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For Week of August 17-23, 1998
C.O.R.N. 98-23
In This Issue:
A) Time to Park the Sprayer!
B) Gray Leaf Spot and Yield Losses on Corn
C) Soybeans with Sudden Death Syndromes
D) "Soybean Cyst Nematode-"Can You Dig It?”
E) Wheat Performance Test *Web Site Updated*
We have reached that time period in which herbicides should no longer be applied to soybeans. Only Roundup Ultra on Roundup Ready soybeans and Basagran can be used as this time on soybeans, as the days to harvest from the herbicide application is 14 and 30 days, respectively. All other herbicides require 45 days or greater until harvest.
You should expect only a slight yield reduction from weeds that are beginning to appear in soybean fields, unless several weeds are present per square foot or if moisture stress occurs. In most cases, with normal rainfall there is no reduction in soybean yields. The only problems usually associated with these late weeds are additional seeds in the weed seed bank, potential harvest problems, and weed seeds in the grain. A "Harvest Aid" application may be considered if weeds are a problem at harvest or you can wait for a hard freeze.
Gray leaf spot development has not increased significantly over the past two weeks in most areas of the state. Although some areas of the state have received significant rain showers, this has not promoted development of the disease. Probably the most significant environmental factor to watch is average daily humidity. Gray leaf spot develops to epidemic levels when we have an extended number of days in July and August with daily average relative humidity above 90%. So far this year most locations in Ohio did not record any days in July with average daily relative humidity above 90%. Gray leaf spot does not need a lot of rain to develop. It does need free water on the leaf surfaces for about 13 hours, but this occurs in fields during the morning hours, especially those that routinely have fog. Gray leaf spot does need a dry period in the afternoon to spread spores that develop on infected tissues. We expect gray leaf spot to continue to develop as the season progresses, but the big questions is how much yield loss will be attributed to it.
A number of researchers have attempted to determine the amount of yield loss in several different hybrids due the the effects of gray leaf spot. Most have found that there is not a direct relationship between yield loss and the percentage of leaf area affected with the disease. This is because actual yield is dependent on a multitude of different factors. Yield losses from gray leaf spot results from reduced photosynthesis by the leaves which reduces grain fill and kernel size. A severe outbreak of gray leaf spot may lead to reduced stalk strength and standability, predisposing the crop to lodging. The amount of yield loss is related to the time of infection in relation to the growth stage of the crop, weather conditions during grain fill, susceptibility of the hybrid, hybrid maturity, severity of lodging from stalk rot and yield potential of the hybrid. Thus, yield loss from this foliar disease is difficult to predict.
There, probably is no way to accurately predict yield losses from gray leaf spot at this time. However, after working with this disease for many years we do have some guidelines. We know that the earlier the leaves above the ear leaf become infected the greater the yield loss. The greatest yield losses occur when over 90% of the leaf area of the plants are covered with disease by a week after tasseling and little yield loss occurs on plants when the upper leaves are not infected until 5-6 weeks after tasseling. I agree with the guidelines developed by Dr. Paul Vincelli, University of Kentucky, for estimating potential yield loss at dent stage based on the percentage blighted tissue on the ear leaf. I have modified Dr. Vincelli's guidelines to include situations with very high levels of disease by early dent growth stage. Please remember that these are tentative guidelines and actual yield losses will vary.
Predicted yield loss due to Gray Leaf Spot.
Percentage ear leaf area
Approximate yield
affected by early dent stage loss expected
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5 % or less
0-2 % loss
6-25 %
2-10 % loss
25-75 %
5-20 % loss
75 % - dead leaf
15-50 % loss
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Corn Gray Leaf Spot. |
Sudden death syndrome of soybean has been identified in a few fields in Ohio this past week. The disease severity is quite dramatic in these areas resulting in total defoliation of the leaflets and the plants are beginning to drop their pods. Sudden death syndrome is a fungal disease of soybeans. The fungus, Fusarium solani fsp glycines, lives in the soil and can be associated with soybean cyst nematode. Soybean cyst nematode is reported to increase the severity of sudden death syndrome symptoms. This disease was first identified in Arkansas in the late 70's and has slowly moved up the soybean production region. Disease severity is greatest when there are high levels of moisture before and during the flowering period. The fungus colonizes the crown and is believed to produce compounds which are phytotoxic to the plant. This results in the foliar symptoms, which are very similar to brown stem rot. These foliar symptoms start out as chlorotic spots, which turn brown, but the veins remain green. In brown stem rot the leaves will wither and stay on the plants, in sudden death they will fall off leaving the petioles. Another characteristic to differentiate the two diseases is to examine the pith. In sudden death the crowns of the plant will be gray and discolored and the pith will be white. In brown stem rot, the pith will be a chocolate brown. Disease management strategies for sudden death syndrome include the use of resistant varieties, manage cyst nematode populations, late planting to escape infections and increase drainage in those fields where sudden death is a problem.
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Soybeans with Sudden Death Syndrome . |
Soybean fields with symptoms indicating very high populations of soybean cyst nematode can be seen now in some areas of Ohio. Stunted, yellow soybeans are the result of the combination of low rainfall in some areas of the state combined with soybean cyst nematode. When examining plants for the presence of soybean cyst nematode - do not pull the plants, but dig the plant up and gently shake the soil from the roots. The adult soybean cyst nematode females are tiny, white/cream in color and much smaller than the nitrogen root nodules.
The best time to see soybean cyst nematode on the roots of plants is four to six weeks prior to harvest. However, fall is often the best time to collect soil samples for cyst nematode. Soybean cyst nematode numbers will be highest when the soybeans are almost mature to shortly after harvest. A soil test is the best means to determine if soybean cyst nematode is present as well as estimate population densities. The population densities of soybean cyst nematodes in a field are used to recommend management guidelines.
This information was compiled from the SCN Coalition Grower Workbook. Ohio State University is part of the Soybean Cyst Nematode Coalition which is funded by the North Central Soybean Research Program via soybean checkoff dollars. To receive a SCN testing kit and management information for Ohio, call toll-free 1-877-SCN TEST (1-877-726-8378) or contact your county Extension office.
In the last edition of C.O.R.N. we published the Web Address for the 1998 Wheat Performance Test. It was later brought to our attention that the information at the web site was from last years tests. This has since been corrected and the 1998 data is now available at the following site: http://www.ag.ohio-state.edu/~perf
Our computer support staff has resolved some of the problems people experienced printing the 1997 trials by setting up more tables for the 1998 version. However, some of these tables need to be printed in landscape mode.
Readers can subscribe electronically to this newsletter bysending an e-mail message to:
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.
Contributors to C.O.R.N. this week include: STATE SPECIALISTS: Pat Lipps & Anne Dorrance (Plant Pathology), Jeff Stachler (Weed Science); EXTENSION AGENTS: Roger Bender (Shelby), Larry Lotz (Fayette), Clark Hutson (Seneca), Barry Ward (Marion), Dennis Baker (Darke), Greg La Barge (Fulton), Howard Siegrist (Licking) and Steve Prochaska (Crawford).
Editor: Clark Hutson Web Editor: David Etzkorn
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868
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