Pale Green Beans

As you drive across the countryside, field corn is not the only yellow crop in our Wisconsin fields. Many of our soybean acres have a pale green hue to them as well. Developmentally soybean does not start fixing nitrogen until the the V2/V3 (two to three trifoliates) crop growth stage, therefore they must rely on soil reserves for their early season nitrogen needs. Cool weather coupled with excessive rainfall has hampered nitrogen uptake by the soybean plant. Once soybean reaches the V2/V3 growth stage and nitrogen fixation begins the the soybean crop will quickly greenup. When inspecting a soybean root for nodulation carefully dig up a soybean plant and wash off the roots. Then with your knife split the nodule in half. If the nodule is firm and white the nodule is alive however nitrogen fixation has not yet begun. If the nodule is bright red as seen below in Image 1 then the nodule is actively fixing nitrogen. If the nodule is soft or black the nodule is dead. In flooded or excessively saturated soils nodulation may be delayed. Remember it takes 10 to 14 days from rhyzobial infection for nodulation to begin.

For more information on nitrogen fixation please refer to my publication entitled: Utilizing Inoculants in a Corn-Soybean Rotation

Image 1. Healthy nodule actively fixing nitrogen.


Head Scab Beginning to Show Up in WI Wheat

Fusarium head blight (scab) was detected this morning (6/17/08) during a routine survey of our winter wheat variety trial located at Lancaster WI. Given the rainfall and temperatures during flowering this was not unexpected. Since the winter wheat head and foliage are still green now would be a good (easier) time to look for head scab in your area. The easiest way to inspect a field is to stand with your back to the sun and look for bleached heads or white spikelets. Once you identify a suspect head look at the base of each spikelet for a salmon color (image 1). Be careful not to confuse head scab with with hail injury or spikelet abortion as seen in Image 2.

Image 1. Head scab in wheat.

Image 2. Hail injury to wheat.

How Long Will Soybeans Survive Under Water?

Soybeans can generally survive for 48 to 96 hours when completely submersed. The actual time frame is dependent upon air temperature, cloud cover, soil moisture conditions prior to flooding, and rate of soil drainage. Soybeans will survive longer when flooded under cool and cloudy conditions. Higher temperatures and sunshine will speed up plant respiration which depletes oxygen and increases carbon dioxide levels. If the soil was already saturated prior to flooding, soybean death will occur more quickly as slow soil drainage after flooding will prevent gas exchange between the rhizosphere and the air above the soil surface.

As a personal frame of reference we were in a wheat field today that was flooded over the weekend and we could already smell rotting tissue. That is not a good sign.

References:

Borges, R. (2004). Soybean management and excessive soil moisture.

Boru, G., T. Vantoai, J. Alves, D. Hua, and M. Knee. 2003. Responses of Soybean to Oxygen Deficiency and Elevated Root-zone Carbon Dioxide Concentration. Annuals of Botany, 91: 447-453.

Neave, S. 2002. Flooded Fields and Soybean Survival. MCCN80. www.plpa.agri.umn.edu/extension.

Wheat Disease Update For June

We are currently in the process of examining the winter wheat variety trials that are at Arlington, Chilton, Janesville and Lancaster for foliar diseases. So far, we have completed assessments at Arlington. The growth stage ranged from Feekes 9 (when the ligule of the flag leaf is visible) to Feekes 10.3 to 10.4 (almost full head emergence). From these assessments, the two main fungal diseases observed have been Septoria leaf blotch and powdery mildew. Tan spot has also been noted in some plots. However, we have also noted the presence of wheat leaf rust and wheat stripe rust. For these assessments, we measured the severity, which is the area of leaf tissue infected on the flag leaf and also the leaf directly below the flag leaf. For wheat leaf rust, symptoms were noted on 40% and 47% of plots the two leaves, respectively, with the severity ranging upwards of 10-15% in some plots. Wheat stripe rust was noted in only a few plots, and with only 3-4 pustules. We will continue to actively monitor for further rust development in the different wheat studies and locations.

Fusarium Head Scab Update

We are entering a critical period for wheat scab development in the state. The most up-to-date risk maps indicates that we have moved into a moderate to high risk in the southern portions of Wisconsin. In the past few days, we have seen temperatures move into the lower 80’s with high humidity and heavy rainfall. As our our wheat assessments late last week, the growth stage ranged from Feekes 9 (when the ligule of the the flag leaf is visible) to Feekes 10.3- 10.4 (heading almost complete). Flowering should be occurring during the next seven days or so, which is also the most favorable period for head scab development and vomitoxin accumulation. Growers should be actively scouting their fields, also for determination of wheat leaf rust and wheat stripe rust. Many of the current fungicides cannot be used after Feekes 10.5 or 10.5.2, so consult the label accordingly if you make a decision to spray. For example, the current labels for some of the representative fungicides indicate the following growth stages for last application:

Quilt: Feekes 10.5 (full head emergence)
Proline: Feekes 10.5.2 (when wheat heads are in full flower growth)
Headline: Feekes 10.5 (full head emergence)
Stratego: Feekes 10.5 (full head emergence)
Quadris: Feekes 10.5 (full head emergence)

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 coolbean.info 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, http://www.apsnet.org/education/IllustratedGlossary/default.htm).

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