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In general, I think we'll have a good crop. Not a bin-buster mind you, but a good crop. The recent rains were refreshing and should help guarantee a good yield.
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The delayed planting of soybean in Virginia has led to a crop that varies widely in development. Usually, by this time, most of our full-season plantings would be past or approaching the R6-stage (full-seed; seed touching each other in the pod). Our double-crop plantings would normally be in the mid seed-fill stage (R5). However, many fields originally planned for May planting gradually became a June planting and many double-crop soybeans were not planted until the middle of July or later. Although soybeans are generally photo-period sensitive (flowering commences when day length is short enough), a delay in planting will still delay development somewhat. See the August issue of the Virginia Soybean Update for more details.
Because of the wide variation in development stages of Virginia's soybean crop, we must remain vigilant in scouting to soybean insect pests, primarily corn earworm. By the time soybeans reach the R6 stage of development, corn earworm is no longer a major threat. The pods become tough enough to resist most damage. Before this stage, soybeans are very susceptible. Because of late plantings, many of our fields are still susceptible.
The photograph above was taken on Sept. 4 from plants grown in the soybean variety test in Suffolk. Note that the maturity group (MG) 5 soybeans that were planted in May are at a fairly advanced stage. The earlier MG 5 on the left is at the R6 stage and is probably safe from corn earworm. The later MG 5 is not quite as advanced, but should be safe in the near future. Now, notice the June plantings. The MG 3 is also in the R6 stage, but the MG 4 and 5 varieties are only in the R5 stage and will continue to need protection for couple more weeks. Now, for the real problem: The MG 4 soybean on the right is still in the pod development stage (R4). Few seeds have begun to form. These plants are still highly susceptible to pod feeders. MG 5 varieties are in an even earlier stage of development. Therefore, keep a close watch on these late-planted soybeans.
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Drs. Ames Herbert & Sean Malone
As anticipated, we are experiencing a late season corn earworm moth flight. Reports from some of the pheromone traps operated by us and several cooperating VCE ANR Agents (Wes Alexander, Glenn Chappell, Keith Balderson, Matt Lewis, and Paul Davis) are indicating catch numbers exceeding any reported earlier in the season. We expected this based on the very-spread-out maturities of the field corn crop - and at this point, some of these moths could be moving out of row crops like soybean and peanuts where they developed as an earlier generation. I fully expect to see corn earworm moths flying until our first killing frost. But the important question is, what threat do they present?
It is rare to see worm populations build up in soybeans in September, even though moths are active. I say rare, but it has happened. Last year many soybean fields did experience a late season onslaught of worms and had to be treated, some for the second time. Fewer fields have been treated this year compared with last year, so fewer have been cleaned of the many beneficial insect populations. That is a good thing and means that beneficials are still available to offset infestations by preying on any deposited corn earworm eggs or small worms. Fields that have been treated will be at greater risk and should be scouted.
There are a lot of soybean fields still at risk to corn earworm attack - many with pods that are not fully developed (see page 1). Much of the full season crop is at or near the point where fields are no longer attractive to moths. Basically, once seeds are full sized and pods toughen up, they are no longer at risk. On the other hand, much of the double-crop planted fields have pods with seeds just beginning to enlarge, and are at risk. These fields should be scouted until moths stop flying, or pods become mature.
We are not hearing of a lot of fields that have reached actual thresholds. We have looked at a lot of fields in the southeastern counties and have not found many at threshold. Calls I am getting from field consultants are much the same - few fields at threshold, but some. Matt Lewis, VCE ANR Agent for King William and King and Queen Counties recently reported that "Corn earworm seems to be much worse in the lower part of KQ and KW counties (West Point and Shacklefords)." "Most farmers have been spraying down there. Worm activity is spotty in the central part of the counties." "We are not seeing much at all in the upper portions of the counties (St. Stephens & Central Garage)."
Bottom line: keep checking until moths stop flying and beans are matured. That is the only way we can be sure of finding fields at risk and keeping spray costs to a minimum.
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Most parasitic nematodes are microscopic roundworms that inhabit the soil and feed on roots. In Virginia, commonly found nematodes include: soybean cyst, northern and southern root knot, sting, stubby root, lesion, ring, lesion, stunt, spiral, and lance. Symptoms vary, but usually include stunting, yellow and/or necrotic leaves, root lesions, root galls, and a general reduction in the size of the root system. These symptoms are easily confused with nutrient deficiencies. For instance, a cyst nematode infestation could easily be confused with manganese deficiency. Do not use above ground symptoms as the only diagnostic tool. Misdiagnosing a nematode problem as a nutrient deficiency could lead to unnecessary sprays and more importantly a future unsolved problem. Above-ground symptoms are many times an indication of a below-ground problem. Accurate diagnosis of nematodes can only be accomplished by a thorough examination of root and soil samples.
Nematodes should always be suspected if plants are stunted and showing any above-ground symptoms, especially if the areas affected are in spots in the field that are elongated in the direction of tillage or planting. Nematodes cannot move very far by themselves. Human activity such as tillage tends to cause most of the spread. If nematodes are suspected, dig the roots from around the affected plants with a spade or small shovel. Never pull the plants from the soil. This will dislodge roots, nodules, cysts, and galls that are important for diagnosing any problem. Closely examine the roots for dark lesions, overall browning of the roots, galls, and cysts. You will likely need a hand lens for proper examination. If any of these signs or symptoms is present, then further diagnosis by a plant pathologist will be necessary to confirm a nematode infestation.
Two of the most common nematode species in soybean are cyst and root knot. Soybean cyst nematode is a big problem throughout the U.S., while root knot nematode tends to be confined more to the southern part of the country. Soybean cyst nematode can be controlled by rotation to a non-susceptible crop, which is any crop other than soybean. Also, planting soybean in double-crop settings helps since the crop is in the field for less of the season. But, never think of small grain as a rotational crop, unless you do not plan to plant soybeans following its harvest. An effective rotation would be at least a full year out of soybeans (i.e., rotate with corn or cotton). In addition, there are soybean varieties resistant to this nematode. The adult females and cysts of this nematode can sometimes be found on the outer surface of soybean roots. These small lemon-shaped structures can be seen with the aid of a hand lens.
There are several species of root knot nematode, two of which are common in Virginia. The northern root knot nematode also attacks peanut, but not corn and cotton; therefore is primarily found in the peanut growing region of Virginia. Southern root knot can infest cotton and corn, but not peanut; therefore, is usually found in soybean rotations that contain these crops. The presence of galls is the characteristic symptom of root knot nematode. The size of the gall will depend on the number of nematodes in the gall.
Diagnostic Services. Virginia Tech's Plant Diagnostic Clinic provides root and soil analysis without charge for identification of crop damage by nematodes. Growers are encouraged to work with their County Extension Agents to collect plant and soil samples from the affected areas. Each sample must be submitted with the proper nematode diagnostic form. Detailed information about the problem should be stated on the form, as this will aid in the diagnosis and recommendation.
Collect living plant and soil samples from the affected area of the field. Do not submit dead plants, since nematodes will only feed and reproduce on living plants. Soil samples should be taken to a depth of 8 inches. Protect both from direct sunlight and heat. Ship plants in loosely folded plastic bags to retard drying. Place and secure soil in plastic or water-tight bags; the soil needs to retain its moisture. Nematode sampling bags are available from your County Extension Agent. If shipping by mail, do not mail near the end of the week because plant and soil samples that sit in a hot post office over the weekend will destroy both nematodes and plants. An overnight courier or hand delivery would be best. If samples must be stored overnight or for any length of time, then place in a refrigerator.
Predictive Nematode Assay. Another method for diagnosing nematode problems, although not as good as the one described above, is a predictive nematode assay. This only requires soil samples and is used to evaluate the risk of nematode damage for the coming year. This would be a good method if you suspect an infestation in a particular field, but that field is currently in corn or another non-susceptible crop. It is also useful in determining if the non-susceptible crop has been beneficial in reducing nematode populations. If a nematode problem is suspected in soybean and soybean are currently being grown, I would encourage you to submit plant and soil samples as described in the preceding paragraph, not just a predictive assay.
The best time for collecting predictive samples is following harvest of the previous crop, but not after mid-November. Information on proper sampling techniques and sample bags can be obtained from your local County Extension Office. A fee of $11 for vermiform nematodes and $19 for vermiform and cyst nematodes is charged for each sample.
Collect 20 to 25 soil cores with a soil sampling tube to a depth of 8 inches in a zigzag pattern over the affected area. Affected area should not measure over 4 acres in size. If only portions of the field are sampled, then focus on the lighter, sandy-textured areas where nematodes cause more of a problem. Once again, I remind you that nematodes will die under hot and dry conditions. Therefore, ship the samples immediately or store in refrigeration.
Nematode Thresholds for Soybeans.
The predictive nematode assay gives the number of nematodes in a given volume of soil (usually 500 cc). But, just because nematodes are present does not mean that they are a problem. Like insects or other pests, nematodes have a threshold where they are or are not an economic problem. Thresholds for various nematodes have been established in Virginia for soybean and other crops. The decision to implement a management strategy is based on the risk level assigned according to the numbers and kind of nematode. The table below lists the risk level for yield loss according to the population density of a specific nematode.
| Risk code based on nematodes/500cc soil (Aug. 1 to Nov. 20)* | |||
|---|---|---|---|
| Nematode | A (Little/None) | B (Moderate) | C (High) |
| Root knot | <50 | 50-170 | >70 |
| Cyst juveniles | <20 | 20-60 | >60 |
| Dagger | <100 | 100-300 | >300 |
| Sting | 0 | 10-20 | >20 |
| Lesion | 100 | 100-300 | >300 |
| Ring | <200 | 200-700 | >700 |
| Stunt | <300 | 300-1000 | >1000 |
| Spiral | <1000 | >1000 | ---- |
| Stubby root | <90 | >90 | ---- |
| Lance | <300 | 300-500 | >500 |
| * A = nematodes are not likely to cause crop damage * B = borderline populations in which crop damage may occur if other factors stress the crop * C = populations that are likely to cause crop damage and a significant loss of yield |
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If the results of the predictive sample indicate a population that is likely to cause yield loss, then control methods will need to be implemented the following year. These methods may include: crop rotation to a non susceptible crop, moving to a double-crop soybean system, using resistant varieties, and/or chemical control. Contact you local County Extension Office for more details on control practices.
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Ohio Soybean Diseases - http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/soybeans1.htm
Soybean Disease Atlas - http://cipm.ncsu.edu/ent/SSDW/soyatlas.htm
Arkansas Soybean Notes (newsletter) - http://www.aragriculture.org/News/soybean_notes/default.asp
(See Sept issue for coverage of soybean diseases)
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Glyphosate
Use Rate: 1.0 to 3.0 lb ai/A
Roundup Ultra Max: 0.8 to 2.4 qt/A
Touchdown: 1.0 to 3.0 qt/A
Others: as specified by label; surfactant may be needed.
Application Instructions and Remarks:
Apply after all pods have set and lost all green color. Allow a minimum of 7 days between application and harvest. Do not graze or harvest treated hay or fodder for livestock within 25 days of last pre-harvest application. Do not apply more than 26 fl oz of Roundup Ultra Max by air. Pre-harvest applications are not recommended for soybean grown for seed as a reduction in germination and vigor may occur.
Paraquat
Use Rate: 0.25 lb ai/A + surfactant
Gramoxone Max: 0.67 pt/A
Application Instructions and Remarks:
Apply in 20-40 gal/A by ground equipment and 5 gal/A by air. For indeterminate varieties (usually MG 4 or less), apply when at least 65% of the seed pods have reached a mature brown color or when seed moisture is 30% or less. For determinate varieties (usually MG 5 or greater), apply when plants are physiologically mature (one normal pod on the main stem has reached a mature color, at least _ of the leaves have dropped, and remaining leaves are yellow). Once physiologically mature, soybeans have accumulated 100% of their dry matter; therefore, yield loss will not occur. Immature soybeans will be injured. Do not apply within 15 days of harvest. Do not graze or harvest for forage or hay. Mature cocklebur, especially drought-stressed plants, is tolerant to paraquat and desiccation may not be complete.
With aerial applications, observe caution and consider the addition of drift control agents. Both herbicides are non-selective and will damage other plants.
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Sincerely,
David L. Holshouser
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