Sudden Death Syndrome is the second most costly soybean disease, second only to Soybean Cyst Nematode. It was first discovered in Arkansas in 1971 and has moved into nearly every state of the central and upper Midwest. A few weeks ago, we covered SCN on the blog, and this week we wanted to cover SDS, particularly as our environmental conditions are set up for the possibility of a heavily infectious year. This blog will cover the symptoms and lifecycle of SDS, helping you identify it in the field. Additionally, we will cover what the overall impact on your yield and crop health will be as well as what management practices can help curtail some of the impact of SDS.
Disease Symptoms and Pathogen Lifecycle
Sudden Death Syndrome is caused by the pathogen fusarium virguliforme. The symptoms of the disease are often very similar in appearance to several other soybean diseases such as brown stem rot or even chemical damage to leaves. Careful identification is necessary to ensure damage is from SDS and not one of these other casual agents. Infection of plants occurs early in the growing season, but visual symptoms don’t show up until the middle of the summer during reproductive stages of the plant. Infected areas generally show up in small patches to begin with but will grow over time.
The symptoms present in the taproot are the ones most useful in distinguishing from other problems. While foliar symptoms are often similar to other diseases or chemical damage, the root symptoms are unique to SDS.
When you split the stem of a SDS infected plant you should see a tan/brown discoloration of the root, but a white pith in the middle. Figure 1 shows the difference between a healthy plant and a plant with SDS. Soybeans infected with brown stem rot will have a whiter colored root on the inside, but a brown pith in the middle. Figure 2 shows in comparison what a split stem of brown stem rot looks like.
Additionally, plants with SDS may have small groupings of light blue masses on the outside of the root system. These are spore masses of the fungus that causes the SDS. These are not normally visible, however they are more common in heavily infested soils.
The foliar symptoms are the first, most obvious symptom of SDS. Interestingly, the pathogen itself does not move out the roots of the plant or above the crown. Any disease symptoms we see on the leaves is due to a toxin the pathogen produces that moves through the plant, causing the damage to the leaves.
Foliar symptoms start as a slight yellowing of the leaves, generally confined to the upper leaves of the plant. This yellowing is fairly mottled initially, then grow together to form chlorotic spots between the veins. These chlorotic spots eventually turn to necrotic, or brown spots as these portions of the leaves die. The classic leaf pattern is brown necrotic leaves, with bright green veins. As symptoms get worse, the leaves will eventually die and fall off the plant, but leave the petiole behind. The retention of the petiole is unique to SDS.
Life Cycle and Host Crops
Once you have SDS in your field, you will not be able to get rid of it. You can only come up with ways to manage it. SDS survives by grouping into chlamydospores that are very durable. These chlamydospores can withstand freezing conditions and extended periods of time between host crops. This makes it extremely difficult to reduce populations of the disease. As temperatures increase in the spring, the chlamydospores will germinate and look for plant roots to infect. A study from Iowa state has shown that SDS can survive on corn kernels and residue, but will not infect the corn plant.
SDS infection is favored by cool, wet conditions. This is particularly true in the early growing season as it is easier to infect the young seedlings than established plants. Areas of the field that are poorly drained or have moderate to severe compaction are also favorable for SDS development. Cool, wet springs are generally associated with high infection years, as well as heavy rainfall near flowering and podset during the summer. Finally, fields with Soybean Cyst Nematode are at a greater risk of SDS as they wound the plant root system and allow entry points for the pathogen.
It’s important to note that if conditions are not favorable, SDS will not develop. In a hot and dry year, levels of SDS will be greatly suppressed or symptoms not even noticeable.
In plants infected with SDS, yield reduction is very common and is very dependent on time of infection as well as environmental conditions throughout the growing season. As much as 50% yield reduction has been reported in severe cases.
As the disease progresses, pods and flowers will start to abort. Pods that are retained may not set any seed, or the seeds will be small and lightweight. These all contribute to the lower yields. The seeds themselves are not carriers of SDS and will not result in any quality issues for long term storage in bins.
Planting tolerant varieties is one of the best ways to manage SDS. While no varieties are completely resistant to SDS, there are many good options that are better able to withstand SDS. Within the last several years, Pioneer has worked on breeding high yielding varieties that are also more tolerant to SDS. In fact, some varieties are now ranked as high as an 8 for tolerance (9 is the most tolerant possible). This dramatically helps combat the effect SDS can have on crop health and yield.
Don’t forget that SCN can exacerbate SDS, so make sure to select varieties that are tolerant to both SDS and SCN for you best possible protection.
Some sources suggest that rotating crops will help reduce levels of SDS. The University of Nebraska states that rotating away from soybeans for two years can help reduce levels. The Crop Protection Network states that a more diverse rotation might help reduce SDS. Including alfalfa or red clover was more beneficial that just rotating with corn. Both Purdue and Iowa State suggest that rotation does not help reduce levels, even switching to continuous corn for several years does not reduce SDS pressure. Only long term rotations show some promise for lowering levels.
There is no real correlation between tillage type and levels of SDS. No-till, conservation tillage, or conventional tillage all had the same amounts of SDS.
A common cultural management practice is to delay planting to ensure warmer soils and more optimal field conditions reduce the risk of early season infection. While this may reduce the risk to some extent, infection can still occur, even when soil temps are 80 degrees. Overall, it is wiser to plant on time or early. Late planting will result in a yield decrease that won’t offset any gain from later infection of SDS. Early planting has been shown to increase yields and this does offset any loss from an early season infection.
You can prioritize your fields planting order. Plant fields with your highest levels of infection towards the end of the planting season. This will give you a slightly later planting date without compromising on yield potential.
Try to reduce compaction, improve soil structure, and maintain good fertility. This will help promote a healthier root system and plant which will be better able to fight against the SDS. If you have problems with drainage, consider tiling your field to improve water filtration through the soil. Improving drainage and reducing compaction should be a priority.
While foliar fungicides don’t have an effect against SDS, a seed treatment from Bayer called ILeVO does work to preserve yield in fields infected with SDS. Studies from Pioneer show as much as a 9 bushel increase in fields with known levels of SDS. This is one of the only known products that effectively preserves yield. The Crop Protection Network explains that a competitive product, Mertect, while also labeled for SDS, is not as effective or consistent at yield preservation as ILeVO has been in research trials.
This year would likely be a great year to consider an ILeVO treatment on your SDS acres. With a cool, wet, spring, infection is very likely. Talk to one of the Pederson Seed team to discuss including ILeVO treatment on your seed.