Stress on R6 Soybean

I have logged many miles across Southern WI the past week and have noticed several pockets of soybean that could use some rain. The U.S. Drought Monitor service verifies my wind shield scouting as it places most of southern WI in the abnormally dry category (Image 1). Across southern and central WI the average soybean field I have been in is at the R5.5 to R6 growth stage (full seed). In WI the R6 growth stage on average lasts ~18 days but will range from 9 to 30 days depending upon the weather. Soybean in this stage use about 1/4 to 1/3 inches of water per day. Lack of sufficient water during this growth stage can cause young pods and developing seed to abort reducing the number of seeds per plant (Images 2 &3).

Image 1.

Images 2 & 3. Stress induced seed and pod abortion at R6 soybean.

Soybean plants can reduce the size of their leaf pore openings to reduce the loss of water vapor. This also reduces the intake of carbon dioxide and the manufacturing of photosynthates which slows plant growth. When normal soil moisture returns, normal growth is resumed. This ability to reduce metabolic activity allows plants to tolerate dry spells without dying or harming their ability to resume growth when normal moisture returns.

If stress has severely affected pod set and seed fill, and if livestock feed is needed, soybeans can be harvested as a forage for ensiling. Highest protein and yields are obtained from soybean harvested at the R6 to R7 growth stage. Harvesting soybeans for forage between the R1 and R5 stage will result in a very high quality silage, but dry matter yields will be reduced significantly. Forage quality will be reduced from R5 soybean forward if a conditioning process is used during harvest. Conditioning will cause significant seed shattering.

Sun Scald, White Mold, and Drought Stress

A survey of our central, north central, and northern soybean variety trial locations was conducted on 8/12/08 and 8/13/08. The soybean crop ranged between the R4 and R5 growth stages. Across most of WI we still remain ~150 gdu’s behind the five year average.

Soybean aphids were present at all trial locations. We have been monitoring our Galesville location for several weeks and yesterday reached the economic threshold of >250 aphids per plant. In a normal year our soybean crop would be developmentally far enough enough along that an application this late would likely not be warranted, however based on growth stage, building aphid numbers, and gdu’s an application was deemed necessary at this location.

Though much of the state has received timely rainfall the central part of WI has not been as fortunate. Several fields are beginning to show drought stress. Drought stress symptoms include a shortened soybean crop, lack of full canopy closure, and flipped soybean leaves (Image 1).

Image 1. Drought stress to soybean.

Sun scald was common across several of our locations. Sun scald or sun burn can be confused with Cercospora. Sun scald symptoms mostly appear on the underside of soybean leaves and often have a distinct line between the healthy and affected areas (Image 2). This line is caused by another leaf shadowing the affected leaf and limiting sun exposure.

Image 2. Sun scald on soybean.

In our irrigated white mold trial we are just starting to see symptoms showing up. Image 3 is an example of early white mold symptoms in the field.

Image 3. White mold.

Lastly, the most obvious weed detected across the state in my windshield survey was volunteer corn. Not only will this weed cause yield loss but also remember that if weed populations are great enough dockage will occur at harvest.

Phytophthora and Stem Canker of Soybean

As we move into the latter part of the soybean season, we have been receiving lots of questions and reports of Phytophthora root rot in soybean, including reports where the Rps1k gene has not been performing well. The weather we experienced this past spring into early summer increased the favorability for Phytophthora. While we have also seen reports about Rhizoctonia root rots, the main other stem disease that can often be confused with Phytophthora is stem canker (Figs. 1 and 2). There are some key differences between the two when scouting fields:

Phytophthora root rot: (i) infected plants are often easy to pull from the ground due to the root rot during the early infection, (ii) stem lesions are the key diagnostic symptom later in the season and include a brown discoloration that progresses 6 to 12 inches from the soil line, (iii) diseased roots are smaller than healthy plants and the taproot and lower stem are internally discolored, and (iv) this can lead to an open areas of the canopy.

Soybean stem canker symptoms: (i) first appear during early reproductive stages as reddish-brown lesions at the base of branches or petioles and are first noted after the petiole has dropped, (ii) as the lesion elongates, it will become dark brown to black forming a canker that is sunken in appearance and the stem often becomes girdled, (iii) foliar symptoms can include an interveinal chlorosis and necrosis due to the production of a toxin, (iv) above and below the site of the canker, the tissue remains green and while the leaves may wither, they will remain attached, and (v) unlike Phytophthora, stem canker does not cause a root rot, and this will also differentiate stem canker from diseases like sudden death syndrome.

In 2007, there were numerous reports of soybean stem canker in Wisconsin and a summary of information can be found in a Wisconsin Crop Manager article published on December 15 (Volume 14, Number 31). We are interested in learning about locations where Phytophthora root rot has occurred and if possible, the source of resistance in the soybean variety being grown. Please feel to contact either Paul or Shawn with reports in your respective areas.

Fig. 1. Phytophthora root rot of soybean (Photo Credit: C. Grau, UW-Madison).

Fig. 2. Soybean stem canker (Photo Credit: C. Grau, UW-Madison).

Soybean Growth Stage Does Not Matter: Soybean Aphid Threshold Should Remain at 250 Aphids per Plant

High commodity prices coupled with receding flood waters have some growers pushing their soybean planting dates well into late July. As these soybeans emerge and develop they will be greeted by an increasing population of soybean aphids. Dr. Eileen Cullen UW Field Crops Entolomogist suggests the following plan of action to manage the soybean aphid on late planted soybean.

From Dr. Eileen Cullen:

For vegetative stage soybeans, if aphids are colonizing, do not treat the soybeans below 250 aphids/plant. Data from yield loss experiments in WI and throughout the North Central region show that there is no yield benefit to decreasing the threshold, even on small soybeans (Journal of Economic Entomology 100: 1258-1267). Keep in mind also that 250 aphids/plant is not the Economic Injury Level (EIL). The EIL is the number of aphids that need to be present for the value of the lost yield to equal the costs of control. There is lead time of 7 days to reach that +/- 674 aphids per plant EIL.

Now that the market value for soybeans has risen, a lowered EIL can be calculated. David Ragsdale, the lead author on the paper describing the existing EIL for soybean aphids, calculated a new EIL for soybeans selling at $15 per bushel, with $8 per acre control cost, and an anticipated yield of 50 bushels per acre. With these values the EIL is lowered to 452 aphids per plant. However, this doe not lead to a decrease in the Economic Threshold of 250 aphids/plant.

Entomological research in six states (including WI) from 19 yield-loss experiments conducted over a 3-yr. period found no detectable yield differences attributed to treating soybean aphids at numbers below the Economic Threshold of 250/plant. Additionally, treating soybean aphids below the ET of 250 aphids/plant increases risk to producers by treating an aphid population that is growing too slowly to exceed the Economic Injury Level, eliminates generalist predator insects, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance.

If the ET of 250 is reached/exceeded on vegetative stage soybeans, treatment is warranted on the double-cropped beans, due to winged soybean aphid dispersal flights and potential outbreak.conditions in affected areas such as Western Wisconsin. Continue to scout soybeans following treatment to watch for recolonization and soybean aphid resurgence.

Economic threshold is being exceeded in many fields in Western Wisconsin. Winged soybean aphids are dispersing from southern MN and westerly, northwesterly winds have the potential to move dispersal flights toward WI. (This is largely happening in parts of Western WI now). For full information on Soybean Aphid Scouting and management recommendations, please vist the Wisconsin Crop Manager Newsletter article “Soybean Aphid Increasing and Variable – Scout Fields Now” at

Image 1. Winged soybean aphid colonizing V2 soybean plant.Image 2. Soybean aphids feeding on V2 soybean plant.
Photos by John Gaska

Dockage for Black Point of Wheat

Bryce Larson, Calumet County Agricultural Agent, reported today that growers were being docked at the elevator for black point disease of wheat. Black point disease is caused by a fungal complex, including (but not limited to) Alternaria, Fusarium, and Helminthosporium. The affected kernels appear black-pointed on the tips. Bryce was reporting that dockage for growers was at $0.06 per point above the 4% threshold. We also had reports of black point disease in 2007. Some of the conditions that favor infection and disease development include warm, humid or wet weather during grain maturation. In considering options to reduce the risk of black point in 2009 include the use of certified seed and use a fungicide seed treatment. A summary of the winter wheat variety trial will appear shortly and will be able to be accessed at

To help us document the severity of dockage issue, if you receive reports and can pass them along, please contact Paul at 608-890-1999 or

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Dockage Due to DON Levels in Wheat is Being Reported in SW WI

Tim Wood, the Director of the Lancaster Research Station, reported today (7/25/08) that a few wheat samples have tested positive for DON (vomitoxin) in SW WI. These samples were collected from wheat fields where low levels of Fusarium head blight (FHB) were noticed prior to harvest. The DON levels in the positive samples were 2.6 and 3.0 ppm. A usual cutoff for DON levels is 2 parts per million or lower in wheat. A local elevator was assessing dockage fees of $0.25/bu for DON levels over 2.0ppm.

Image 1. Scabby and Tombstone Kernels
(Photo courtesy of Laura Sweets)

As a point of reference the UW Winter Wheat Variety Trial located at the Lancaster Research station was our most uniform and prevalent FHB site. Our ratings at Lancaster showed a FHB index range from 0.6 to 8.8 among the varieties. This FHB index is based on a range from 0 to 100 where 0 indicates no signs of scab infection and 100 means all heads in the field are completely infected. The 0.6 rating was found in the public variety “Truman”, which has resistance to head scab.

Image 2. Spikelet infected with FHB.
(Photo courtesy of Laura Sweets)

As we continue with wheat harvest and grain delivery remember that having FHB does not automatically mean that the grain will have a concentration of DON, and second, there is evidence that healthy looking kernels can test positive for DON. Therefore, if you are concerned that there is mycotoxin contamination, consult the Pest Management in Wisconsin Field Crops – 2008 (UW-Extension, A3646) in the corn disease section where a list of laboratories that conduct mycotoxin tests is listed. DON infected wheat can be blended with non-infected wheat to reduce the overall concentration.

Please contact Paul Esker if you learn of positive finds in your county at: or 608-890-1999.

Preliminary Wheat Yield Estimates

The 2008 winter wheat harvest is a little behind schedule due to decreased heat units, delayed maturity, and green stems. The advantage of this delayed harvest however will likely be noticed once the combine hits the field. Preliminary yield reports from Dane (Dave Fisher), Winnebago (Nick Schneider), and Green Lake (Carla Hargrave) counties show winter wheat yields ranging from the low 70’s to 105 bu per acre with test weights in the 58 to 62 pound range. In our three wheat fungicide tests we have mean yields of 88, 94, and 111 bushels per acre. If the weather continues to hold look for our winter wheat variety trial results to be published by 8/8/08.

Soybean Crop Starting to Bloom

Across many areas of Wisconsin the soybean crop is just starting to bloom. According to the USDA crop progress report (7/6/08) the WI crop is slightly behind the 5 year average for bloom progress. This is due to two factors: fewer accumulated GDU’s in May and June and later soybean plantings. As we enter the soybean reproductive growth phase there are a few things to keep in mind. The first is that soybean will produce flowers for ~3 to five weeks, depending upon planting date and environment. Soybean will abort anywhere from 20 to 80% of the flowers that they produce. Generally it is the first and last flush of flowers produced that are most likely to be aborted.

Second of all the timing window for our glyphosate applications in soybean are quickly closing. Chris Boerboom, UW Extension Weed Scientist states that:

“Monsanto has clarified the definition of “throughout flowering” in regard to the latest stage that glyphosate can be applied to glyphosate-resistant soybean. Soybean flowering is defined as the R2 growth stage. The R3 growth stage begins when one of the four top nodes with a fully developed leaf has a 3/16 inch long pod. With this definition, glyphosate can be applied through the R2 growth stage, but not after the R3 stage begins. Applications made after the R3 stage are off-label applications.”

On average it takes ~ 4 days to move from R1 (beginning flower) to R2 (full flower) and ~10 days from R2 to the start of R3 (beginning pod). That means we have ~2 weeks for glyphosate applications to occur.

Lastly, wheel track damage made from ground applications may start to reduce yield. Sprayer wheel traffic from first flower (R1) through harvest can damage soybean plants and reduce yield (Hanna et al. 2008). Our research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April though mid-May can compensate for wheel tracks made when a field is sprayed at R1. Yield loss can occur, however, when wheel tracks are made at R1 or later in thin soybean stands (less than 100,000 plants per acre) or late planted soybeans. Regardless of stand, plants could not compensate for wheel tracks made at R3 (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). In our trials yield losses averaged 2.5, 1.9, and 1.3% when sprayer boom widths measured 60, 90, and 120 foot, respectively. Multiple trips along the same wheel tracks did not increase yield loss over the first trip.

For more information on spraying fungicides in soybean please see my Extension article entitled: Managing Fungicide Applications in Soybean.

Image 1. Soybean crop beginning to bloom.

Early Season Soybean Diseases: Don’t Rush to Spray

We have started to receive some questions regarding the need for foliar fungicides in soybean. In walking our research fields at Arlington and West Madison this week, we noticed evidence of brown spot, otherwise known as Septoria leaf spot (causal organism: Septoria glycines) and bacterial blight (causal organsim: Pseudomonas syringae). Given the spring conditions we have had, with cooler and wet, and now more warmer and wet conditions, this find was not a surprise. Brown spot typically occurs yearly in soybean fields and has rarely been shown to cause an economic yield reduction. Brown spot can be easily differentiated with bacterial blight in that brown spot is found in the lower and older leaves while bacterial blight can be found in the upper canopy. Research has shown that if a fungicide is applied to slow down the severity of brown spot, applications made from bloom to pod fill are the best timing.

Figure 1. Symptoms of brown spot in the lower soybean canopy at Arlington on June 23, 2008.

Figure 2. Symptoms of bacterial blight in the upper soybean canopy at Arlington on June 23, 2008.

Soybean Aphids Starting to Show Up

When scouting your soybean fields over the next few weeks pay close attention to any elevated ant activity as an indicator of soybean aphid arrival (Image 1). Krista Hamilton (DATCP Entomologist) reported finding soybean aphids in Columbia and Dodge counties last week. This morning John Gaska and Branden Furseth (UW Soybean Extension and Research Program) found soybean aphids in our soybean seed treatment experiment at the Arlington research station (Image 2) as well as at our soybean research trials located at the West Madison research station. Fall suction trap counts suggest that this may likely be an off year for aphids, however remember to scout and be vigilant as some fields may still reach threshold. For more information on soybean aphids please visit the Wisconsin Crop Manger newsletter or the Soybean Aphid publication authored by Dr. Eileen Cullen (WI Extension Field Crop Entomologist).

Image 1. Ants on soybean plant.

Image 2. Soybean aphids located at new soybean growth.