What a gloomy sounding topic to start my column with today.It sounds like the title of a bad science fiction movie.Unfortunately it is not fiction at all.Sudden death syndrome (SDS) is caused by the fungal organism, Fusarium solani f. sp. glycines, which overwinters as spores on crop residue or freely in the soil.These spores will infect the soybean root as temperatures warm in the spring.Visual foliar symptoms of SDS start with mottling and mosaic of the leaves, and generally begin to occur sometime after flowering during reproductive development.Visual symptoms begin with leaf tissue between the major veins turning yellow and then brown, with the tissue near the major veins remaining green.Later, the leaves will die and fall, leaving the petiole (the stem of the trifoliate leaf) attached to the stem.
SDS has been breaking out in a major way throughout Iowa since early August, and in some parts of southern Iowa since mid-July.It started appearing in far southern Minnesota in mid-August.It can be readily seen in small areas throughout a soybean field while driving by on the road.However, I strongly suggest walking into the field to observe plants close up, in order to identify the inter-veinal necrosis and the green tissue along the veins in the leaflets.Yield losses of 10% or more can occur, with some documented losses up to 30%.Timing of symptom development, i.e. – leaflet necrosis and premature plant death, is critical.Earlier infection and symptom development can lead to greater yield losses.If symptom development occurs after most of the seed fill has already occurred, yield losses will be minimal.From what I have seen to date thus far, yield losses in Iowa will be substantially larger than losses in southern Minnesota due to the earlier development of the disease in Iowa.SDS has been widespread in southern soybean growing areas for many years, but is relatively new in northern areas like Minnesota, Wisconsin, and Michigan. SDS was first reported in Iowa in 1994 and in Minnesota in 2002, which suggests that the disease is gradually spreading north from year to year.
There is nothing that can be done to treat an SDS-infected crop once infection has occurred.There are currently no fungicide seed treatments or foliar treatments commercially available to effectively control SDS.The fungus invades young soybean roots when soil conditions are cool and wet.However, development of foliar symptoms during reproductive development seems to depend on warm and wet environmental conditions.Early planting has been shown to increase the likelihood of SDS development.However, in 2009 Iowa experienced serious exceptions to this rule.Soil compaction has also been associated with SDS development.In fact, often SDS-infected areas can be seen more often on headlands and other compacted areas of a field than in other areas.However, symptoms can and do sometimes show up nearly anywhere in a field.There also seems to be a correlation in occurrence of SDS infection and soybean cyst nematode (SCN) infestations.Often fields that develop an SCN infestation problem, also develop a potential for SDS development a few years later.However SDS can also be found in fields where SCN has not been detected.Unfortunately, significant exceptions can and have been found to virtually all environmental interactions with SDS, like the ones just described above.These exceptions make it rather difficult to predict and manage.
What then can be done to manage SDS?Several things can be done to reduce the probability for exposure to SDS.First of all, take and maintain accurate notes of fields and areas in fields where SDS is a problem in order to develop a field history.Since the fungal spores that cause SDS can reside in the soil for several years, it seems likely that those fields could potentially show symptoms again in the future. Plant those fields later, when the soil may be warmer and drier, in order to reduce both soil compaction and root infection rate. Also, use varieties on those acres that have higher levels of resistance to SDS.Most pathologists would agree that use of SDS-resistant varieties is the best SDS management option.Unfortunately, no completely SDS-resistant varieties are currently available in the industry.However, partial resistance or tolerance to SDS is readily available in the industry, including Galena Genetics.We select for resistance to SDS at all stages of our breeding and testing program whenever our testing environments provide an opportunity for selection.A number of Galena Genetics varieties are tolerating this year’s SDS onslaught very well compared with the rest of the industry.However, even the most resistant varieties can show symptoms in a severe SDS year like 2010.Genetic resistance to SDS is complex and quantitative.That is, resistance appears to be controlled by many genes in the soybean plant.SDS resistance is difficult to select, because the causal organism is difficult to culture and grow artificially in the lab, and difficult to use for inoculation.In addition, it is difficult to know exactly when and where large areas of naturally-occurring SDS are going to occur in a given field in order to plan and plant effective SDS-resistance experiments.
Don’t blame SDS for everything!
Although SDS is widespread in Iowa and appears to be spreading rapidly in Minnesota as of this writing, don’t assume every problem observed in your soybean field is SDS.I was talking to a couple of well-respected farmers late last week about crop conditions.One of them said he had some areas in his soybean fields that were looking pale and off-colored because of drought stress.The other farmer claimed the problem had to be SDS because there was too much rainfall this summer for drought stress to be happening on August 20.The fact was that neither one of them really knew for sure because they had not gone out into the field to closely examine the cause of the problem.
Drought Stress.I decided to make a closer look for myself.As it turns out, there are definitely some drought-stressed areas showing up in fields late in August.From the road, these areas are currently looking pale green to bluish-green to gray in color.I was generally finding them on gravel veins, very sandy knobs, and bank-cuts along road ditches.It may seem surprising that drought stress could be occurring with the ample amounts of rainfall experienced earlier in the summer.However, there is a logical explanation for it.(Please bear in mind that I am referring here to luxurious amounts of rainfall, but not the flooding kind of rainfall experienced in many areas of Iowa.Flooding is a whole different issue.)Ample rainfall earlier in the summer did not require a large root mass to develop in order to supply adequate water and nutrients to the plant, either soybean or corn.Later, both soybean and corn were using large amounts of energy to fill pods or ears, leaving little energy available to allow root growth in order to seek more moisture when soils started drying.When soils began to dry out, as was the case in the sandy soils I observed, the plants began to suffer.The high temperatures and sunny skies in August exacerbated the stress in those soils.The surrounding heavier soils, with higher water-holding capacity, were able to continue providing adequate moisture to plants with relatively small root systems.
Flooding.Large amounts of rainfall and subsequent flooding, especially in many areas of Iowa throughout the summer, have caused significant damage to soybean and other crops as well this summer.It is a bit unusual to see soybean fields or corn fields in standing water in August.Of course the higher the level of standing water and the longer the duration of standing water, the more severe the yield losses will be.Little research is available to know the ultimate impact on the quality of the soybean or corn harvested from late-flooded fields.If water was standing above developing soybean pods or corn ears, the harvest crops from these fields will likely need to be monitored for mold and other damage prior to and during storage.
Soybean brown stem rot.Brown stem rot (BSR), caused by the fungal organism Phialophora gregata, can be easily confused with SDS in the field.Both diseases can exhibit symptoms of yellow and brown leaf tissue between the veins on the leaflets (inter-veinal necrosis), and green tissue surrounding the veins.The best way to distinguish between SDS and BSR is to pull a plant from the soil and split the stem longitudinally. An SDS-infected plant will have white (normal) tissue in the center of the stem, but may have a bluish hue on the root, caused by the fungal growth on the root.A BSR-infected plant will have brown tissue in the center of the stem, and will not have a bluish tint on the root.
Grant Metz, PhD
Research Director
Galena Genetics, LLC
501 Main St.
PO Box 548
Ormsby, MN 56162
(grant_metz@rabbeusa.com)
