Volume 6, No. 8, October 2003

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Index


Dear Reader,

Hurricane Isabel topped the news in September. Here in Suffolk, trees were down, power was out, and our crops suffered significant damage. The TAREC finally got power restored 1 week after the storm. Generator power enabled us to run the pump on our well, a few lights, computers, and phone system. Unfortunately, there are still many without power and putting up with damaged roofs.

Wet weather and field conditions once again seem to be a challenge. If the wetness continues, we could face similar problems as last year. Symptoms of certain disease are already showing up. I've dedicated this issue to the hurricane and those disease concerns.

I also want to remind everyone that the Virginia Soybean Association will be sponsoring a soybean yield contest. There is some good yield potential out there in spite of the adverse weather. If interested, contact your local County Extension Office for details and forms.

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Hurricane Isabel Will Likely Lower Soybean Yields


Hurricane Isabel roared into Virginia with a vengeance that will likely lower yields of many of our crops. Although not damaged as bad as other crops such and corn and cotton, soybean yields will likely suffer where severe lodging has taken place. It looks as if southeastern Virginia has the most damage. I've evaluated our variety tests in Suffolk, Painter, and Blackstone. Suffolk is clearly the most damaged with most of the crop severely lodged (> 45O from vertical), followed by Painter with plants leaning slightly to moderately, and Blackstone with only slight lodging.

Things could have been much worse. Yield is most severely affected by lodging when the lodging occurs in the R5 (beginning seed) development stage. Most of our soybean crop was in the R6 (green seed filling the pod cavity) stage or later. Although yield is still affected at R6, yield losses are only half as severe at this stage. However, there are many July-planted soybeans that are only in the R5 stage throughout the state. This late-planted crop was usually much shorter and did not have the severe lodging that the more full-canopied crop that was planted in May or June. Therefore, I'd expect a similar yield loss for the late-planted crop.

So, what's my estimate on the amount of loss that we'll incur? First, we have to distinguish physiological yield loss from combining loss. Combining loss can vary anywhere from 3-10% depending on many factors. In some cases, we may have to harvest the most severely lodged soybeans in one direction. But, with that said, I expect the soybeans to stand up quite a bit as soon as leaves begin falling. I've even seen some recovery only after a week.

There is little data on physiological yield loss, but what's out there seems to be pretty consistent. What do I mean by physiological yield loss? That's the loss in yield from lodging if all of the soybeans that are now on the plant can be harvested. In controlled studies in which the researchers simulated lodging and compared it to a crop that was artificially supported, losses have ranged from 0 to over 30%. Why such a range in yield loss? It depends on the severity of lodging and the stage of development in which the lodging occurred.

Let's first address the severity of lodging. Soybean researchers have traditionally rated lodging on a scale of 1 to 5 as follows:

1.0 = almost all plants erect

2.0 = either all plants leaning slightly, or a few plants down

3.0 = either all plants leaning moderately (45O angle), or 25-50% down

4.0 = either all plants leaning considerably, or 50-80% down

5.0 = all plants down

As you may expect, a rating of 4.0 to 5.0 is very severe lodging. I have seen this in a couple of locations, but at this time I'd rate most of the lodging between 3.0 and 4.0. Yield loss will be minimal unless most plants are leaning at a 45O angle or more. Otherwise, yield losses can range from 10-35%, depending on the stage in which the lodging occurred.

Why does lodging cause yield loss? It's not completely clear, but the generally accepted reason is a reduction in net photosynthesis. With less photosynthesis, there is less energy going to the developing pods and seeds. When plants are lodged, relatively less of the upper leaves and more of the lower leaves are exposed to sunlight. The upper leaves are more photosynthetically active and the lower leaves are less active. When lodging occurs, the entire energy-producing mechanism is disturbed. In other words, we are now exposing less of the most productive leaves and more of the least productive leaves to the sun. So, yield will decline.

So, let's assume that lodging rated above 3.0 will cause a 10-30% loss. Now the severity of the yield loss will depend on the development stage that the soybean plant was in. As I said earlier, there's little hard data on this subject, but a few older experiments give us some information. In a study conducted in 1972-73, S.J. Woods and M.L. Swearingin of Purdue University indicated that the R5 stage was the most critical time for lodging to occur. At this stage, yield was reduced by 18-32%. At stages R3 and R6, yield was reduced by 12-18% and 13-15%, respectively. Details of that experiment shown in the next column:

In that study, the plots were manually lodged with a long aluminum bar at the indicated soybean stage. Although lodging ratings were not given, I would consider it to be in the 3.5 to 4.0 range. Two varieties were tested. ŚCorsoy' was more susceptible to lodging, but was able to branch more; therefore, it yielded higher when lodged. ŚWells' is more resistant to lodging, but did not branch as much; therefore, was unable to compensate as much for the lodging. In the natural lodged plots, only slight (2.0 or less) lodging occurred.


From the above data and a few other studies, I'd estimate that where we had moderate to severe lodging and the soybean were in the R6 stage, we'll probably reduce our yields by 10-15%. Less lodging will result in less yield loss. If the plants were in the R5 stage and lodging was severe, then losses could be 20-30%. But, as I said earlier, the later-planted soybeans are not as lodged due to their smaller height.

One more thing must be mentioned. If the soybean were in later stages, then yield loss will be less. Our full-season maturity group 3 soybeans are completely mature (R8) and many of the maturity group 4 soybeans are physiologically mature (R7; one pod on the plant has reached mature color). Once a plant reaches physiological maturity, 100% of the dry matter has accumulated; so there will be no yield loss. Plus, the plants with fewer leaves lodged less. On the other hand, nearly all of our double-crop plantings will be affected.

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Cercospora Blight Showing Up Again in 2003


Due to the continued wet weather, egged on by Hurricane Isabel, I am once again seeing a significant amount of Cercospora Blight showing up in our soybeans. The most striking symptom is the light purple areas that develop on the leaves that are most exposed to the sun. The above photo taken from our soybean variety test in Painter shows the symptoms. (The photo also shows some marginal leaf desiccation; this is not Cercospora, but salt spray damage from the hurricane.) This discoloration can spread over the entire upper surface of leaves, giving them a leathery texture. As the disease spreads, defoliation can occur on upper part of the plant. Some may mistake this for natural leaf senescence, but it is not. The lower leaves will remain green. Remember, this is just opposite of the natural senescence process. You may even think that some type of insect is defoliating the plant. You could also see some reddish purple lesions develop on the leaf petioles. Lesions on the petioles tend to hasten defoliation.


Significant yield losses can occur on heavily infected plants. This is the same disease that causes purple seed stain. However, infected plants do not insure that seed will be stained. Seed staining does not reduce yield, but heavy seed discoloration can lead to dockage.

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Harvest Soon After Maturity to Avoid Seed Decay Problems

The warm and humid conditions that we're now experiencing are setting us up for another year of poor seed quality. Aside from the potential for purple seed stain, we risk seeing Phomopsis Seed Decay raise its ugly head again in 2003. Phomopsis Seed Decay arises from the disease, Pod and Stem Blight, which develops primarily after maturity and the crop begins drying down. Two fungi cause this disease: Phomopsis longicolla and Diaporthe phaseolorum.

We usually don't have a problem with the disease progressing to the seed decay phase unless an early maturing variety is used, conditions of warm and wet weather are present during late seed fill, and harvest is delayed. Diseased seed will have a white, chalky, and shriveled appearance. They are often cracked and elongated. They may not germinate or are delayed in germination. Infected seed produce lower quality oil and meal. Seed may not show symptoms, but still be infected and lead to damping off the following spring. Severely infected seed can lead to grade reductions and lower prices. General overview and symptoms. The fungi can be found in most Virginia soils and survives on crop debris. This year, we had a significant amount of seed infected with this disease. Fortunately, most used a seed treatment to help control the disease. Infected seed is an important, but not the primary way that the disease is introduced. Still, infected seed can result in infected plants.

The fungus first colonizes the seed coat, then the cotyledons (seed leaves). Unless young seedlings die from damping off, the disease remains dormant until seed have formed and the plant is close to maturity. Occasionally, plants will die from the disease in the early reproductive stages, but this is rare. Infection spreads to other parts of the plant in the form of small, one-celled spores called conidia. These spores are splashed by water onto other parts of the plant where infection is initiated. The disease remains dormant and only after plant tissue dies or the crop matures will you begin to see small speck-sized picnidia (fruiting bodies), usually arranged linearly up and down the stem. The black picnidia can also be limited to small patches, commonly near the nodes. The extent of spread of picnidia is related to the environment. During a wet fall, the disease will produce picnidia simultaneously over the entire plant; during dry falls, the picnidia will be localized near the soil or at nodes. These picnidia will also be found on the pods. Where spores are splashed onto developing pods, seed can become infected. Most seed infection occurs during or after the R7 (yellow pod) stage. Not all infected pods produce picnidia, but mature pods with picnidia will contain infected seed. I have seen some picnidia begin to form on our earliest maturing varieties that are already mature.

Prolonged periods of wet and warm (>70O) weather during seed maturation will favor the spread from the pod to seed. The disease if more prevalent on pods located at the bottom of the stem. More seed decay occurs from plants deficient in potassium (K), virus infected plants, and plants heavily attacked by insects. The longer that harvest is delayed, the heavier the infestation.


Control/Prevention. There is little you can do to prevent this disease from occurring. As I stated earlier, it is endemic to Virginia; it resides in the soil and on infected residue. Later maturing varieties will delay maturity until a cooler part of the harvest season when rate of infection is slowed. Using a double-crop soybean system will also delay maturity into a cooler part of the fall. Most importantly, harvest in a timely manner. The disease will become worse as harvest is delayed.

It looks as if the weather is setting us up for another year of poor seed quality. I hope I'm wrong. Maybe, the weather will change and we'll dry off some. One real problem is the wet soils that will not let us into the fields. At the time of writing this article, they're calling for more rains over the weekend. Timely harvest may be a challenge. But, give it your best shot. Harvesting soon after maturity can prevent seed quality problems.

For more information on Pod and Stem Blight and Cercospora Blight, refer to Soybean Update Vol. 5 No. 7. It can be found on the web at: http://www.vaes.vt.edu/tidewater/soybean/soybeanup/0210/0210.html

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Be On the Watch for Soybean Rust

Soybean rust has not yet been found in the United States, but has caused serious yield losses in South America and Africa. Many are now saying that it's not if we will have a rust problem, but when. The disease could be a significant problem to an already marginally profitable soybean crop. Early diagnosis and treatment is critical. As of right now, there are no varieties on the market that have resistance to the disease, but research is continuing. It can be controlled with timely fungicide applications, but these only add to the cost of growing the crop.

Early symptoms appear as small lesions on the leaves that become brown to reddish brown. It can be easily confused with other diseases, such as bacterial pustule. Under closer examination, tiny bumps can be found on the lesions. These bumps are spore structures called uredia and give the leaves a rusty look. Rust-infected leaves eventually turn yellow and fall off. Infected leaves cause the crop to have fewer pods and seeds and lower seed weight.

Masses of microscopic urediospores are released from the uredia and are transported to nearby plants and fields. The fungus spores can be transported for some distance by wind.

Below are some symptoms to look for. These photos were taken from the USDA Animal and Plant Health Inspection Service (APHIS) web site. This is a very good web site for more details on this disease. The web address is: http://www.aphis.usda.gov/ppq/ep/soybean_rust/.


If you suspect rust in your field, contact your local County Extension Agent. The agent can collect leaf samples and send to Virginia Tech's plant disease diagnostic lab for confirmation.

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Sincerely,

David L. Holshouser


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