http://www.ag.ohio-state.edu/~corn/

CORN
Crop Observation and Recommendation Network

May 1-7, 2000
C.O.R.N. 2000-11

In This Issue:

A) Alfalfa Weevil and Leafhopper Resistant Alfalfa
B) Options for Cutworm Management in Corn
C) Wheat Growth Stage Update and Disease Predictions
D) Wheat Getting too Large for Herbicides
E) Burndown Considerations

A) Alfalfa Weevil and Leafhopper Resistant Alfalfa - Hal Willson CORN Questions

Questions have been raised regarding alfalfa weevil infestations on leafhopper resistant varieties compared to non-resistant varieties. In addition, a report was received last week indicating that weevil injury appeared to be greater in a field leafhopper resistant alfalfa compared to an adjacent field of non-resistant alfalfa.

During the past week, we sampled three sets of replicated plots of leafhopper resistant and non-resistant alfalfa in Clark County. In a replicated on-farm trial, weevil numbers in leafhopper resistant and non-resistant alfalfa were equal (ca. 4.5 larvae per stem), but estimated defoliation in the leafhopper resistant alfalfa was significantly greater than that in the non-resistant alfalfa (ca. 57% vs 40%). In a 1997 planted trial on the Western Branch station, similar results were observed, but weevil activity was lower (ca. 2.8 weevil per stem) and differences in defoliation were not statistically significant. In a 1998 planted trial, differences in defoliation were marginal and not significant.

The difference in defoliation due to weevil between leafhopper resistant and non-resistant varieties can probably be attributed to differences in stand height due to the slower spring growth of the leafhopper resistant varieties due to their greater winter dormancy. In each trial, stand height of the leafhopper resistant variety was less than that of the non-resistant varieties. As a result, the ratio of weevil larvae to stem length was higher in the leafhopper resistant varieties and defoliation was intensified.

A third trial at the Western branch planted in 1999 exhibited vigorous growth and minimal weevil injury. Presumably, the effect of dormancy in the latest generations of leafhopper resistant alfalfa have been reduced and differences in weevil injury among varieties may not be as evident as in earlier generations of leafhopper resistant alfalfa.

When inspecting alfalfa fields for weevil, one should note the relative impact of weevil injury in various locations of a field. Wherever alfalfa growth has been limited due to either available nutrients, soil conditions, or age of stand, injury by weevil will likely be more intense. Population density of weevils may differ significantly from one area of a field to another, but growth of the alfalfa host crop will also be a key factor in regard to the severity of weevil injury.

B) Options for Cutworm Management in Corn - Hal Willson CORN Questions

Adult moths of the black cutworm have been collected in abundance in pheromone traps at the Western Branch station during the past two weeks, and broadleaf weed growth is fairly heavy in some fields of soybean stubble which will likely be planted to corn during the next week or two. The combination of cutworm moths flying in from the south and attractive green egg laying sites may result in potential stand losses due to black cutworm.

There are basically three options for managing black cutworm in corn, namely: (1) application of a soil insecticide at planting, (2) application of a pre-emergence insecticide, or (3) application of a post-emergence rescue treatment when scouting justifies a treatment. A case may be made for each of the options depending on the field history and the effectiveness of a scouting program.

Research at the Western Branch station, where natural infestations of cutworms periodically occur, has demonstrated that application of a granular soil insecticide with a high level of cutworm control (e.g. Aztec, Force or Lorsban) at planting time will provide optimal stand protection against cutworm infestations. However, such treatment of first year corn may not be cost effective if a field does not have a history of significant cutworm activity over time.

Application of a pre-emergence insecticide broadcast treatment (e.g. Ambush, Asana, Pounce, Lorsban or Warrior) will also prevent stand losses due to cutworm. Such treatments often applied as a tank mix with herbicides are not as effective in preventing injury from heavy cutworm infestations as granular treatments applied at planting time, but they are less expensive.

The third option is to scout a field closely for cutworm injury and apply a rescue treatment only when needed. This option eliminates the expense of applying a treatment in years when there is minimal cutworm activity. However, in years of severe cutworm infestations, a rescue treatment is only effective if it is applied in a very timely manner. The results of a rescue treatment may be marginal compared to the preventive treatment options, especially if the infestation is detected after a significant level of injury has already occurred.

Application of preventive treatments (at planting or pre-emerge) should be based on a field history of cutworm problems and preferably on a few years of strip trials to determine whether a benefit is achieved or not over a period of time.

In last year's trials at the Western branch, detection of a cutworm infestation before the entire stand had emerged enabled timely application of rescue treatments that achieved stand and yield results comparable to the pre-emergence broadcast treatments. If the rescue treatment had been delayed a week or two, the results of rescue treatments would have been equivalent to that of the untreated check plots which had a 25% stand loss.

At this point in time, one should consider how a potential corn field appears a cutworm moth carried aloft by a weather front. If a field of soybean stubble is lush with weeds, it is likely a favored cutworm egg laying site. If a field of soybean stubble is weed free or early tilled, it is likely to be passed over by the migrating cutworms. If preventive treatment is under consideration, weigh the costs against the potential benefits. If a wait and see action is taken, then walk the fields as soon as the corn begins to emerge.

C) Wheat Growth Stage Update and Disease Predictions - Pat Lipps CORN Questions

Wheat growth rate has been very difficult to predict this year. By early April, the wheat crop was nearly three we ahead of schedule. Over the past two weeks wheat growth has slowed so that now the wheat crop is no more than 5 days earlier than normal. In southern Ohio the crop is in beginning boot growth stage (growth stage 9 to 10) and in northern Ohio most of the crop is in the two node stage (growth stage 7) or flag leaf emergence (growth stage 8). Head emergence is still probably two weeks away.

Dry weather for the past week and a 10 day forecast of below normal precipitation indicates that disease spread in fields will be limited. Powdery mildew does not require precipitation for spread, just high relative humidity. Dense stands of wheat are wet for extended periods of time with just a heavy morning dew. To be on the safe side continue to monitor wheat fields that have powdery mildew. The lack of precipitation will definitely limit any spread of Stagonospora leaf blotch. Within the next weeks we will begin to monitor spores of Fusarium in the air and provide predictions for head scab. Continued dry weather will limit the threat of this important disease.

D) Wheat Getting too Large for Herbicides - Jeff Stachler CORN Questions

Wheat in southern Ohio is near the boot stage, meaning that herbicides can no longer be applied to wheat. In northern Ohio the wheat is around flag leaf emergence, meaning that Peak Harmony Extra, Curtail and 2,4-D can no longer be applied to wheat. Buctril and Stinger can still be used for ragweed and thistle control with Stinger in northern Ohio until the wheat reaches the boot stage. More than likely by the middle of next week nearly all wheat in Ohio will be to advanced to have herbicides applied.

E) Burndown Considerations - Jeff Stachler CORN Questions

We are noticing knotweed in no-till fields where Atriplex can be found. If you identify knotweed in a field apply a glyphosate product at 1.0 qt/A or Touchdown at 1.6 pt/A, since 2,4-D and Gramoxone will not control it.
With the current forecast of warm weather towards the end of the week, foxtails and other annual grasses will grow rapidly and will quickly become too large for products like Sencor, Canopy, Canopy XL and atrazine products to effectively control the grasses. Glyphosate products or Touchdown will need to be added to burndown herbicide mixtures when the grasses get taller than 1 inch or when there are perennial grass weeds or hard to control weeds in the 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.

Contributors to C.O.R.N. this week include: State Specialists: Pat Lipps and Anne Dorrance (Plant Pathology), Jeff Stachler (Weed Science), Hal Willson (Entomology), Ron Hammond (Entomology), Peter Thomison (Corn Specialist); Extension Agents: Andy Kleinschmidt (Van Wert), Roger Bender (Shelby), John Barker (Knox), Clark Hutson (Seneca), Barry Ward (Champaign), Gary Wilson (Hancock), Greg LaBarge (Fulton), Ray Wells (Ross)and Steve Prochaska (Crawford).

Editor: Steve Prochaska Web Editor: Nathan Watermeier

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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.