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.

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.