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I bring this up because fall is a good time of the year to be planning ahead for next year. Take note of problems, sample your soils and for nematodes if necessary and just think of ways to make your operation more efficient and productive. In the end, it'll likely pay off.
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Over the last couple of weeks, I've brought my truck to some rather abrupt stops as I was driving down the road. What caught my attention? At first, you might mistake it for the normal maturation process, but at second glance, you notice that the lower leaves are still green (lower leaves usually yellow first, not last). Plus, the color just didn't look right. Instead of yellowing, the crop seemed to be browning. It looked like the effects of an early frost.
After pulling over and getting into the field, I recognized the symptoms as frogeye leaf spot, a disease that we've seen a lot of in Virginia this year. The unusual thing about these fields is that I'd estimate that well over half of the leaf area was covered with lesions. I've never seen the disease this bad. Upon examining the rest of the plants, I found that there are a sufficient number of pods, but the seed were extremely small. This was not unexpected after seeing the leaf damage. Once soybean reaches the R6 stage (green seeds completely filling the pod), yield loss due to stress is considerably less than in earlier stages. Stresses will however cause significant yield reductions for about 10-14 days after R6. This is mainly by reductions in seed size, but plants can still drop pods. Only after the seed and pod has lost all of its green color has the plant stopped adding mass to the seed.
Unfortunately, there is nothing to be done to salvage this year's crop. Yes, an earlier fungicide application would have helped. But, hindsight is always 20/20. I never suspected that we'd get this much damage from the rather light early infestations.
Still, there is something that we can do. First, rotate away from soybeans for at least 2 years. Secondly, many varieties have some resistance to frogeye leafspot. So, if you have one of these fields, then note the variety. And let me know; I'll flag those varieties in my future recommendations.
Will we continue to have problems with this disease? That's the million dollar question. Although it has occurred in the past, and will continue to occur in Virginia, this year's warm, humid weather was the reason that it's been so destructive. Will we have a similar year next year? Other than disease, it's been a pretty good year for row crop yields. So, I don't wish dry weather on anyone. We can be ready with rotation, variety selection, and fungicides if needed.
The following recommendations for controlling frogeye were listed in the August newsletter, but I'll list them again:
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Nematodes should always be suspected if plants are stunted, maturing early, or 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.
You may also want to consider a predictive nematode assay. This requires soil samples and is used to evaluate the risk of nematode damage for the coming year. It can also be useful to determine if a non-susceptible crop has been beneficial in reducing nematode populations. If a nematode problem is suspected in soybeans and soybeans are currently being grown, I would encourage you to submit a sample.
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.
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 | |||
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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Phomopsis Seed Decay. This disease arises from the disease, Pod and Stem Blight, which develops 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.
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.
Black picnidia forming on soybean stems & pods
There is little you can do to prevent this disease from occurring. 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. Harvesting soon after maturity can prevent seed quality problems.
Purple Seed Stain. Unlike Phomopsis seed decay, purple seed stain is not more prevalent in earlier maturity groups nor does it result in small and shriveled seed. Still, germination will likely be delayed when the area of discoloration exceeds 50% of the surface. Seed with nearly 100% of the surface discolored tend to have lower oil content and higher protein versus unstained seed.
The causal organism is Cercospora kikuchii. The fungus can infect soybean seed, pods, stems, and leaves. However the stem, leaf, and pod infection, known as Cercospora blight and leaf spot, is not strongly correlated with the purple stain found on the seed. Symptoms are, as the name implies, a pink to purple to dark purple staining of the soybean seed coat. The discolored area will range from specks to large blotches that may envelope the entire seed. Interestingly enough, infected seed may not show any of these symptoms.
Cercospora blight and leaf spot symptoms generally appear at the R5 (seed development) stage and afterwards. The upper leaves will take on a purplish appearance. Reddish purple, angular to irregular lesions will occur later on the upper and lower surface of the leaves. High levels of infection will result in a premature defoliation of the upper leaves. This could be mistaken for senescence, but the normal progression of soybean senescence is from the bottom up. Reddish purple discoloration and lesions on the stems and petioles will also be noticed. The photograph below shows the infected stems and petioles. Although this photograph shows staining of the seed, foliar and seed symptoms are not always correlated.
For control, rotate soybean with a non-legume crop and plant high-quality seed, free of visual staining. If seed show symptoms, treat with a fungicide. For seed production, fungicides can be applied during the pod and seed set development stages.
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We Need Your Help!
The program committee needs your help in planning the winter soybean conference. We want to include topics that will increase your soybean profitability. Please take a moment to fill out this form and return it to the Virginia Soybean Association or to David Holshouser. This is your meeting! Your comments will be given serious consideration.
Please rank [1= not very important; 5 = very important] the following topics:
1. Increasing Soybean Yields _______
2. Marketing Your Soybean Crop _______
3. Weed Management _______
4. World Soybean Outlook _______
5. New Uses for Soybeans (biodiesel, ink, etc.) _______
6. Food-Grade Soybeans or other new markets for VA _______
7. Equipment _______
8. Soybean Rust _______
9. Soybean Aphid _______
Please list other topics of interest that you would like to see:
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
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Return by October 22
| To: | Dick Atkinson Virginia Soybean Association 151 Kristiansand Drive Suite 115 E & F Williamsburg, VA 23188 FAX: 757-564-8165 soybean@visi.net |
David Holshouser Virginia Tech Tidewater AREC 6321 Holland Road Suffolk, VA 23437 FAX: 757-657-9333 dholshou@vt.edu |
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Sincerely,
David L. Holshouser
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