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.

Low Test Weight in Wheat

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

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

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

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.