CORN
Crop Observation and Recommendation Network

May 8-14, 2000
C.O.R.N. 2000-12

In This Issue:

A)  Can Corn Survive Leafing-Out Underground?
B)  Wheat Growth Stage Update and Disease Predictions
C)  Wheat Fungicide Labels
D)  Four Virus Diseases in Wheat
E)  Efficacy of Alfalfa Weevil Treatments
F)  Detection of Cutworms and in Surface Residues
G)  Slugs are Active Earlier than Normal
H)  Greenhouse Results from Studies on ALS-Resistant Giant Ragweed

A) Can Corn Survive Leafing-Out Underground? - Peter Thomison CORN Questions

When certain unfavorable environmental conditions (especially dry, cloddy soils) occur, germinating corn seedlings may start unfurling leaves below ground. This premature leafing out may also be associated with twisted shoots (coleoptiles). When the problem is widespread across a field, replanting is often necessary.

Normally the coleoptile is pointed and quite stiff, it can spike or push its way through soil during emergence. However, in a cloddy field where soil coverage of seed is poor and irregular, sunlight can reach the seedling and induce leaf emergence beneath the soil surface. Other factors (or combinations of factors) can also result in abnormal unfurling symptoms. Heavy rains after planting can cause a hard crust which makes emergence of small seedlings very difficult. As a result, bending and twisting of the seedling below the crusted layer often occurs. Planting the seed too deep, which may cause poor germination and emergence, may also result in premature unfurling of the corn. Certain herbicides such as Lasso and Dual, and the premixes that contain their active ingredients, can show similar symptoms (i.e. twisting, abnormal growth) when excessive rates are applied pre-emergence. Besides excessive rates, improperly closed seed furrows can allow the pre-emergence herbicide to come in direct contact with the seed. Excessive soil insecticide dosage or in-furrow placement of insecticide phytotoxic to seed can also cause twisting sprouts and abnormal leaf expansion underground. In addition, anhydrous and aqua ammonia fertilizer injury has been associated with these symptoms. Certain corn hybrids are also more prone to premature unfurling during emergence.

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

Wheat growth rate has been rapid over the past four days. In southern Ohio the crop is in beginning to head (Growth Stage 10.1) and in northen Ohio most of the crop is in flag leaf emergence (growth stage 8)or early boot stage (growth stage 9).  The wheat crop has now advanced to that critical stage when disease can be a significant problem. The ten day forecast is for continued moderate temperatures (lows in mid 50s and highs in low 80s) and high humidity with normal precipitation. These conditions will favor the development of several wheat diseases.

Powdery Mildew - will continue to spread due to the mild temperatures and high relative humidity. Any fields that had powdery mildew earlier in the season should be scouted again right now. If powdery mildew is beginning to show up on the leaf below the flag leaf and the variety is rated as susceptible, a fungicide application could be made to control further yield loss. Make the decision to spray based on your estimates of overall yield potential of the field and the current price of wheat. Check out our past articles on CORN (CORN2000-10) for guidelines for fungicide applications for powdery mildew control.

Stagonospora Leaf Blotch - is beginning to show up in some fields that have had sufficient rain over the past two weeks. The disease is still confined to the lowermost leaves in most fields, but will begin to spread up the plants as rain showers splash spores. The warmer temperatures expected over the next 10 days will also favor infection and disease development. Again, scouting fields of susceptible varieties will be important.

C) Wheat Fungicide Labels - Pat Lipps CORN Questions

Tilt fungicide from Novaritis currently has a SLN 24(c) registration in Ohio. This label permits application of Tilt (4 fl. oz./A) by ground or aerial applications through Feekes Growth Stage 10.5 (full head emergence). The restrictions include:

1. Do not apply more than 4 fl oz per acre per season

2. Do not apply within 40 days of harvest

3. Do not graze or feed livestock treated wheat, straw, forage or hay

4. Do not cut the green crop for hay or silage

5. After harvest, the straw may be used for bedding

This SLN label was issued April 29, 1998 and is good for 5 years. 

Bayleton fungicide from Bayer, has not had small grains on its label for several years. Growers can use any product that was manufactured and packaged during or prior to 1997. Thus, if you have a stockpile of Bayleton with the old label and it has small grains on the original label, you can still use it on wheat.

D) Four Virus Diseases in Wheat - Pat Lipps CORN Questions

This year we have already seen four different virus diseases in wheat. These include Wheat Yellow Mosaic (wheat spindle streak mosaic), Wheat Soilborne Mosaic, Wheat Streak Mosaic and Barley Yellow Dwarf.

1. To date the most widespread is Wheat Yellow Mosaic, with Wheat Soilborne Mosaic being much less frequent. Both of these virus diseases are vectored by a fungus that lives in the soil. Also, they are greatly affected by temperature. As the weather warms in the spring the symptoms observed in the leaves of affected plants generally declines and plants appear to recover. Thus, yield losses by these two diseases is generally limited, being 5-10% losses for Wheat Yellow Mosaic and 10-30% for Wheat Soilborne Mosaic. Plant varieties with resistance to these virus diseases if they occur on your farm.

2. Wheat Streak Mosaic is the worse virus problem because of its ability to cut yields. This virus has not been common in wheat in Ohio, but when it has occurred losses have ranged from 30-60%, depending on the proportion of the field affected. Symptoms of Wheat Streak Mosaic will continue to increase in severity as the growing season progresses. Affected fields generally look very uneven, stunted and ragged. Severely affected plants may not produce heads. Upper leaves of plants have thin pale streaks between the veins giving the leaves a pale yellow color. This virus disease is spread by the wheat leaf curl mite that survives on volunteer wheat from wheat harvest until the new crop emerges in the fall. As the volunteer wheat dies the mite moves to the new wheat crop. The only way to control Wheat Streak Mosaic is to kill volunteer wheat soon after harvest.

3. Barley Yellow Dwarf has been seen on scattered plants in early planted fields. The most notable symptom is the yellowing and reddening of the tips of the flag leaves on affected plants. Barley Yellow Dwarf is spread by aphids. The most severe symptoms occur on early planted wheat infected in the fall. This year we are only seeing scattered plants in fields affected, thus yield losses will be minimal. Spraying in the fall for aphids has not been as effective in controlling Barley Yellow Dwarf as delaying planting until after the Hessian Fly Safe Date.  You may still have a few plants infected when planting on the Hessian Fly Safe Date, but this is much better that planting early and having the entire field affected.

E) Efficacy of Alfalfa Weevil Treatments - Hal Willson CORN Questions

Calls have been received during the past week about the relative efficacy of treatments available for control of alfalfa weevil. Due to the fact that significant infestations of alfalfa weevil only occur about every five years, we have minimal Ohio based data on the relative efficacy of treatments currently labeled. However, we have just collected some data from a trial currently in progress at the Western Branch station which provides some insight on products currently being applied for control of alfalfa weevil. Pre-treatment and post-treatment (7 days after treatment) data recently collected is presented in the following table.

It should be noted that both the high and low rates of Baythroid and Warrior provided effective control of weevil. Thus, application of these new synthetic pyrethroid products may be applied at rates that should be cost effective.  In the case of Pounce (permethrin), control of weevil was not as effective as the other treatments included in the trial.  Questions have been received from the field during the past week regarding the efficacy of Pounce as a weevil treatment.  However, it should be noted that stand height was not significantly reduced in the Pounce treatment relative to the other treatments applied.

Alfalfa weevil in central Ohio as of May 4th were predominantly in the 3rd instar stage, and pupation of 4th instar larvae have been detected.  Given the stage of weevil development and alfalfa growth, early harvest should be the optimal action to be taken if treatment of weevil is under consideration. However, if early harvest is not possible due to various reasons (corn planting priorities, etc.), then treatment may be justified where economic infestations are present.

F) Detection of Cutworms and in Surface Residues - Hal Willson CORN Questions

A close inspection of the soil surface under a chickweed stand in central Ohio prior to planting will likely lead to detection of numerous cutworms this season. Such cutworms are usually rather large (an inch or longer in length) and have clear chevron markings and longitudinal stripes. Most of these cutworms are dingey cutworms, which overwinter in the larvae stage and are likely to pupate before corn germinates or emerges. In addition, other species of cutworms may be detected which do not readily fit any description in most manuals generally used to identify cutworms.

G) Slugs are Active Earlier than Normal - Ron Hammond CORN Questions

Sampling indicated that slug eggs in Wayne County have begun to hatch, approximately 2-3 weeks ahead of schedule. This early hatch was caused by warmer than normal winter temperatures. Sampling last week indicated that many, if not most, slug eggs had hatched within the past week. Normally eggs in this area of Ohio hatch in mid-to-late May, followed by slugs causing significant feeding injury in late May and early June. It should be mentioned that egg hatch might have occurred earlier in more central and southern Ohio. With this early egg hatch, growers should be aware of the potential for slug injury much earlier than normal, perhaps in the next few weeks. The greatest concern for growers is that the corn and soybeans that are currently planted, or being planted, will be emerging soon and will be at a very susceptible growth stage when the slugs begin feeding. Thus, it will take fewer slugs to cause economic injury to the smaller plants that have just emerged. There might be a short time period between the time when slugs begin feeding and when significant economic injury occurs. Growers who have had a history of slug injury over the past years should be advised to begin monitoring their crops for slug injury now, rather than waiting until late May. Both corn and soybean will show feeding injury, and visits at dusk might indicate the presence of slugs. Growers should also be advised to check underneath the crop residue to see the condition of seedlings that have yet to emerge. If significant injury is occurring, a molluscicide bait should be applied. The main molluscicide bait in Ohio is Deadline MPs. This bait should be applied as a broadcast application between 7.5 to 10 lbs per acre. If the lower rate is used, extra care has to be taken to achieve even coverage. Trials in Ohio indicate that these rates do a good job of controlling slugs when the baits are applied evenly.

H) Greenhouse Results from Studies on ALS-Resistant Giant Ragweed - Jeff Stachler CORN Questions

We have tested 53 populations of giant ragweed in the greenhouse and found that 21% of the populations are ALS-resistant. We used FirstRate at 0.6 oz/A (2X rate) to test for the resistance, since it is the best ALS-herbicide to control giant ragweed. Sixty-two percent of the 53 tested populations were susceptible with the remaining 17% of the populations containing 1-2 plants that may be resistant or gaining resistance. The following counties have one or more populations of ALS-resistant giant ragweed based upon these studies: Auglaize, Clinton, Crawford, Delaware, Hancock, Pickaway, Union, and Wyandot. Two of the ALS-resistant giant ragweed populations came from fields that also have confirmed ALS-resistant common ragweed! Stay tuned for more information as we approach the post-emergence herbicide season.

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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 (Plant Pathology), Anne Dorrance (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Hal Willson (Entomology), Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Barry Ward (Champaign), Steve Prochaska (Crawford), Bruce Clevenger (Defiance), Greg LaBarge (Fulton), Gary Wilson (Hancock), Howard Siegrist (Licking), Glenn Arnold (Putnam), Ray Wells (Ross), Clark Hutson (Seneca), Roger Bender (Shelby) and Andy Kleinschmidt (Van Wert).

Editor: Clark Hutson        Web Editor: Nathan Watermeier

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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