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August 27 - September 2, 2002
C.O.R.N. 2002-28
In This Issue:
A) Soybean Diseases Still Prevalent
B) Grasshoppers On Soybeans
C) Drought Increases Potential For Stalk Rot And Lodging In
Corn
D) Bean Leaf Beetle Pod Feeding
E) Herbicide Carryover Concerns For Winter Wheat
From samples and surveys last week we were able to diagnose a number of soybean diseases. The soybean pathogens are really making a difference in the drought-stricken areas. In fields which have been hardest hit by the drought, the areas of the fields that have high populations of soybean cyst nematode or high incidence of Phytophthora root and stem rot look even worse. The beans in these "double" hit areas are only ankle high and have 1 to 2 pods that will produce 1 or 2 seeds. As you are scouting your fields, focus on the areas that look the worst.
For SCN, the females were easily seen with the naked eye. Pick the best-looking plants of the worst area in the field, dig up the roots, and shake off the dirt and look. The females will look like white pearls and are much smaller than the Rhizobium nodules.
For Phytophthora, there were large numbers of dead standing plants throughout the field. To confirm, look for plants that are wilting, then pull these plants and look for a canker that goes from the roots up the stem. The symptoms for Phytophthora stem rot do not look like the picture in the book this year. Many of the cankers are at the soil line when the plant dies. In previous years, the stem cankers are typically at the third or fourth node before the plant dies. There are also signs on the root system, where it looks like the plant has tried to produce more roots. The tap root will be totally dead as will most of the other roots.
Diaporthe stem canker was also spotted in southern Ohio this past week. To separate Diaporthe stem canker from Phytophthora, Diaporthe begins as a sunken lesion, usually on the second or third node, which girdles the plants. The area below this canker is green. In contrast, Phytophthora will colonize the plant from the ground up and if you split the stems, you will be able to verify this.
Many of the fields look sad, but it is always good to know what and where the problems are. You will need this information in the future to make better decisions on which crop or variety to plant in these fields next year. Wise decisions on variety selection will mean more $ in your pocket despite the weather conditions.
As Pat Lipps indicated in his C.O.R.N. article last week ("Premature Death in Corn and Stalk Quality", http://corn.osu.edu/archive/2002/aug/02-27.html) drought conditions this year are likely to increase the potential for lodging and stalk rot problems in corn. When stalk rot occurs late in the season as it often does, it has little or no direct effect on yield. Nevertheless, stalk lodging can have such an impact on harvest losses that many plant pathologists consider stalk rots to be the most significant yield limiting disease of corn.
For a corn plant to remain healthy and free of stalk rot, the plant must produce enough carbohydrates by photosynthesis to keep root cells and pith cells in the stalk alive and enough to meet demands for grain fill. When corn is subjected to severe weather stress, photosynthetic activity is sharply reduced as leaves roll tightly and plant growth slows. As a result, the carbohydrate levels available for the developing ear are insufficient. The corn plant responds to this situation by removing carbohydrates from the leaves, stalk, and roots to the developing ear. While this "cannibalization" process ensures a supply of carbohydrates for the developing ear, the removal of carbohydrates results in premature death of pith cells in the stalk and root tissues, which predisposes plants to root and stalk infection by fungi. Even mild, early season water stress during the pretassel stage of development can significantly increase root infection by stalk rot fungi and result in greater stalk rot at maturity. As plants near maturity, this removal of nutrients from the stalk to the developing grain results in a rapid deterioration of the lower portion of corn plants in drought stressed fields with lower leaves appearing to be nitrogen stressed, brown, and/or dead.
Other plant stresses which increase the likelihood of stalk rot problems include: loss of leaf tissue due to foliar diseases (such as gray leaf spot), insects, or hail; injury to the root system by insects or chemicals; high levels of nitrogen in relation to potassium; compacted or saturated soils restricting root growth; and high plant populations.
Most hybrids do not begin to show stalk rot symptoms until shortly before physiological maturity. It is difficult to distinguish between stalk rots caused by different fungi because two or more fungi may be involved. Similarly, certain insects such as European corn borer often act in concert with fungal pathogens to cause stalk rot. Although a number of different fungal pathogens cause stalk rots, the three most important in Ohio are Gibberella, Collectotrichum (anthracnose), and Fusarium. For more information on stalk rot in corn, consult the OSU Plant Pathology web site "Ohio Field Crop Diseases" (http://www.oardc.ohio-state.edu/ohiofieldcropdisease/) for more details and pictures of the disease symptoms associated with these pathogens.
A symptom common to all stalk rots is the deterioration of the inner stalk tissues so that one or more of the inner nodes can easily be compressed when squeezing the stalk between thumb and finger. It is possible by using this "squeeze test" to assess potential lodging if harvesting is not done promptly. The "push" test is another way to predict lodging. Push the stalks at the ear level, 6 to 8 inches from the vertical. If the stalk breaks between the ear and the lowest node, stalk rot is usually present. To minimize stalk rot damage, harvest promptly after physiological maturity (about 30% grain moisture). Harvest delays will increase stalk rot and result in more lodging.
Although bean leaf beetle (BLB) populations do not seem to have been that large this past summer in Ohio, fields probably exist where their numbers are high. The concern with the bean leaf beetle should now have shifted to the second generation that will be feeding on pods. Pod injury could be very evident in soybean fields where the green pods have become more attractive to the BLB adults than the foliage.
When pod injury occurs on 10-15% of pods, seed injury will become evident and yield losses are possible. If the pod injury occurs during periods of wet conditions that enable infection by seed diseases, the development of moldy bean seeds may lead to a loss in seed quality. However, if dry field conditions continue when pod injury due to BLB feeding occurs, the potential for development of moldy seeds in injured pods may be minimal.
Rescue treatment to prevent excessive development of seed damage may be warranted when pod injury exceeds 10-15% and adult BLBs are still present and actively feeding enough to cause additional pod injury. The assessment of a field infestation depends on (1) determination of the current level of pod injury, (2) the abundance of adult BLB activity using a sweep net, (3) consideration of weather factors that may enable infection of the damaged pods by disease agents, and (4) the amount of time remaining before total leaf drop and dispersal of a BLB population from the field. Growers also should be aware of the harvest interval with the insecticide that is used. Many of the materials have waiting periods of between 45 and 60 days, which would probably limit their use for use at this time.
Growers should start sampling their fields weekly for the remainder of the summer. The percentage pod injury is determined by random sampling about twenty plants, and then counting the total number of pods and those with feeding injury. Adult BLB population should be estimated by taking 10 sweeps with a sweep net at three to four locations in a field. Less than five BLB per sweep are unlikely to cause significant injury. Five to ten BLB per sweep indicates a potential problem possibly warranting rescue treatment. More than ten BLB per sweep will result in significant injury especially if two or more weeks remain until leaf drop. When the foliage dries and drops, beetles exit the field. Thus, the time remaining for BLB feeding is a key factor in the occurrence of pod injury.
Dry weather this summer in many areas could increase the risk of injury to wheat from corn and soybean herbicide carryover. Abnormally dry soil conditions can result in a reduced rate of degradation of some long-residual herbicides. Dry soil conditions generally affect the rate of herbicide degradation by microorganisms to a greater extent than the rate of degradation by chemical reaction (hydrolysis, etc). As a result, herbicides such as Scepter and Pursuit for which microbial degradation is important may be more likely to carryover under dry conditions, compared to herbicides such as atrazine and chlorimuron (Classic, Canopy XL), for which hydrolysis is the most important degradation mechanism. The carryover potential is also determined by the timing of dry soil conditions. Carryover is more likely when dry weather occurs immediately after herbicide application, while wet conditions for at least several weeks following application should greatly reduce the carryover potential.
Carryover of soybean and corn herbicides to wheat has not been much of an issue in Ohio previously, even in years when rainfall has been scarce. Wheat is considerably more tolerant of most persistent herbicides, compared to corn, forages, and many minor crops. Diagnosis of wheat injury from herbicide carryover has been somewhat difficult. The major symptom of carryover, especially of ALS inhibitors, seems to be a general reduction in vigor in the spring. Consequently, herbicide carryover is identified as the cause of slow wheat growth only when all other possible causes have been ruled out. Since many persistent herbicides will inhibit root growth, it is possible that a slight degree of root inhibition could interact with poor fall growing conditions to reduce the vigor and development of the wheat. The overall result of this interaction might be reduced winter hardiness. For this reason, we suggest using the best management practices possible during wheat establishment to reduce any problems with herbicide carryover. These include timely planting, preplant control of weeds with tillage or herbicides, and ensuring good seed to soil contact.
Information on the waiting period between herbicide application and wheat planting is available in Table 15 on page 141 of the current OSU Weed Control Guide. We suggest contacting manufacturers' reps for more information on the risk of carryover where there is a concern. Conducting a herbicide bioassay can also provide helpful information when assessing the risk of carryover. The bioassays need to be started at least 3 to 4 weeks before wheat planting to allow enough growth of the indicator plants to get an accurate assessment. The general procedures for conducting bioassays follow:
1. Collect at least two soil samples from the suspect field. Collect a third sample from the same field or another field close by that is of the same soil type and where no herbicides were applied this season. This sample is called the check and is important in determining herbicide carryover. For continuous no-tillage fields, collect soil to a 3 to 4 inch depth and for conventional tillage wheat collect soil up to 6 inches deep. Collect enough soil to fit the containers that will be used to grow the indicator crops. If possible, collect soil when it is in a fairly friable condition, and further crumble it by hand so that it is suitable for planting.
2. Obtain rectangular containers that are at least ten inches wide, 20 inches long, and 3 inches deep to grow the indicator plants. Drill holes at the bottom of the container to allow water to drain. Fill one container per sample.
3. Plant the indicator crops in rows across the width of each container. For imazaquin carryover, plant wheat, Clearfield corn, regular corn, and sunflowers. For chlorimuron carryover, plant wheat, regular corn, and sunflowers (these species should work for other ALS inhibitors also). For atrazine, planting wheat alone should be adequate. Obtain wheat seed from each of the varieties you will be planting and plant 10 seeds for each variety in a single row in the container. Plant 6 seeds per row for corn and sunflowers.
4. Place the containers in full sunlight outdoors. Water accordingly, but most importantly, do not over-water because seed diseases will destroy the bioassay. Apply some fertilizer after plants emerge.
5. Let plants grow for at least 3 weeks or until plants are at least 4 inches
tall. To evaluate injury, compare the color and height of the plants grown in
the check soil to those in the suspect soil. Imazaquin and chlorimuron may cause
interveinal chlorosis (yellowing) and stunting. Carefully dig up the roots of
at least two plants per row and compare the size and/or weight of the roots
between the check and the suspect sample. If significant herbicide residues
are present, the sunflowers may have brown growing points, with the possibility
of some severely stunted or dead plants. If there appears to be little difference
in corn and wheat growth between the check and suspect sample, wheat should
be able to be safely planted. If the sunflowers die and the wheat and corn are
stunted, we suggest not planting wheat in those fields.
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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
State Specialists: Pat Lipps & Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux & Jeff Stachler (Weed Science), and Bruce Eisley & Ron Hammond (IPM). District Specialist: Ed Lentz (Northwest) Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska (Crawford), Greg Labarge (Fulton), Gary Wilson (Hancock), Howard Siegrist (Licking), Glen Arnold (Putnam), Ray Wells (Ross), and Clark Hutson (Seneca).Editor: Dave Jones Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
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