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Seed quality has been the biggest issue this fall. It's probably the worst I've ever seen. Two diseases, phomopsis seed decay and purple seed stain, are the main culprits. But, other problems are likely present as well. I've spent a great deal of time on the phone and issuing email alerts on this subject. There's little we can do about this now, but I felt that I should devote this issue to the subject. In this issue, you'll learn more about these two diseases and ways to minimize them in the future.
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"In all my years of breeding soybean, I've never seen seed quality as bad as this year." ...Glenn Buss, retired Virginia Tech Soybean Breeder.
The above photograph was taken of samples obtains from our soybean variety tests. The seed in the center is one of the few plots where we had good seed quality. The seed to the left represents plots where phomopsis seed decay was rampant. The seed on the right is an example of purple seed stain infection. These samples probably represent the extremes; we haven't seen too many soybeans worse than the seed on the left and right. Still, they represent a significant proportion of our plots.
Understanding the causes and effects of poor seed quality will help you understand our situation and should help you minimize, although not prevent, these problems in the future. In this issue of Virginia Soybean Update, you'll learn about two diseases that have caused the above problems, how environment affects their severity, and ways to reduce their damage in the future. Let us first explore both diseases in depth.
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The disease, Pod and Stem Blight, can be found in Virginia soybeans almost every year. However, we do not usually see the disease progress to the phomopsis seed decay phase unless an early maturity group is used, conditions of warm and wet weather are present during late seed fill, and harvest is delayed. We met two of the above conditions and, in some cases, all three. Two fungi cause Phomopsis seed decay: Phomopsis longicolla and Diaporthe phaseolorum. The relative prevalence of each will vary with location and season. 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. Infected seed is an important, but not the primary way that the disease is introduced. However, as stated above, 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 usually remains dormant until seed have formed and the plant is close to maturity. We have occasionally found plants that have died 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.
Prolonged periods of wet and warm (>70°) 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.
Will volunteer soybean be a problem in next year's crop? As stated earlier, heavily infected seed will be shriveled and chalky white in appearance. These seed will be light and, in many cases blown out the back of the combine. The risk of germination of the seed is low. Therefore, I would not be overly concerned about these seed resulting in volunteer plants for next year. This would be of special concern if Roundup-Ready seed were used and you were rotating to another Roundup-Ready crop. On the other hand, not harvesting this year's crop due to low yield or severe seed quality problems will likely result in some volunteer soybean plants next year. If this is the case, selection of herbicide for next year's crop will be important.
Still, some infected seed will not show symptoms or the symptoms will be minimal. Saving these seed for next year's planting (non Roundup-Ready seed only) will entail some risks. Germination will be reduced and damping off will be more likely. Seed treatments, which I'll discuss later in the issue, will improve your success in getting a good stand. But, seed treatments are no substitute for using good quality seed. Storage will reduce phomopsis infection, but only after extended storage (more than 1 year) will the seed be free of the disease.
Control/Prevention. There is little you can do to prevent this disease from occurring in the future. As I stated earlier, it is endemic to Virginia; it resides in the soil and on infected residue. Tillage to bury and speed decomposition of residue is an option, but a bad one; tillage causes more harm than good in most cases. Rotating with corn will help. Plant high- quality seed and late-maturing varieties. 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. This is the main reason I do not recommend MG 4 varieties for cotton and peanut farmers. MG 4 varieties will inevitably begin maturing during the warm and humid months of September and October, before cotton and peanut harvest begins. And rarely will soybean be harvested before cotton and peanuts. The result is a mature soybean crop remaining in the field for longer than it needs to be. Of course, timely harvest this year was a challenge. There was little that we could have done to lower the disease incidence. Foliar fungicides will lower the incidence of infection if applied to pods during the early pod (R3-R4) and seed (R5-R6) development stages. But, this is an insurance treatment only. The disease may never develop, especially if harvest is timely. However, for seed production, the practice should be considered.
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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. Infection may be latent (dormant) with no symptoms expressed on the stems, foliage, or pods; yet purple seed stain will occur. The disease should not be confused with Cercospora sojina, the causal organism for frogeye leaf spot. Frogeye leaf spot is rare in Virginia, but could be quite devastating to yield. We have varietal resistance to frogeye leaf spot, but very little to purple seed stain. Like phomopsis seed decay, maximum infection of purple seed stain occurs under warm and humid environments. Hence, the main reason we had higher infection rates in Virginia in 2002.
Symptoms. 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.
Infected seed can result in diseased seedlings and reduced stands. Cotyledons will become dark purple, shrivel, and fall off. This can spread to the stem, producing necrotic areas that can girdle the stem and kill seedlings. Less affected seedlings are stunted.
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. I observed these symptoms in our variety tests in Suffolk. Although the symptoms varied greatly between replications, some varieties were definitely more susceptible. The photo below is taken from a plot that was heavily infected with the fungus. Note that the upper leaves have dropped, leaving behind the petioles, but the lower leaves remain. On the next page is another photograph showing the infected stems and petioles. Although this photograph shows staining of the seed, foliar and seed symptoms are not always correlated.
Disease Cycle, Causes, and Distribution. C. kikuchii overwinters in diseased leaves, stems, and seed. Infection by seed is not the primary means of infection. In the case of severely infected seed coats, the fungus will reduce stands or actually girdle and kill a young seedling. Of the remaining plants, the fungus only infects a small percentage. Later, secondary infection from infected plant tissue begins at flowering (earlier than Phomopsis). Splashing water and wind spreads the disease. Temperatures of 80-85° F along with extended periods of high relative humidity, favor disease development. The fungus grows through young pods, through the vein connecting the seed to the pod, then through the hilum into the seed coat. Purple seed stain, unlike most seedborne pathogens does not increase with delayed harvest.
Control. First, rotate soybean with a non-legume crop. 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. Although there may be some varieties that are less susceptible to the disease, insufficient data is available to make this judgment. After this year however, we may be able to select some with tolerance.
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Soybean mosaic virus. Infected seed will be smaller and lighter than normal and have a black to brown mottling that seems to "bleed" from the hilum. Yield can be substantially reduced. If infected seed germinate, they will produce diseased seedlings and plants that are usually somewhat stunted with crinkled, puckered, and mottled leaves. The virus is then spread to non-infected plants by aphid feeding. Prevention can be achieved by planting non-infected seed and through the selection of resistant varieties.
Anthracnose. This is another disease that occurs during warm, humid conditions after prolonged rainfall. All plant parts can be infected, but a characteristic feature is the small, spiny, black fungal fruiting bodies (resembling tiny pincushions) on the stem. The disease may lead to premature defoliation. When it attacks developing pods, pods may be empty or the seed may be shriveled, moldy, and dark brown in color. The fungi survive in crop debris and seed. Seed treatments at planting or a foliar fungicide at early pod set will lower the disease incidence.
Stinkbug. This damage is caused by the mouthparts of the insect piercing the developing pods and seed. If damage occurs early, pods may fall off. But if the injury occurs later, seed size is reduced, the seed may be discolored and shriveled, or the seed may have a flat appearance. If infestation occurs after the seed has reached its normal size, injury will appear as splotches on the seed coat. Scouting and treating if the pest exceeds economic thresholds accomplish control.
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Seed Quality Ratings. Let's first compare this year's seed quality to previous ones. The table on the next page is the average seed quality rating of all varieties tested in the official soybean variety tests over the past four years. Seed quality is rated from 1 to 5 according to the following scale:
1.0 = very good
2.0 = good
3.0 = fair
4.0 = poor
5.0 = very poor
The seed shown on the left in the figure on the first page would likely be rated a 4.5 to 5.0. Although we do not determine the disease causing the poor seed quality in these ratings, it is likely that phomopsis seed decay is the culprit.
Climatic conditions during late seed fill and post- maturation affect seed quality. The first thing that you may notice in Table 5.1 is that seed quality is usually better for double-crop than full-season systems. Remember that phomopsis seed decay is most severe when warm and wet weather conditions
Table 5.1. Average seed quality of soybean varieties tested in Virginia's official variety tests, 1999-2002. 2002 data is limited; all tests had not been harvested at this time.
| Maturity Group | ||||||
|---|---|---|---|---|---|---|
| Year | System | III | Early IV | Late IV | Early V | Late V |
| 1999 | FS | 2.4 | 1.8 | 1.8 | 1.4 | 1.4 |
| DC | 1.7 | 1.7 | 1.7 | 1.3 | 1.3 | |
| 2000 | FS | 1.7 | 1.8 | 1.6 | 1.2 | 1.0 |
| DC | 1.6 | 1.8 | 1.5 | 1.0 | 1.4 | |
| 2001 | FS | 2.1 | 2.3 | 1.7 | 1.2 | 1.3 |
| DC | 1.8 | 1.9 | 1.5 | 1.3 | 1.2 | |
| 2002 | FS | 2.9 | 2.8 | 2.6 | 1.9 | na |
| Average (99-01) | FS | 2.1 | 1.9 | 1.7 | 1.3 | 1.2 |
| DC | 1.7 | 1.8 | 1.6 | 1.2 | 1.3 | |
are prevalent during late seed development and maturation stages. Full-season soybeans are maturing in September and October; double-crop soybeans are maturing in October and November. A warmer climate during late seed fill and maturation will worsen seed quality. In general, our variety tests are harvested on a timely basis; therefore weathering has had minimal effect on the seed quality. Still, you can see the difference that planting date makes. This is also evident with maturity group. The later the maturity group, the better the seed quality for the same reason. Later maturity groups are exposed to cooler temperatures.
I have limited data from this years test due to the harvest delays that we've encountered this year. But, you'll quicken notice that seed quality is worse. Continuous rain during the fall has been the culprit. You can seed the difference that maturity group makes. The Eastern Shore region has experienced the worst seed quality and the Northern Piedmont the best. This makes sense; the more humid areas should have poorer seed quality.
Timeliness of harvest affects seed quality. While the above data gives an indication of the pre-harvest environment on seed quality, it does not give any indication of timeliness of harvest. Even if the environment is conductive of poor seed quality, one can avoid problems by harvesting soon after maturation. The data below illustrates the effect of weathering associated with delayed harvest on germination of 'Hill' (Group IV) and 'Bragg' (Group VII) soybean. Note that if the soybean were harvested soon after maturity, little decline in germination occurred. But, most importantly, note that the MG IV soybean did not tolerate weathering nearly as well as the MG VII variety. Seed moisture fluctuated with each rainfall event. But, unless temperatures were warm during post-maturation, germination was not affected greatly. These data show a clear effect of temperature during weathering, and the rate and severity of seed deterioration.
Table 5.2. Effect of weathering on moisture content (M.C.) and germination of seed of 'Hill' and 'Bragg' soybean.
| Harvest Datea | Hill (MG IV), % | Bragg (MG VII), % | ||
|---|---|---|---|---|
| M.C. | Germination | M.C. | Germination | |
| 9/15 | 26 | 96 | ||
| 9/22 | 13 | 97 | ||
| 9/29b | 17 | 90 | ||
| 10/6b | 20 | 78 | ||
| 10/13 | 11 | 76 | 26 | 98 |
| 10/20b | 19 | 71 | 18 | 98 |
| 10/27 | 12 | 53 | 13 | 93 |
| 11/3b | 14 | 37 | 14 | 92 |
| 11/10 | 14 | 92 | ||
| 11/17b | 20 | 89 | ||
| 11/24b | 13 | 86 | ||
| 12/1 | 15 | 87 | ||
| 12/8 | 11 | 84 | ||
| 12/15b | 14 | 84 | ||
| aSeed was hand-harvested and threshed, then cleaned with hand screens and aspirator before germination test. bOne or more rains during preceding week. |
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Source: Potts, H.C. 1978. Proc. 8th Soybean Seed Res. Conf., 8:33-42.
Purple Seed Stain and Environment. Purple seed stain also increases under warm and moist environments. However, remember that this disease attacks the crop at an earlier stage, beginning as soon as flowering. Therefore, the moisture conditions at time of infection are probably more important since temperatures are generally optimal for infection during earlier developmental stages. For that reason, maturity group has no consistent correlation with severity of infection. The limited data from this year's soybean variety tests (Table 5.3) verifies this point. In this case, MG III varieties have less purple seed stain than MG IV varieties. Why? When moisture conditions were high, MG III varieties were exposed to the disease over a shorter time period than later-maturing varieties. In fact, MG III varieties were mature by the time the continuous rains began. MG IV varieties did not mature until October, therefore were more susceptible to the disease during the wet fall environment.
Also of note is how location influenced the degree of infection. Purple seed stain was most severe at Painter (Eastern Shore), followed by Chesapeake (Tidewater area), Suffolk (southern Coastal Plain), and Orange (northern Piedmont). Disease incidence was lower with decreasingly humid environments.
Table 5.3. Average percent purple seed stain of 2002 Virginia Soybean Variety Tests.
| Maturity Group | |||||
|---|---|---|---|---|---|
| Location | System | III | Early IV | Late IV | Early V |
| ---------------(%)--------------- | |||||
| Chesapeake | Full-season | --- | 13.0 | 10.5 | 6.7 |
| Orange | Full-season | 0.1 | 0.5 | 1.1 | 6.1 |
| Painter | Full-season | 11.2 | 14.2 | 21.8 | 17.0 |
| Suffolk | Full-season | 1.7 | 1.9 | 5.4 | 6.7 |
Other issues that may have affected seed quality in 2002. Although our environment was the major cause of our seed quality problems, we need to remember other problems that we faced this year, primarily insects. Corn earworm numbers were high. This insect generally leaves a large hole in the pod that allow rain and dew to enter. I've notice that these insect-damaged pods had worse seed quality. Although the insects were controlled when thresholds were met, some damage still took place. The literature states that disease severity is higher when confounded with insect damage, viruses, or other stresses. I did not note that virus was higher in 2002. But, insects (corn earworm, soybean loopers, beet armyworm, etc.) were definitely higher and the drought put an unusual amount of stress on the crop.
In summary, the warm and wet environments during seed development and post-maturation were the primary cause of increased disease and lower seed quality. However, our own actions of planting an earlier-than-recommended variety may have contributed in some cases. Timely harvest was not possible this year, but weathering effects are less problematic if adapted varieties are used.
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Timely Harvest. This is a factor that we had little control over this year. Still, timely harvest is not just a problem in wet years. Soybean tends to take second place to peanut and cotton harvest, and occasionally to wheat planting. I stress that harvesting in a timely manner will prevent seed deterioration.
Variety Selection. I not talking about selecting resistant varieties within maturity groups here, but instead, selecting the appropriate maturity group for your cropping system and environment. In general, we take a risk by growing MG III, and maybe early MG IV varieties in eastern and southern Virginia. They mature early and quick. I've seen MG III varieties deteriorate from a good seed quality (rarely do I see excellent seed quality with MG III soybean) to a very poor and un-saleable crop in just 7-10 days. I'm not saying they do not have a place; just make sure that you can harvest them soon after they mature. In the northern Piedmont or the Valley, MG III and early IV fit well however. In cotton and peanut growing areas, I only recommend MG V, unless the individual is willing to stop harvest of cotton and begin soybean. Again, double-crop systems generally have less seed quality problems.
Is there varietal resistance? I don't know and there's little information regarding this. However, we should have a good data set this year.
Should you save and replant your seed for next year? First, saving Roundup-Ready seed is illegal. With that out of the way, you may want to re-consider saving seed this year considering the seed quality problems. In addition, soybean was being harvest at high moistures this year due to the limited days that we were able to get into the field. To insure high quality seed (even if disease-free), soybean should be stored at about 12% moisture. If you're considering saving seed, I'd suggest that you have them tested at harvest and again before planting.
Will seed treatments help next year? My general recommendations regarding seed treatments are that they are beneficial if planting early into cool, wet soils. But, will they help with poor quality seed? Yes, they will and significantly. But, seed treatments are no substitute for good, high-quality seed. Dr. Pat Phipps and I established two tests this year evaluating seed treatments on good- and poor-quality seed. Unfortunately, we have not analyzed the data yet. We'll provide this data upon request and it will be published later. But, I doubt if it will change my above conclusion. Still, with the weather conditions this year (many parts of the country are experiencing the same), I'm leaning towards seed treatments for our full-season plantings next year. Try to find out all you can in regard to where your 2003 seed beans were grown in 2002.
Are foliar sprays beneficial? Foliar sprays would have been beneficial in Virginia this year; on average, they are not. This relates to soybean grown for the open market; not seed. For seed production, the practice will insure high quality seed. Foliar fungicides, such as Quadris, Bravo, and Topsin-M will decrease the incidence of pod and stem blight (phomopsis/diaporthe complex), stem canker Cercospora leaf spot, purple seed stain, and frogeye leaf spot. Fungicides are used regularly in the Delta states where they have shifted to earlier-maturing varieties and encounter warm and wet harvest conditions on a more regular basis. Usually, two applications are suggested for maximum yield and seed quality.
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Sincerely,
David L. Holshouser
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