Impact of Cool Temperatures on Soybean Seed Fill

Delayed crop development due to decreased heat units coupled with forecasted cool nighttime temperatures over the next week have many questioning the fate of the WI soybean crop. The soybean fields that I have scouted over the last week range from the R5.5 growth stage to the R6 growth stage (full seed: pod containing a green seed that fills the pod cavity at one of the four upper most nodes on the main stem with a fully developed leaf). The minimum temperature range required for soybean seed ripening is 46.4 to 48.2 °F; whereas the optimal range is 66.2 to 68°F (Holmberg 1973). A survey of the literature suggests that though cool temperatures during grain fill may adversely affect yield, the more serious impact to soybean would likely be increasing the number of days to physiological maturity and the threat of a killing frost (Image 1).

Image 1. Average date for first killing frost in Wisconsin.

On average a soybean plant remains in the R6 growth stage for 18 days; however the range can be as short as 9 days or as many as 30. The risk to WI growers is that the longer we remain below the optimal temperature range for seed development the longer our crop may remain in the R6 growth stage. As we move through the R6 growth stage and into R7 soybean (physiological maturity) seed moisture declines thus decreasing the risk of yield loss due to frost. Judd et al. (1982) found that seed in green pods which contain 65% moisture are injured at 28 °F whereas seed found in brown pods at 35% moisture was not injured at 10 °F.

Though chilling may adversely impact soybean yield a more important issue may be the effect of chilling on seed quality of yellow hilum soybean. Morrison et al. (1998) found that seed coat discoloration in yellow hilum soybean increased with the accumulation of daily minimum temperature ≤ 59 °F during seed development.

Literate cited:

Holmberg S. A. 1973. Soybeans for cool season climates. Agric. Hort. Genet. 31:1-20.

Judd, R., T.M. Tekrony, D. B. Egli, and G.M. White. 1982. Effect of freezing temperatures during soybean seed maturation on seed quality. Agron J. 74:645-650.

Morrison, M. J., L. Pietrzak, and H Voldeng. 1998. Soybean seed coat discoloration in cool-season climates. Agron J. 90:471-474.

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