Downy Mildew of Soybean

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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:


The Year of the Yellow Bean

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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

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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.

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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 80o 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

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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.

Low Test Weight in Wheat

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As wheat harvest is finally getting into full swing we are hearing scattered reports of low weight and subsequently significant dockage. As these questions arise there are a few factors to consider that may be causing the low test weights.

  1. Genetics: There is a wide range in test weight among the common wheat varieties grown in WI. In a “good” year these differences may not appear significant however in a “bad” year these differences can cost growers significant dollars. The University of Wisconsin Variety Test Program not only provides growers with yield information, but we also report test weight differences. Our variety trial results will be published in the next two weeks. Go to www.coolbean.info for these results.
  2. Weather: The wetting and drying action of frequent rainfalls on dry wheat can drop test weight quickly. Thus timeliness of harvest is critical. Wheat can also swell following a rainfall and remain swollen even after the grain has dried back down. This increased kernel size displaces space and leads to lowered test weight.
  3. Thin stands: Thin stands due to winterkill in parts of WI have led to late tillering, green stems and immature heads in the field. This may be causing a resource allocation issue in some fields (e.g. some photosynthates maybe going to growth instead of grain fill). If possible avoid these parts of the field if you are experiencing low test weight.

K Deficiency and the Soybean Aphid

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Given the high input costs for P and K in soybean many growers either cut back or as I have been told “took a year off”. I wanted to remind growers as well as crop consultants of the relationship between K deficiency and soybean aphid fecundity. Below please read a blog contribution from Dave Hogg: Professor of Entomology, University of Wisconsin, Madison.

Soybean aphid infestations remain at sub-economic levels throughout Wisconsin. Will they stay there, or will we have a repeat of last year’s late season aphid buildup? We will have to wait to see what aphid populations do over the next several weeks. However, one thing we do know – that soybean fields with suboptimal potassium levels are at greater risk of soybean aphid population increase and yield loss.

Following the 2000 discovery of the soybean aphid in Wisconsin, entomologists and agronomists noticed that soybean aphid infestations seemed to be more severe in K deficient soybeans. [The below photo of a soybean field in Grant County (taken by John Wedberg in August, 2000) illustrates this. The yellow beans on the left were literally dripping with soybean aphids and were presumed to be K deficient, whereas the healthy beans on the right had few aphids and were thought to have adequate K. The demarcation line follows the field contour.]

Subsequent research has proved this observation to be correct, plus we now have a better understanding of why this occurs. What happens is that low K actually makes soybean more nutritious for soybean aphids, promoting higher aphid reproduction and leading to more rapid aphid population increase. To give an idea of how this might work, under field conditions in a K deficient field an aphid infestation can increase from 10 per plant to 230 per plant in 10 days; in a field with adequate K, that same population would increase from 10 to 150 aphids per plant. Further research suggests that K deficient beans have a greater percentage of asparagine in the plant phloem where the aphids are feeding. Asparagine is known to be an important amino acid for aphid nutrition.

Finally, we think the yellowing of K deficient soybean leaves may preferentially attract migrating soybean aphids, placing K deficient fields at a further disadvantage. The color yellow has been shown to be highly attractive to a number of aphids.

Bottom line, maintaining adequate K levels in soybean goes a long way toward managing soybean aphid.

Estimating Yield Loss in Hail Damaged Soybean

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Violent storms over the weekend caused significant hail injury to many Wisconsin soybean fields.

Image by Tim Wood

As growers, county extension agents, and crop consultants walk fields and assess crop damage it is important to remember the following key points.

1. Do not do anything to the field before you call your hail adjuster and have the claim inspected.

2. Remember in soybean the crop injury often looks worse than it really is. A soybean can add a new trifoliate ever 3-5 days so canopy coverage will reoccur in ~2 weeks.

3. Most of the soybean crop in Wisconsin is in the R3 growth stage or earlier. If defoliation only occurred expected yield loss ranges from 0 to 33% (33% yield loss may occur at 100% defoliation). If stem breakage or node removal occurred then the following yield losses may be encountered (Table 1).

4. There is no evidence to suggest a fungicide application to hail damaged soybean will prove beneficial.

Table 1.Yield loss from simulated hail injury.

Crop Growth Stage when Injury Occurred

Percent main-stem node removal

V2

V6

R3

—————Yield Loss—————

20

5%

7%

15%

40

11%

14%

24%

60

12%

22%

37%

80

17%

40%

59%

100

19%

99%

100%

*Conley, S. P., Pedersen, P., and Christmas, E. P. 2009. Main-stem node removal effect on soybean seed yield and composition. Agronomy Journal: 101:1-4.

Preliminary Scab Results – Winter Wheat Variety Trials

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During this past week, Karen Lackermann (MS Student in Plant Pathology) collected field data to estimate the incidence and severity of Fusarium head blight in the Winter Wheat Variety Trials in Wisconsin. This work is a part of our program to develop an integrated management platform for wheat disease management in Wisconsin. Data were collected on a plot basis, with 100 heads assessed per plot (50 consecutive heads in one border row; 50 consecutive heads in the second border row). Incidence was defined as the presence of Fusarium head blight on a wheat head, while severity was based on the rating scales of Engle et al. and Stack and McMullen. In addition, an index value was calculating as percentage incidence x percentage severity and dividing that number by 100.
Preliminary results:
On average, FHB was low across the variety trial locations (Figure 1) in Wisconsin. The highest levels of scab (incidence, severity, and index) were observed at the Lancaster trial location, however, even there, the index values were less than 5%. A more formal analysis will be shortly conducted and summarized for the variety trial book. Furthermore, post-harvest testing of grain for evidence of Fusarium graminearum and DON will be conducted.
Figure 1. Incidence of Fusarium head blight at the winter wheat variety trial locations in Wisconsin. The scale for incidence (y-axis) is 0-100%.
Figure 2. Severity of Fusarium head blight at the winter wheat variety trial locations in Wisconsin. The scale for severity (y-axis) is 0-100%, although the figure only shows from 0-20%.

Figure 3. Fusarium head blight index, a measure of incidence x severity. Index values range from 0-100%, although the scale shown is only from 0-10% for illustration.

Brief Weather and Disease Update – June 23-29, 2009

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Weather conditions last week were rather hot and humid early in the week, as temperatures were in the low 90s throughout many our research project areas. Late in the week, temperatures did fall back into the 80s and now into the 70s and 80s. Rainfall was light, in particular at our winter wheat variety trial locations. Weekly rainfall totals were:

  • Arlington: 0.01 inches
  • Chilton: 0.11 inches
  • Janesville: 0.01 inches
  • Lancaster: 0 inches
Winter wheat is past flowering for the state – the risk window for Fusarium head blight. To determine if FHB was a problem in your field, assess fields during the soft dough growth stage. We are currently taking field notes and will update you shortly on the preliminary results.
Soybean diseases to date have been low, primarily bacterial blight and brown spot. Remember though, this is an excellent time to scout for early season stress that may be caused by soilborne pathogens. Also, take advantage of the free SCN testing program, sponsored by the Wisconsin Soybean Marketing Board. For further information, please contact Colleen Smith at clsmith8@wisc.edu or 608-262-7702.