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Our double-crop soybeans are another story. There are many areas where they are just not growing off. There's simply too much water. I hope the rain will slack up (but not shut off) and the sun will return. With some sunshine and soil drying, I would expect to see a doubling of growth in a short amount of time.
Insects and diseases are of concern this time of year. While the traditional corn earworm problem may not be as bad as we first thought, stink bugs are numerous. We haven't found soybean rust, but some unexpected diseases such as frogeye leaf spot are heavier than normal.
It's just another day in the life of farming in Virginia; there's always something to keep us on our toes.
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We are experiencing more stink bugs than normal in our soybean crop. We are finding two species, green and brown, and both undergo about 3-5 generations per year. They feed on a lot of different weed hosts and crops, moving from one to another to seek the freshest food source. Populations have been unusually high this spring and summer, with large numbers in corn and cotton. Now they have moved into soybeans. They damage soybeans by their feeding when they pierce the pod and seed to feed on plant juices, simultaneously injecting digestive enzymes. Feeding can cause shriveling and abortion of young pods, and shriveled discolored seed on older pods. Both seed quality and yield can be reduced if populations exceed threshold levels. Our threshold is set at 1 stink bug per foot of row (if using a drop cloth) regardless of row spacing, or 2.4 to 3.6 per 15 sweep net sweeps in 7 to 21-inch, or greater than 21-inch row spacing, respectively. Stink bugs are the primary soybean pest in Georgia and their thresholds are higher, set at 2 per foot of row when seeds are just forming, and 3 per foot of row after mid pod fill. Stink bug feeding can also result in maturity delay. Pod damage during pod set to pod fill can slow maturity and result in green fields or areas of fields at harvest. We are recommending that growers scout fields using either the drop cloth (in 21-inch or wider row spacing) or the sweep net (row spacing less than 21 inches) and consider treatment if thresholds are met. We also recommend using pyrethroids. Pyrethroids will do a good job on any corn earworm populations, a good job on green stink bug, and a fair job on brown stink bugs if higher rates are used. To maximize control, use the high rates listed for stink bug (Ex. Karate at 1.92 oz, Baythroid at 2.8 oz, Mustang Max at 4.0 oz, Prolex at 1.54 oz).
Update on corn earworm. So far, we seem to be escaping any major problems with corn earworm in soybeans. We have scouted many fields in southeastern counties and so far have found none at, or even near, threshold. With these two newest tropical storms and the heavy rains, we may not see much develop. But, there is always the potential as moths are still being caught in blacklight and pheromone traps. Even in low pressure years, some fields will get hit. We recommend that growers search fields carefully over the next 2-3 weeks just to be sure.
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The above report was a little surprising. The estimate came in 88 million bushels below the pre-report trade guesses and down 63 million bushels from USDA's July projections. The Midwest is expecting big yield increases from 2003. For instance, Iowa's estimate is up 10 bushels/acre from last year's poor-performing 32 bushel crop. Other states are predicting similar increases.
Some growers are not so optimistic however. Problem areas are occurring throughout the Midwest. Although soybean aphid has not been much of a problem, late planting and cool wet conditions have delayed the crop in the Dakotas and Minnesota. If yield projections are to be realized, the first killing frost will have to hold off until mid-October. From my experience in northeast Nebraska, this would be unusual. So, keep a close eye on the weather in the upper Midwest. Unexpected surprises could move the markets.
A web site that I'd suggest you look into regarding markets and conditions for all Midwestern crops is www.agweb.com. I particularly like to track the Crop Comments section of this site where growers give reports on their crop. By reading several of these comments, you can sometimes get a pretty good reading of what's happening in the Corn Belt. To go to this web page, click on the AgWeb News button on the AgWeb.com home page. Then look for AgWeb Crop Comments. I would also encourage you to participate by sending in your comments.
In Virginia, we are expected to harvest 490,000 acres, virtually the same as 2003. Predicted yields are now pegged at 36 bushels per acre, up from 34 bushels per acre last year. If realized, Virginia will produce 17.6 million bushels of soybean, which is near record levels (at least over the past 15-20 years).
However, keep in mind that the most critical period of soybean development will take place within the next month. Although it seems as if we could never run out of water this year, it only takes 10-14 days of no rain to place soybeans under drought stress on some soils. Therefore, let's not count our chickens before they hatch. Still, the season looks as if it's shaping up into a good one.
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Pathologist A survey of soybean fields in Virginia for early detection of soybean rust has been underway since June 14, 2000. The survey was funded by the Virginia Soybean Board to provide an early warning of disease outbreaks. Currently, soybean rust is known to cause major damage to soybean in the South American countries of Argentina, Brazil, Argentina, Paraguay and Bolivia. No occurrences of the disease have been reported in the United States or north of the equator in the western hemisphere.
Leaf samples from soybeans showing symptoms of various diseases were collected from fields in 27 counties across eastern Virginia from June 14 through August 16 (see Fig. 7.6.12). A total of 100 samples were processed in the plant pathology lab at the Virginia Tech, Tidewater Agricultural Research & Extension Center in Suffolk. The most common diseases were downy mildew, frogeye leaf spot, brown spot, and anthracnose.
The good news is that none of the samples showed symptoms or signs of soybean rust. A somewhat alarming concern, however, was the heavy occurrence of frogeye leaf spot on several field samples. As we enter the final phase of the growing season when pods are being filled and maturation begins, foliar diseases will have their greatest impact on yield. This is also the time when soybeans are most susceptible to soybean rust. Surveys for the disease will continue until normal defoliation and maturity of the crop.
Below and on the following page are some photographs of samples that were processed in our lab. Note that some could be confused with rust to the untrained eye. Keep in mind that rarely can a disease be diagnosed by visual observation in the field. Only a trained plant pathologist with the appropriate laboratory equipment can verify the presence of a disease. The below photographs should only be used as a guide, but not for confirmation of the disease.
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*All photographs taken by Nathan O'Berry, scout for Virginia's soybean rust & aphid monitoring program
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Frogeye leaf spot is one of the diseases that we are seeing in quite a few fields throughout the state and is one that we should be concerned about. The disease can be a yield robber, but information on thresholds is rather sparse. In the Delta states, they commonly have this disease in combination in aerial blight and Cercospora leaf blight. There, fungicide applications are not uncommon. From the limited information that I could get regarding thresholds, it seems that 5 to 10 leaf spots per leaflet in combination with weather predictions that promote disease development would trigger a spray. However, I don't think that this represents the entire picture.
The disease basically causes the loss of leaf area. So, the yield reductions are due to the loss of photosynthetic material. In my opinion, we should probably treat this disease much like we treat insect defoliators. However, unlike insect defoliators, dry weather can stop the disease in its tracks. Sometimes you'll see lower leaves that show symptoms of frogeye, but new leaves not infected. This is not the case this year; the disease has continued to spread and develop. Still, if we would use the insect defoliation thresholds as a guide, we might be able to predict some yield loss.
If loss of leaf area is the problem, then the size of your canopy should make a difference. This should relate well to our leaf area index (LAI) work of the past. We generally have large full-season canopies this year, so I would think that we could tolerate 15-20% leaf area loss. Double-crop plantings don't have as large of canopy, so we would probably only tolerate 10-15% leaf area loss in that system. One other point regarding leaf area should be mentioned. When lesions are numerous enough to cover about 30% of the leaf area, a blighting phase often will occur. Leaves will then wither quickly and fall prematurely. Leaf drop of course results in 100% loss of the leaf area; therefore yield loss under this high level of infection would be much greater. Yields have been reduced 30% when extensive leaf blighting occurs. You don't want to let the disease progress to this stage.
So, should you be treating for this disease? This is not an easy question. Remember, most fungicides are preventative in nature. In other words, they will prevent further disease development, but not cure what's there. Therefore, applications after significant disease development will limit any yield response to a fungicide. Another thing to keep in mind is the soybean development stage. If soybeans have moved beyond the R6 (full seed) stage, then fungicide benefits will be limited. But, if you are seeing significant amount of frogeye leaf spot (5 to 10 spots/leaflet), the soybeans are in the R2 (full flower) to late-R5 (beginning seed formation), and the future weather predictions indicate a continuation of what we've experienced so far, then a fungicide will likely be beneficial.
More important than the fungicide application is the soybean variety that you're using. If you have the disease, note the variety. In next year's "Characteristics of Soybean Varieties Grown in Virginia", I'll include frogeye resistance. Another very important option is to rotate out of soybeans for at least 1 year (2 years would be best). While plowing under residues is another option, such a practice causes more long-term problems such as soil degradation and erosion.
In summary, the following steps are suggested to manage frogeye leaf spot:
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Sincerely,
David L. Holshouser
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