CORN
Crop Observation and Recommendation Network

July 2 - July 8, 2002
C.O.R.N. 2002-20

In This Issue:

A) Dry Weather And Weed Control
B) Reminder Of Flexstar Carryover Potential
C) Survey Concludes Low Levels Of Wheat Head Scab In Ohio

A) Dry Weather And Weed Control - Mark Loux CORN Questions

Some areas of the state are experiencing dry conditions, and the resulting moisture-stressed weeds can reduce the effectiveness of postemergence herbicides. Our experience with dry weather and weed control can be summarized as follows:

- Under dry conditions, control of large weeds can be especially difficult. Postemergence herbicides seem to maintain their effectiveness on small weeds even under dry conditions. If you are delaying application to control later-emerging weeds, keep in mind that emergence of additional summer annual weeds should be minimal this late in the season.

- Antagonism between grass and broadleaf herbicides, which reduces grass control, is more likely under dry conditions. Antagonism is most problematic when trying to control yellow foxtail, branyardgrass, or crabgrass, but can occur with giant foxtail. To reduce the risk of antagonism, increase grass herbicide rates, apply when grasses are small, and/or apply grass and broadleaf herbicides separately. Grass herbicides are most effective when applied with crop oil concentrate, and use of surfactant will increase the risk of antagonism.

-Regarding the decision to wait for a rain before applying herbicides, we suggest the following strategy: If rain is likely within the next few days, waiting may improve control. However, if rain in the near future is not likely, go ahead and apply before weeds become larger and even more difficult to control.

B) Reminder Of Flexstar Carryover Potential - Mark Loux CORN Questions

While carryover of Flexstar to the following years's corn has been rare in Ohio, it can occur when Flexstar is applied in mid to late July. Dry soil conditions, which reduce the rate of herbicide dissipation in soil, can increase the risk of carryover. Be cautious about late-season applications of Flexstar, and do not apply the full rate more than once per season.

C) Survey Concludes Low Levels Of Wheat Head Scab In Ohio - Pat Lipps & Dennis Mills CORN Questions

For the past two years we have been attempting to predict the level of wheat head scab occurrence in Ohio based on a weather based forecasting system. The head scab forecasting system predicted low to moderately low levels of disease for the majority of locations where weather information was obtained. This low disease prediction was due to cooler temperatures during the time when the wheat crop was in flower in late May. In order to validate the forecasting system, and to determine the accuracy of the predictions, OSU Extension agents surveyed wheat fields in 23 counties to determine the actual incidence of scab. Agents assessed scab incidence within a field by counting the number of diseased heads and healthy heads per foot of row in each of 10 locations in each field. Incidence data was recorded from 1 to 10 fields per county for a total of 125 fields being surveyed. Results of the survey indicated that 25% of the fields had less than 1% of the heads with disease, 57% of the fields had from 1 to 5% incidence, 14% had 5 to 10% incidence, 2 % had 10 to 15 % incidence and only 2% had over 15% incidence. Overall, the average incidence of fields surveyed was 3.3% with a range of 0% to 24.2% incidence.

The disease forecasting system uses two computerized models to predict the level of scab that may occur based on weather conditions. The threshold level used by the models is based on an estimated economic level of 10% severity of head scab. In other words, the models calculate the probability of head scab occurring at the 10% severity level. It should be noted that the County Agent survey reported the incidence of head scab (% of heads with disease) and not the severity of the disease. The overall severity of scab was much less because most florets on heads were not diseased (100% severity would be all florets on all heads within a field with scab). Based on the model predictions and the actual levels of disease in the fields as documented by the County Agent survey, the models have been relatively accurate over the past two years. Additionally, they appear to be useful tools to use in future years as an early warning system for head scab.

You can view the results of the County Agent survey on the Forecasting Fusarium Head Scab web site at: www.oardc.ohio-state.edu/ohiofieldcropdisease/

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

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.

Editor: Clark Hutson        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.

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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