Sudden Death Syndrome (SDS) and Soybean Cyst Nematode (SCN)

Images and phone calls related to Sudden Death Syndrome (SDS) have been coming into our offices over the last 7-10 days. Many of the suspect fields are being positively diagnosed with SDS. Paul Esker and I walked a field this morning with Matt Hanson, Doge County Crops and Soils Agent, and found a field that was most likely SDS. The symptomology in this field was odd in that it was appearing in small rings across the field and only in certain varieties. Recent literature however has suggested a positive relationship between soybean cyst nematode and SDS. Therefore we would recommend that anyone finding SDS also check for SCN. Remember the WI Soybean Marketing Board provides free testing for SCN for Wisconsin soybean growers. (For more information on this test please click here).

For more information on SDS please refer to the following links.

Soybean Disease Update

Common Soybean Diseases in Wisconsin

Soybean Disease Update

Sclerotinia Stem Rot: Over the past week, we have continued to receive numerous questions regarding Sclerotinia stem rot (aka, white mold) and the control of this disease using foliar fungicides. In most soybean fields, we are at the R4 growth stage, or full pod. Fungicides are not recommended at this time for managing white mold. The key point to always remember is that infection by the pathogen that causes Sclerotina stem rot occurred during the flowering period. Sclerotinia stem rot is a monocyclic disease. This means there is only a primary cycle for infection. There is no secondary spread of the disease; although plants that are adjacent or touching an infected plant may become diseased.

Sudden Death Syndrome: Also in the past week, we have also received reports and have seen symptoms associated with Sudden death syndrome (SDS), caused by Fusarium virguliforme. As a reminder, this is a relatively new disease in Wisconsin and the foliar symptoms of SDS can easily be confused with Brown stem rot (BSR) (Figure 1). Symptoms of SDS include a yellow to brown discoloration of the leaves around veins. These often begin as small, circular spots. A differentiating characteristic to BSR us that roots may be black and rotted with a slightly blue hue (growth of the fungus). Conditions in Wisconsin during the 2009 growing season have been favorable since there was ample soil moisture during early vegetative growth in many parts of the state as well as the cool temperatures around flowering.

Figure 1. Images showing symptoms of Brown stem rot (top) and Sudden death syndrome (bottom).

Downy Mildew of Soybean

We have been receiving lots of reports of downy mildew of soybean throughout the state. Downy mildew is common disease of soybean that rarely affects soybean yield in the state. Based on our own observations and comments from growers and consultants, the increased incidence of downy mildew appears to be somewhat variety dependent. Downy mildew is caused by the fungus Peronospora manschurica. Symptoms of downy mildew are typically found on the upper surface of young soybean leaves. Spots of downy mildew are green to light yellow that can enlarge into bright yellow spots. Also, during periods of high humidity, the spots will look slightly gray and fuzzy when looking at them from below. As the lesions age, they may become brown with a yellow border. Depending on soybean variety, there may be a leaf distortion that may look like virus symptoms.

Figure 1. Symptoms of downy mildew as they appear on the upper and lower leaf surface (Image Source: C. Grau, UW-Madison).

Fungicides are not recommended for control of downy mildew given that it rarely reduces yield. Many of the questions we have received have focused on the use of an insecticide-fungicide tank mix and we would like to remind growers that it is important to base decisions for use of insecticides for soybean aphid on the established threshold 250 aphids per plant and that decisions for use of fungicides should be based on active scouting, identification and recognition of diseases that may reduce the soybean yield.

For further information:

Considerations for Spraying Foliar Fungicides in Soybean

Considerations for Spraying Insecticides for Soybean Aphid in Soybean

The Year of the Yellow Bean

The 2009 soybean growing season has been filled with questions and concerns regarding the yellow "state" of many soybean fields. This topic has been building from emergence through the current R4/R5 growth stage. I have been in many soybean fields over the past 6 weeks and have seen many of the same culprits. Below is a short list of the main issues I have seen.

1. Variable emergence: Variable seeding depth was a major contributor to some of the early season "off color" soybean fields that I investigated. In these fields you would see odd patches of yellow beans. Once you carefully excavated the soybean plants you would see that the yellow beans were planted <1/2 inch deep. Following planting these seeds did not receive enough moisture to stimulate germination. The seed sat in dry soil until a significant rainfall triggered emergence. The delayed emergence and subsequent cool environmental conditions held the soybean plants back developmentally, hence when their neighbors were lush green they appeared light green to yellow. This difference was simply a function of delayed developmental stage (N fixation).
   
2. Cool temperatures and delayed development: Please refer to the following article to address this topic. "Yellow soybeans and nitrogen fixation"
   
3. Potassium (K) deficiency: Many soybean growers either cut-back on their K rate or flat out "took a year off". This has lead to an increased number of fields showing K deficiency. This not only has a direct impact on yield but may increase soybean aphid populations (Please see "K deficiency and the soybean aphid"). To further confound this issue Sale and Campbell (1987) found a differential response to K deficiency among soybean cultivars. This may explain why some varieties or fields are showing greater symptoms than others.
   
4. Drought: Many soybean fields in Wisconsin have experienced droughty conditions. Research has shown that nitrogen fixation in soybean is extremely sensitive to drought and that this sensitivity to soil drying constitutes a serious constraint to N fixation (Serraj et al., 1998; Sinclair et al., 1987).
   
5. Poor inoculant application method: Lastly, I have walked several fields where an inoculant was either not applied or improperly applied to "new" soybean ground leading to variable nodulation (one plant would have nodules and the neighbor lacked nodules) across a field.
   

Cited article:

Sale, P. W. G. and L. C. Campbell. 1987. Differential responses to K deficiency among soybean cultivars. Plant and Soil. 104:2:183-190.

Serraj, R., T. R. Sinclair, and L. H. Allen. 1998. Soybean nodulation and N₂ fixation response to drought under carbon dioxide enrichment. Plant, Cell, and Environment. 21:491-500.

Sinclair, T. R., R. C. Munchow, J.M. Bennett, and L.C. Hammond. 1987. Relative sensitivity of nitrogen and biomass accumulation to drought in field-grown soybean. Agronomy Journal. 79:986-991.

First Report of Dockage for DON

Yesterday (8/6), both Shawn and I received the first report of wheat grain being rejected at the elevator for too high of level of DON. DON, which stands for deoxynivalenol, is a vomitoxin that can affect humans and animals. Because of its risk, most grain handlers or processors have set a limit of 2 ppm. Under 2 ppm, no discounts are usually applied for acceptance of the grain. Please also keep in mind that these limits may be more rigid depending on the expected end-use for the grain, or if it may be shipped overseas.

Throughout much of the 2008-2009 growing season, conditions were low for risk of Fusarium head blight (FHB) (Figure 1). However, this did not mean that we would not have scab in the state. Also, the current iteration of risk models is for FHB and not DON. It is known that symptoms of FHB in the field do not always correlate with DON, meaning that asymptomatic kernels could still be infected. Also, there is some newer evidence that also suggests infection by Fusarium graminearum can occur after flowering, although how much this affects DON is work that is in progress.

Figure 1. Fusarium head blight of winter wheat.

In the meantime, we very much would like to hear about new reports of either dockage or rejection of wheat grain due to the DON. In order to have a better idea of the current DON situation in the state, if you would be able to provide information regarding the location where dockage/rejection occurred, the level of DON, and any other information about the production setting, we would be most appreciative. This will help us move forward to better develop management recommendations that use both the current forecast model, plus what we know is occurring in the state.

Options for hail damaged corn assessed as a total loss.

Hail in late July severely damaged much corn in Wisconsin. Some fields will be assessed as total losses. Corn that was broken off at the ear will not continue to grow. What options remain for those planning on silage?

1. If the crop was insured, check with insurance adjuster to ensure that any action does not cause a greater loss in payment than the value of forage produced.
   
2. Consider the value of the nutrients if the crop is simply disked down.
   
3. Harvest the remaining forage for silage as the whole plant moisture dries down. Make sure the forage to be ensiled is at the proper moisture. The lower stalk and leaves will ferment if harvested at 60 to 70% (moisture depending on storage type) and produce a low quality silage adequate for heifers and dry cows.
   
4. A common question is: what can be planted to produce more tonnage yet this year? Frankly the options are few this late in the season.
   
   1. Absolutely do not plant sorghum-sudangrass or sudangrass. This is a warm season annual that will grow only very little when the average daily temperature falls below 80^o F. Since little growth will occur in September, the result will be low yield.
      
   2. Corn planted August 1 can be expected to yield about 0.7 to 2.8 t/a dry matter in Southern Wisconsin. These yields were achieved in 2006 and 2005 when a killing frost hit on October 12 and October 26.
      
   3. Oats planted during the first two weeks of August can be expected to yield 1 to 2 t/a dry matter in Southern Wisconsin and less as one moves north.
      
   4. Other small grains will yield less because they will not head this year.
      
   5. Some acres may be prepared for winter wheat production.
      

Dan Undersander, Joe Lauer, and Shawn Conley, Agronomists, University of Wisconsin

Scout Soybean Fields for Stressed Plants

We have started to receive some reports of stressed fields where the verbal description of the field situation is very similar to 2008. As a frame of reference, as well as to get a feel for what was learned in 2008, please click here.

Also, samples have been submitted to the Plant Disease and Diagnostic Clinic that has tested positive for Phytophthora, Fusarium spp, and Rhizoctonia.

Similar to what we emphasized earlier this year, when we recommended assessing plants from the V2 to V4 growth states to look for early indications of stressed soybean plants, follow up assessments beginning now during the reproductive period is important for understanding the possible complex set of interactions. Start by checking areas of the field where there may be higher compaction, such as the headlands. In 2008, we saw a variety of symptoms in the field. Check soybean plants for petiole retention; also scrape away at the epidermis to look for signs of organsims like Macrophomina phaseolina; lastly, split and examine stems and determine if there are necrotic areas to the stem or as we move into the later reproductive period, brown stem rot. Based on our observations from 2008, there is not a single pattern to these composite sets of symptoms in field as we saw anything from circular-like patches to patterns that followed down the rows.

If you have suspected samples, submit those to the Plant Disease and Diagnostic Clinic. For further information about the different soybean diseases that may be found in Wisconsin, consult Soyhealth.