incidence levels (>50%) are considered unusual for AMV in commercial soybean fields. Therefore, incidence of leaf cupping or other abnormal leaf growth at incidence levels of 90% or 100% are more likely suggestive of an abiotic disorder, such as herbicide injury.
Co-infection of soybean by AMV and Soybean mosaic virus (SMV) can also occur (Malapi-Nelson et la., 2009). Co-infection can make symptoms of AMV much more severe. In cases where severely infected plants are identified with AMV, tests of SMV may also result in confirmation of that aphid-transmitted virus too.
How is the AMV Spread?
AMV is transmitted by mechanical wounding and also by aphids (several species, not just soybean aphid). AMV can also be transmitted at low levels in soybean seed.
Management
No in-season management is recommended. Spraying insecticides to control aphids in order to reduce virus transmission has shown to be unsuccessful. One reason for this is that many species of aphids can move into a soybean field and transmit the virus. The best solution for managing AMV (and also SMV) is to choose a soybean variety with the best resistance to AMV and SMV you can find in your area. Remember, spraying aphids below threshold with an insecticide will only control the vector and won’t solve your virus problem.
To learn more about AMV and SMV click here.
References
Giesler, L. J., and Ziems, A. D. 2006. Incidence of Alfalfa mosaic virus, Bean pod mottle virus, and Soybean mosaic virus in Nebraska soybean fields. Online. Plant Health Progress doi:10.1094/PHP-2006-0424-01-HM.
Malapi-Nelson, M., Wen, R.-H, Ownley, B.H., and Hajimorad, M.R. 2009. Co-infection of soybean with soybean mosaic virus and alfalfa mosaic virus results in disease synergism and alteration in accumulation level of both viruses. Plant Dis. 93:1259-1264.
Mueller, E.E. and Grau, C.R. 2007. Seasonal progression, symptom development, and yield effect of Alfalfa mosaic virus epidemics on soybean in Wisconsin. Plant Dis. 91:266-272.
Tolin, S.A. 1999. Alfalfa Mosaic. In: Compendium of Soybean Diseases, 4th Edition. G.L. Hartman, J.B. Sinclair, and J.C. Rupe, eds. APS Press.
Phytophthora Root Rot of Soybean
Anette Phibbs, Plant Pathologist with the Wisconsin Department of Agriculture and Consumer Protection, reports that the 2014 survey of early vegetative soybeans shows high levels of Phytophthora root rot disease caused by Phytophthora sojae. Nearly half of soybean fields sampled from June 6 to July 16 in 35 surveyed counties were infected with this fungus-like pathogen. Lab testing of root samples showed 26 out of 53 (49%) fields tested positive for P. sojae. Fields that tested positive were found in the following 15 counties: Barron, Clark, Dane, Green, Jefferson, Kenosha, Lafayette, Manitowoc, Marathon, Ozaukee, Rock, Sheboygan, St. Croix, Walworth, Winnebago. Counties were the problem was not encountered should not expect to be free from the disease. This is the highest prevalence of soybean root rot since the start of this survey in 2008. During the flood prone spring of 2010 the pest survey team found 38% of fields infected. This high prevalence of Phytophthora root rot throughout the surveyed area is no doubt due to heavy rainfalls causing saturated soils and relatively low temperatures this spring which have been very conducive to this water mold. A relatively new Phytophthora species, P. sansomeana, was detected in soybean roots in Calumet, Dunn and Eau Claire Counties. This pathogen was first detected in Wisconsin soybeans in 2012 in Jefferson, Marathon and Sheboygan counties; again in 2013 in Dane, Green, Outagamie and Sheboygan counties. Research into P. sansomeana’s potential effect on soybean and corn are ongoing.
For more about Phytopthora root rot of soybean, visit an informational webpage by clicking here and scrolling down to “Phytophthora Stem and Root Rot” or download a UWEX fact sheet by clicking here. Specific questions can be directed to Damon L. Smith, Field Crops Extension Pathologist, University of Wisconsin-Madison at dlsmith26@wisc.edu.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin
Over the last several weeks, soybean samples have been coming into the Plant Disease Diagnostic Lab with symptoms of Cercospora leaf blight (CLB). I have also received numerous questions about the disease and have observed CLB in several soybean fields around the state.
Figure 1: Bronzing of soybean leaves caused by Cercospora leaf blight
What does Cercospora leaf blight look like? Symptoms of CLB often appear during the reproductive phase of soybeans, but can appear earlier if conditions are conducive. Typically the upper leaves of the plant that are exposed to the highest levels of sunlight will show symptoms. These symptoms include a bronze (Fig. 1) to purple (Fig. 2) blotchy appearance on the leaves. Subsequent angular lesions can manifest on the upper and lower leaf surfaces. Lesions can grow together causing leaf death and defoliation under severe cases. The fungus that causes this damage is Cercospora kikuchii, which is the same fungus that causes purple seed stain of soybean (Fig. 3). Heavily infected seed can cause seedling blight and reduce stands. Less severely infected seed can survive but may be stunted or show reduced vigor. Typically in Wisconsin, damage by CLB is often not severe. Leaf discoloration is the typical symptom and defoliation is usually limited. However, frequent scouting during the early reproductive phase of soybean can help with monitoring the severity of CLB during seed set.
Figure 2: Purpling of a soybean leaf caused by Cercospora leaf blight
How does Cercospora leaf blight spread? Primary inoculum can come from infected seeds or from old soybean debris on the soil surface. Infections are favored by humid conditions that result in heavy dew events. Warmer air temperatures (at or above 82 F) favor conidial formation and dispersal, which can result in secondary infection. Incidence of the seed stain phase of disease has been correlated with higher levels of spore dispersal during the season.
How should Cercospora leaf blight be managed? Soybean cultivars vary in their resistance to CLB. In fields were CLB has been a problem, cultivars with resistance should be chosen. Residue management is also important. Fields with short rotations and/or reduced tillage can have higher levels of CLB. Good quality seed should also be planted. Seed lots with high levels of purple seed stain should be avoided. Some recent data suggests that the severity of CLB might be reduced by the use of a foliar fungicide in fields with high
Figure 3: Purple seed stain caused by Cercospora kikuchii
incidence of disease. For information on efficacy of products for CLB click here. Note that many products are labeled as just “fair” and no product was rated “good” for CLB. Therefore, a CLB management plan SHOULD NOT focus solely on foliar fungicide use, but should use an integrated management approach. In addition, severity from CLB in Wisconsin will often not be high enough to justify fungicide treatment. Scout carefully!
Reference
Compendium of Soybean Diseases, 4th Edition. G.L. Hartman, S.B. Sinclair, and J.C. Rupe, eds. APS Press.
Wilting and plant death as a result of Sclerotinia stem rot. Photo Credit: Craig Grau.
Damon Smith, Extension Field Crops Pathologist, University of Wisconsin
Kiersten Wise, Extension Specialist for Field Crop Diseases, Purdue University
Martin Chilvers, Extension Field Crops Pathologist, Michigan State University
Carl Bradley, Extension Plant Pathologist, University of Illinois
Daren Mueller, Extension Plant Pathologist, Iowa State University
Farmers in the Great Lakes area of the U.S. may be concerned about white mold (also called Sclerotinia stem rot) in soybean this year. The disease, caused by the fungus Sclerotinia sclerotiorum, is not common every year in in the Great Lakes region, but farmers that have battled the disease in the past will want to assess the risk of white mold development as soybeans approach flowering (growth stage R1 – plants have at least one open flower at any node).
White mold development is favored by cool, cloudy, wet, humid weather at flowering. The disease is more problematic in soybeans in high-yield environments where high plant populations, narrow row spacing, and an early-closing canopy are commonly used. No single management strategy is 100% effective at eliminating white mold, and in-season options for at-risk fields are limited. For more information on white mold, the disease cycle, and additional management options click here and scroll down to “White Mold.”
There are fungicides available for in-season management of white mold, however not all commonly used fungicides are labeled for use against white mold in soybean. For information on which fungicides are labeled for disease control and recommendations on fungicide efficacy, please click here. Fungicide recommendations are developed by the NCERA-137 national soybean disease committee, and recommendations are based on replicated research data collected from University trials.
In Wisconsin in 2013 numerous products were evaluated for white mold control in soybean. Results of this trial can be viewed by clicking here and scrolling down to pages 6 and 7. Consistent with results of the NCERA-137 research, our Wisconsin research identified several products having a rating of ‘good’ for white mold management, including Aproach, Endura, and Proline. If using fungicides for white mold management, keep in mind that efficacy may be based on the ability of the fungicide to penetrate into the canopy, and the timing of the fungicide application. Fungicides will be most effective at reducing the impact of white mold when applied at, or close to, growth stage R1. Wisconsin research data indicates that fungicides applied up to growth stage R3 (early pod – pods are 3/16-inch long at one of the four uppermost nodes) may be effective, but later applications will likely not be effective at reducing disease. Once symptoms of white mold are evident, fungicides will have no effect on reducing the disease. Fungicide applications for white mold management may be most useful on fields where varieties rated as susceptible to white mold are planted in a field with a history of the disease.
If a soybean field is diagnosed with high levels of white mold, this field should be harvested last. This will help reduce the movement of the survival structures of the white mold fungus by harvesting equipment, to fields that are not infested. Also, be sure to clean all harvesting equipment thoroughly at the end of the season to avoid inadvertent infestation of fields. Rotations of 2-3 years between soybean crops can help reduce the level of the fungus causing white mold in fields. Using corn or small grains crops such as wheat, barley, or oats in rotation with soybean is recommended.
There are several resources available to help farmers and agribusiness personnel manage white mold. Extension plant pathologists across the North Central Region have developed a useful resource in collaboration with the North Central Soybean Research Program to describe the disease and optimal management strategies. This resource, along with downloadable fact sheets, can be found here.
This group also developed a podcast series to facilitate learning about white mold on-the-go. This series can be accessed by clicking here.
There is also a University of Wisconsin Cooperative Extension video that shows symptoms of white mold and discusses management options for the disease. The video can be found on YouTube by clicking here.
