Slow Growth of Crops Caused by Less Than Normal Heat Units

Heat units are a major driving force behind soybean and wheat development. Since May 1st of 2008 we are ~100 heat units behind our 20 year average and almost 200 heat units behind 2007. The long-term implications for this delay are dependent upon the rest of the growing season as we can easily catch up during a warm June. The biggest concern at this point in soybean would be early season soybean diseases. Paul Esker published an article in the WI Crop Manager discussing the potential disease issues in soybean. The biggest risk for cool wet soils would be pythium. In winter wheat the cooler than normal temperatures may lead to delayed harvest, however cooler than normal temperatures through grain fill may also lead to increased grain yield.

Rust Report for Wheat and Soybean

Wheat: Wheat leaf rust is on the increase in the Southern U.S., and recent reports have the severity > 10% in the far portions of the wheat areas (south of Missouri). Wheat leaf rust severity is in the 0-10% range, reaching northern Missouri and south to south-central Illinois. We continue to monitor our wheat plots for the presence of leaf rust. Wheat stem rust has been documented from Texas up towards southern Missouri, with severity typically less than 10%.

Soybean: Active scouting continues in the Southern U.S., but no new finds have been recently documented. A summary of the current conditions and forecast for the South indicates that large portions of the Southeast are dry, with isolated storms forecasted. Wind directions in Florida are blowing such that spore movement is being confined within the state. While winds in Texas are gusting to the north, clear and dry conditions are reducing the likelihood of spore deposition or survival. The 1-2 and 3-5 day forecasts do not suggest much change in the rain forecast or for spore movement.

Flag Leaf Emergence – Decision Time

Much of the winter wheat in the state is at the flag leaf growth stage. This is a critical time to consider the need for foliar fungicide applications. Shawn and I recently posted on the website (UWEX Soybean Extension) and to be published in the Wisconsin Crop Manager a new article that poses four questions that should be asked to help guide a decision to spray a fungicide. These questions focus on disease identification, knowing what the variety is resistant against, understanding if disease pressure is increasing, estimating if the potential final yield warrants the extra cost of spraying a fungicide, and finally asking if the crop is under stress that is non-disease related. Most importantly, these questions really emphasize the need to actively scout your fields to be able to answer the necessary questions accuractely.

Soil Temperatures and Soybean Emergence

In our soybean seed size experiment we have been monitoring soil temperatures since planting on May 8th. Mean daily soil temperatures have ranged from 52 to 60 degrees F over this time. Preliminary research suggests that soybean requires ~145 (air) and ~165 (soil) GDD‘s base 50 degree F for emergence. The large difference between these two measurements is related to the temperature swing difference between air and soil temperatures in the spring. We have accumulated ~80 GDD based on soil temps since planting. This experiment is planted at Arlington WI on worked ground.

Wheat Diagnostics: Tan Spot and Septoria Leaf Blotch

For those who have been scouting wheat this spring and also read the Wisconsin Crop Manager and Wisconsin Pest Bulletin will note that there has been reference to observations for both Septoria leaf blotch and Tan spot this spring. We have also noted these two diseases but feel it is important to discuss the differences in symptoms.

Septoria leaf blotch: symptoms start as a light green to yellow spot that is between leaf veins on the lower leaves. This is due to contact with the soil. As these symptoms elongate, they become irregularly shaped and become more tan to red-brown. A key diagnostic sign to different Septoria leaf blotch with Tan spot are black speckles on the lesions. These are called pycnidia, which is formally defined as asexual, globose or flask-shaped fruiting body of certain imperfect fungi producing conidia (Source: Illustrated Glossary of Plant Pathology,

Tan spot: symptoms are typically small tan spots that are lens-shaped, however, symptoms may be tan to brown, round to slightly elongate spots that are surrounded by a yellow halo. Often, the center spot will appear diamond-shaped. The variability in symptom type is dependent on the wheat variety.

Image Credits: Craig Grau, UW-Madison, and American Phytopathological Society Image Gallery

May Wheat Rust Outlook

In checking wheat around the state, I have found no evidence for wheat leaf rust overwintering to date. But, we need to keep a watchful eye out in the southern U.S. as wheat development progresses. Right now, wheat leaf rust has been on the increase in parts of Kansas and Oklahoma, and the weather fronts that have been moving across Wisconsin can lead to spore movement from those areas. We will continue to monitor wheat as we move closer to flag leaf emergence for wheat rust in order to make the most effective recommendations.

Photo credit: C. Grau, UW-Madison

What About Soybean Rust?

I have been monitoring reports from the southern U.S. to determine what we might see as a risk for soybean rust in 2008. Also, thanks to a message from David Fischer, UW-Extension-Dane County, we know that growers are paying attention to the possible risk for soybean rust as they consider their planting options for switching from full-season corn hybrids to either soybean or shorter-season corn hybrids. As it stands right now, soybean rust activity in the south has been quiet. The last documented detections were in mid-April and conditions in southern Texas (which would be a primary source of inoculum for Wisconsin) has been dry. Questions have been raised regarding the situation in Mexico, and to the best of our knowledge, the last documented detections there were in January. Thus, the risk for soybean rust in Wisconsin remains low. We will continue to monitor the situation in the south, especially Texas and Louisiana over the next few weeks. Supported by check-off dollars and the USDA, Wisconsin will have 15 sentinel plots in 2008 to help us in our monitoring efforts for soybean rust. Updated information can also be found at

Early Season Soybean Diseases

As I stare out my window, watching rain fall, I write this short note to remind everyone to pay attention after planting for early-season seedling diseases in soybean. The cool and wet weather favors a disease like Pythium, but it is critical to make a good field assessment to determine which soybean seedling problems may be found. Often, the effects of seedling diseases is missed because symptoms are not expressed early in the season, but over the growing season, early-season soybean diseases can impact plant vigor during the reproductive stages. In the recent April 24, Wisconsin Crop Manager, Nancy Koval, Craig Grau, and I discuss the differences between seedling diseases and how to accurately diagnose them. Furthermore, updated information is now available at regarding soybean seedling diseases, which has information summarized across a multi-state area.

Aphid Numbers in Wheat are Low

Spring aphid treatments were applied to our Barley Yellow Dwarf Virus experiment on 5/6/08 at our Arlington, Lancaster, and Janesville sites. Wheat growth stage varied among our planting dates and ranged from Zadoks 14,22,30 (Feekes 4-5) “pseudostem erection” to Zadoks 16, 29, 31 (Feekes 6) “1st detectable node”. Across all of our plots we found only one Oat Bird Cherry aphid. The established spring threshold for this pest is 50 aphids per linear foot of row.

Oat Bird Cherry Aphid

Drawing by Dennis Murphy – MU

Scouting for Wheat Diseases

Winter wheat in much of Wisconsin is around the jointing stage.  This is an excellent time to start scouting for wheat diseases, especially those that are known to overwinter in Wisconsin like Septoria and Powdery mildew.  We have detected those two diseases in our initial scouting around the state, but their presence is highly dependent on the wheat variety.  In our scouting efforts, we have found these diseases in more susceptible varieties.  Severity of Septoria and Powder mildew is still very low at the moment.  Have a look through the April 10 Wisconsin Crop Manager for two articles that describe scouting for wheat diseases and also the decision process for understanding the risk for a disease outbreak and the use of foliar fungicides.