CORN
Crop Observation and Recommendation Network

April 10-16, 2000
C.O.R.N. 2000-8

In This Issue:

A)    What’s the Deal with Chickweed and Purple Deadnettle?
B)     Stage Wheat Before Applying Herbicides
C)    Weed Emergence Report
D)    Be Aware; Wheat Growth Stage is Greatly Advanced
E)     Wheat Disease Update-Powdery Mildew and Virus Diseases
F)     Soybean Seed Treatment – What are the Options

A) What's the Deal with Chickweed and Purple Deadnettle - Mark Loux CORN Questions

We are having another banner year for chickweed and purple (or red) deadnettle populations in areas of the state.  The combination of warm temperatures in late fall and early spring has resulted in above-average growth, and some of these populations are dense and thriving.  Chickweed and dead nettle seem to be gradually increasing in population over time in some areas, but it is difficult for us to determine whether this is due to warm fall weather in recent years or other factors.  Anecdotal evidence suggests that a switch from preplant/preemergence herbicide programs (Squadron, Canopy etc) to Roundup Ready and other postemergence programs could be one cause of the problem, since some dealers see winter annual weed problems showing up more often following postemergence programs.  If so, this may indicate that the preplant herbicides are either: 1) preventing seed production by these weeds in the spring, or 2) persisting into the fall at rates that are high enough to reduce winter annual emergence.  Continued problems with winter annuals may warrant reconsideration of the utility of total postemergence programs.

Chickweed, deadnettle, mustards, and most other winter annual weeds complete their life cycle in the spring, going to seed around the time of crop planting or shortly thereafter.  However, some winter annuals can demonstrate summer annual weed characteristics under certain environmental conditions.  We have occasionally observed mustard and marestail emerging with the foxtail, lambsquarters and other summer annual weeds over the month or so following corn or soybean planting.  Because they generally are completing their life cycle in late spring, one management strategy when winter annual populations are low is to apply burndown herbicides and plant the crop regardless of how effective the herbicides were on the weeds. This can be an adequate strategy, since the winter annuals are most likely reduced in vigor and are unlikely to interfere with early crop establishment.  However, as crop planting is moved earlier and winter annual populations increase, a more aggressive management strategy will be needed.  Early crop planting may coincide with an earlier growth stage of the weeds when they are more competitive and can interfere with crop establishment. Dense populations of winter annuals can interfere with the planting process and in some cases, prevent the soil from drying and greatly delay planting. Some winter annuals, such as marestail, also are more competitive with the crop because their life cycle is not complete until early summer.

We conducted research at three sites this year, with a number of fall- and spring-applied herbicide treatments.  One site had a dense chickweed population and the other two had moderate to dense populations of purple deadnettle. Most treatments included 2,4-D amine, but it contributed essentially no control of chickweed and poor control of dead nettle (but 2,4-D ester is cheap enough that it should probably be included in any treatment of this type). Results varied by weed species, making it somewhat difficult to develop a single management strategy for both weeds. Overall, late fall applications (early November) were a far better approach than spring applications (applied about three weeks ago), with regard to both effectiveness of control and minimizing interference with soil drying and planting.

Fall treatments providing at least 90% control of chickweed included Steel, Command, Roundup (1 pint), and Banvel (1 pint).  Fall-applied Sencor and Sencor+Python combinations were more variable, with control ranging from about 60 to 80% depending upon rate (the 8 oz rate of Sencor provided close to 80% control).  Spring treatments providing the most effective control included Roundup (94%), Gramoxone (80%), and Sencor (74%).  For control of existing chickweed populations at this time, our results thus show Roundup to be an effective chickweed herbicide, or possibly a combination of Gramoxone plus Sencor where more rapid burndown is desired.  The Gramoxone and Sencor plots were characterized by large areas of completely dead chickweed plants, interspersed with very healthy plants.  This pattern may indicate that effective spray coverage and proper adjuvant use with contact herbicides is critical to maintain their effectiveness on chickweed.

Control of dead nettle varied by site and size of the weeds at the time of application.  Plants were considerably smaller in the spring at our site near South Charleston, compared to our site near Amanda. At South Charleston, fall application of Python+Sencor, Canopy (2 or 4 oz), Steel, Command, Sencor (8 oz), or Roundup provided good to excellent control (although Sencor was somewhat more variable than the other treatments).  At Amanda, good to excellent control occurred with fall-applied Canopy (4 oz), Command, or Sencor (8 oz).  Fall-applied Roundup and lower rates of Canopy and Sencor were more variable at Amanda, but still provided at least 80% control. For spring treatments at South Charleston, adequate control resulted from Sencor (98%), Canopy (89%), or Python+Sencor (88%). At Amanda, the most effective spring-applied control was 79% with Sencor or Canopy. Gramoxone applied in the spring provided some control at both locations(59 to 69%).  Roundup was much less effective in the spring than in the fall. As with chickweed, the combination of Gramoxone with a product containing at least 4 oz of Sencor (and 2,4-D ester) may be a good strategy at this time of the year. It should be noted that we did not include Lorox in this research, and it may provide control similar to Sencor. 

Our observations of producers' fields south of Columbus treated with a variety of herbicides over the past month indicate that effectiveness of the above-mentioned treatments may be highly variable with weather conditions. While we observed good to excellent control with several spring-applied herbicides in our plots, control with these same treatments was much reduced in some producers' fields. This could be due to both weather and application factors, but we suspect that weather is the main factor responsible for variability in herbicide activity. 

We recommend that application of burndown herbicides be targeted as much as possible for periods of warm, sunny weather. Activity of Gramoxone is highly dependent upon sunlight, and activity of Roundup and other glyphosate products is highly dependent upon growth status of the plant.  Weather conducive for plant growth is also good for glyphosate activity, and application during periods of cold weather can greatly reduce control.  Maintain high spray volumes for Gramoxone/Sencor treatments and include crop oil concentrate. Include ammonium sulfate with glyphosate treatments (and surfactant if recommended by the product label).  Where drying of soil is a concern, apply as soon as possible to allow time for plants to die and dessicate. Also, keep in mind that these weeds will eventually die and dessicate anyway, and if this appears to be happening early enough in the spring relative to your intended planting date, than specific control measures may not be warranted at this time.

B) Stage Wheat Before Applying Herbicides - Jeff Stachler CORN Questions

Many wheat fields across Ohio, especially south of U.S. Route 30, have begun to joint! Before applying Banvel, Curtail, MCPA and 2,4-D, double check the stage of the wheat. Application of these products, especially Banvel, can cause significant yield loss once the wheat has begun to joint. The labels of Curtail, MCPA, and some 2,4-D products allow application up to early boot stage, but risk of injury is increased compared to application before jointing. Curtail, MCPA and 2,4-D are safer to wheat beyond the jointing stage than Banvel. The use of 2,4-D amine is also safer to wheat than 2,4-D ester, but 2,4-D ester can provide more effective weed control. To minimize the risk of injury with 2,4-D after jointing, use water only as the carrier and do not apply more than ½ pint of ester or 1 pint of amine.

Now is the time to apply herbicides in wheat, as winter annual weeds are abundant, summer annual weeds have started to emerge, and a number of herbicides can still be used safely. The most broad-spectrum weed control program in winter wheat is Harmony Extra plus 2,4-D ester, especially at or shortly after jointing. A Harmony Extra plus Buctril/Moxy combination is nearly as effective, but not as good on dandelion, Canada thistle, and marestail. Wheat herbicides can generally be applied in nitrogen fertilizer solution when the wheat is top-dressed. This may increase injury somewhat, and some labels recommend adjusting surfactant rates to minimize injury. The Harmony Extra plus 2,4-D combination should only be applied in water if the wheat has begun to joint.

C) Weed Emergence Report - Jeff Stachler CORN Questions

Atriplex, giant ragweed, smartweed, common lambsquarters, and Canada thistle are up in fields and growing well. Some giant ragweed have 2 pairs of leaves already and some Canada thistle are 3-4 inches tall. Take this into consideration when applying burndown herbicides.

D) Be Aware: Wheat Growth Stage is Greatly Advanced - Pat Lipps CORN Questions

The warm temperatures we have experienced in February and March have pushed the growth of wheat to stages not seen in recent history. Those varieties that break dormancy early, like varieties adapted to southern regions, have had considerable time for re-growth during late winter and are most advanced. By the first of April, the more advanced fields in southern Ohio were beginning the stem elongation growth stage when the first node is detected on the main stems just above the soil surface (Feekes’ growth stage 6). Earlier this week we found some of our most advanced wheat plots with plants at growth stage 6 in Wooster. Check plants for this growth stage by pulling a plant or two from the field and separate out a main tiller from the plant keeping the roots attached to the tiller. Strip the outer, lower leaves and leaf sheaths down to expose the inner stem. At growth stage 6 you should see a node on the stem about one half to one and a half inch from the base (where the roots come out) of the tiller. The node is a definite structure so you should not have to hunt for it.  

Our historical records indicate that this growth stage is 2 to 3 weeks earlier than normal for these locations. This growth stage is important for several management decisions. 1) Some herbicides, like Banvel, should not be applied at or after this growth state. Be sure to examine plants in each wheat field before applying herbicides with growth stage restrictions. 2) Stem elongation signals the time of increased Nitrogen uptake for the wheat plant. If you have not applied your spring top-dress of nitrogen, then make plans to apply the appropriate amount as soon as possible. It is best to have the full amount of nitrogen on a period of time prior to stem elongation growth stage. 3) Growth stage 6 is a good time to begin to scout fields for diseases. It is the perfect time to determine if powdery mildew is started in the field and if you have early season virus diseases. 4) The advanced stage of the wheat at this time of year will increase the potential for cold or freezing injury to the crop. We will be monitoring the crop closely, especially during cold periods, provide our predictions in future C.O.R.N. communications.

E) Wheat Disease Update - Powdery Mildew and Virus Diseases - Pat Lipps CORN Questions

It has been an interesting season so far for the wheat crop. We have barely begun the season and already diseases are out there.

Powdery mildew has been found in many early planted fields of susceptible varieties. It would be wise to check fields of susceptible varieties at this time to note if powdery mildew is present or not, then come back at flag leaf emergence to determine if an application of fungicide is needed based on the movement of powdery mildew up the plant. Susceptible varieties include Pontiac, Cardinal, Dynasty, Patterson, 529W, 539W, 558W, Classic RW151, Classic RW1517, 25R26, RS901, RS927, SG1545, SG1550, Benjamin, Podach, Reino SR204, TS5020, TS8040, W9830 and possibly others.

Two virus diseases have been found in the state so far this season. Both appear to cause wheat to be yellow and are mistaken to be nitrogen deficiency. Wheat yellow mosaic (formerly wheat spindle streak mosaic) has been found in fields of susceptible varieties through the state. This disease has a long history in Ohio, being very important in the 1970's and 1980's. The development of resistant varieties has limited the extent of the problem during the 1990's. There are varieties planted today that are susceptible and show the typical yellowing symptoms with small yellow dashes and streaks in the upper leaves of the plants. Yellow mosaic symptoms tend to become less prominent as temperatures increase later in the spring. Susceptible varieties include: GR942, Sawyer, Pontiac, 544W, 558W, Classic RW151, 2571, 25R57, RS927, SG1550, Podach, SR204, TS4020, TS5020 and possibly other varieties. Yield losses are related to the severity of symptoms and the length of time symptoms are present in the flag leaves. Warm weather in May will help prevent yield losses.

Wheat Yellow Mosaic

We have much less information about the second wheat virus disease showing up in the state because this is only the second year we have recognized it in Ohio. We are still doing the proper tests to confirm our diagnosis, but we are tentatively calling this disease Wheat Soilborne Mosaic because the distribution of affected plants in the field is associated with low areas and the younger leaves of the plants show a mosaic (leaves with green areas mixed in a yellow background) symptom and plants of some varieties are severely stunted.

Wheat Soilborne Mosaic Symptoms

Wheat Soilborne Mosaic at Field Scale

Like Wheat Yellow Mosaic, Wheat Soilborne Mosaic is affected by temperature. The optimum temperature for symptom development is 60 F. Above 68 F disease progress stops. Symptoms typically appear in early spring right after green-up. During the time when wheat is rapidly increasing in height, mosaic symptoms fade, but stunting may persist until maturity. Yield losses are related to the size of the area affected in the field and the length of time plants show symptoms. A long cool spring prolongs the symptomatic period. Losses may range from 10-50%, but are usually highly variable due to soil conditions and varietal differences. We have little information on the susceptibility of different wheat varieties grown in Ohio to Wheat Soilborne Mosaic. Our observations indicate there is considerable variability in the level of resistance among varieties. This disease has been a problem in the Great Plains wheat growing areas for some time. We have used information from Kansas State University for this article (http://www.oznet.ksu.edu/dp_path-ext/factsheets/wheat/wheat4.htm)

F) Soybean Seed Treatment - What are the Options - Anne Dorrance CORN Questions

Seed Treatment fungicides can help protect soybean seeds from some seed-borne fungi such as Phomopsis and soil-borne fungi such as Phytophthora sojae, Pythium, Rhizoctonia and Fusarium. Commercial applicators have several options and these are outlined in Bulletin 639A, Efficacy of Seed Treatment Fungicides for Agronomic Crops in Ohio which is available from County Extension Offices and Ohioline. Several seed treatment products contain a fungicide that is a skin irritant, so proper handling and personal protective clothing is essential. In addition, many of the fungicides are applied at very reduced rates requiring precise application to seed. Commercial application equipment for seed treatment is more suited for low rate applications than traditional hopper box treatments.

There are some fungicide treatments available to producers to treat their seed on-farm either as a hopper box treatment or with systems designed for an auger conveyance system. For all of these seed treatments it is very important that producers read and follow the label directions. Be sure to wear the protective clothing that is listed on the label in that some of these products are skin irritants. Be sure that the seed and seed treatment compounds are thoroughly mixed to ensure even distribution of the material on the seed. Even coverage of the seed is necessary for maximum protection of the seed and seedling.

E=excellent, N=no control, G=good, F=fair, P. sojae=Phytophthora, Rhizoc.=Rhizoctonia

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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 & Anne Dorrance (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science); District Specialists: Ed Lentz (Agronomy); Extension Agents: Roger Bender (Shelby), Steve Bartels (Butler), Clark Hutson (Seneca), Barry Ward (Champaign), Dennis Baker (Darke), Gary Wilson (Hancock), Glenn Arnold (Putnam) and Steve Prochaska (Crawford).

Editor: Greg LaBarge        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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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.

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