CORN
Crop Observation and Recommendation Network

September 3 - September 8, 2002
C.O.R.N. 2002-29

In This Issue:

A) Controlling Dandelion With Fall Herbicide Applications
B) Control Dandelion Before Wheat Emergence!
C) Late- Season Corn Leaf Disease Diagnosis
D) Premature Plant Death In Corn And Effects On Yield
E) Minimizing Harvest Losses In Drought Damaged Corn Fields
F) 2002 Farm Science Review
G) Tri-County Agronomy Day

A) Controlling Dandelion with Fall Herbicide Applications - Mark Loux, Jeff Stachler, and Dobbels CORN Questions

Common dandelion has become extremely problematic in many corn, soybean and wheat fields across Ohio over the past several years. We believe there are several reasons for this, including the following:

• No-tillage and reduced tillage systems, which allow dandelion and other perennials to become established and increase in population density.
  
• Inconsistent control of dandelion with spring burndown treatments.
  
• Reduced use of preplant herbicides with residual activity in soybeans, and an increase in total postemergence herbicide programs (or burndown without residual followed by postemergence programs). Since the emergence of new dandelion seedlings occurs primarily in May, use of preplant herbicides with residual activity probably helps control dandelions emerging from seed within the month or so after soybean planting.
  
• Incomplete control of seedling or established dandelion with postemergence soybean herbicides. Dandelion plants that survive postemergence treatments become well established by late fall, and are extremely competitive and difficult to control the following spring.
  
• The wind-borne movement of large quantities of dandelion seeds, which results in further spread.

Like many perennial weeds, dandelion is most effectively controlled with fall herbicide treatments. In OSU research, we have been able to achieve near complete control of low to moderate dandelion populations in the fall with several herbicide treatments. Some suggestions/observations on fall herbicide treatments for dandelion control follow. The 2,4-D rates shown are based on a 4 lb/gallon formulation. Where a rate range is shown for glyphosate, the first number refers to the Roundup UltraMax rate, and the second rate refers to Touchdown, Glyphomax, and other 4 lb/gal glyphosate products. Roundup UltraMax is being replaced by Roundup WeatherMax, which has a higher glyphosate concentration. The conversion: 26 oz of UltraMax = 22 oz of WeatherMax.

• Across all of the dandelion studies we have conducted in fields where soybeans will be planted, the most consistently effective dandelion control resulted from application of: glyphosate (26 oz/A of UltraMax or 32 oz/A of other glyphosate products); a mixture of glyphosate (13 to 16 oz/A) plus 2,4-D (1 pt/A); a mixture of Canopy XL plus Express plus 2,4-D; or 2,4-D alone (1 qt/A). While all of these are effective in low to moderate dandelion population densities, glyphosate or glyphosate plus 2,4-D has been the most effective treatment in very high dandelion populations in OSU research.
  
• In fields where dandelion and other winter annuals are the primary target of a fall herbicide treatment (but chickweed is not a problem), application of 2,4-D at the rate of 1 qt/A is an effective and inexpensive treatment.
  
• In OSU studies, mixing glyphosate and/or 2,4-D with Scepter, Pursuit, Valor, or Sencor has occasionally reduced dandelion control, compared to application of glyphosate and/or 2,4-D alone. Backdraft is a premix of Scepter plus glyphosate.
  
• With all treatments, the effectiveness of control and reduction in the dandelion population will be dependent upon the population density. In low to moderate population densities, the treatments mentioned above are capable of controlling all or most of the dandelion present. This level of control may not be possible where dandelion population densities are extremely high.
  
• In our limited comparisons of various 2,4-D products, the 2,4-D rate appeared to be more important that the type of 2,4-D applied. When applied alone, the rate should be the equivalent of at least 1 lb ae per acre (1 qt of 4 lb/gallon products). In combinations with glyphosate or Canopy XL/Express, a 2,4-D rate of 1 pt/A appears to be adequate.
  
• Where corn will be planted the following spring, consider one of the following treatments: glyphosate (26 oz of UltraMax or 32 oz of other glyphosate products; glyphosate (13 to 16 oz/A) plus 2,4-D (1 pt); 2,4-D alone (1 qt/A); Basis (0.5 oz); or Basis plus 2,4-D. Simazine can be added to these treatments to improve control of some winter annuals or extend the period of residual control, but simazine will not have much activity on dandelion. The most economical and broad-spectrum fall treatment for corn may be simazine plus 2,4-D (1 lb ai/A plus 1 qt/A).
  
• We recommend adding 2,4-D to all fall herbicide treatments, including glyphosate, because a mixture of herbicides with several different sites of action should delay the development of herbicide-resistant weed populations. A 2,4-D rate of 1 pt/A should be adequate in mixtures with glyphosate, Basis, or Canopy XL/Express. 2,4-D will also help control a number of winter annual and perennial broadleaf weeds. Mixing 2,4-D with glyphosate can reduce control of Canada thistle and some perennial grasses, compared to glyphosate alone, but this type of antagonism should not be a problem with other weeds.
  
• If possible, wait until after a light frost to apply herbicides for dandelion control. Applications too early this fall may not result in the most effective control due to the extended dry conditions across much of the state. Mid-to late-October appears to be an appropriate period for herbicide application, but we have achieved effective control with November applications also. Apply before a hard freeze.
  

B) Control Dandelion Before Wheat Emergence! - Mark Loux CORN Questions

Many weed problems in no-tillage wheat can be managed with a preplant/preemergence application of glyphosate. Dandelion and winter annual weeds such as chickweed and purple deadnettle have been serious problems in wheat in recent years. Producers are often not aware that these weeds are a problem until late fall or early spring. However, many winter annual weeds have emerged by late September and are easily controlled with relatively low rates of glyphosate prior to wheat planting. This can be a more effective treatment for dandelion and winter annuals, compared to the herbicides that can be applied broadcast to wheat in late fall or early spring. A dense population of winter annuals or dandelion may have already suppressed wheat growth by the time a spring treatment can be applied.

Dandelion is especially problematic because most wheat herbicides have limited effectiveness on this weed. Exceptions include Stinger and Curtail, which are effective on dandelion but must be applied in the spring. Express, Harmony Xtra, and 2,4-D have some activity on dandelion also. Dandelion should be controlled with tillage or glyphosate prior to wheat emergence, since options after emergence are less effective. Tillage must be thorough enough to completely disrupt dandelion plants, and this may not be accomplished with one pass of an implement designed primarily for seedbed preparation. Higher glyphosate rates will be required for dandelion control compared to winter annual weeds. We suggest the following glyphosate rates for dandelion: Roundup UltraMax - 26 oz per acre; Touchdown, Glyphomax, and other glyphosate products - 32 oz per acre. Include ammonium sulfate in glyphosate treatments, and the appropriate amount of surfactant if specified by the product label. We suggest delaying treatment until late September or early October, and after significant rain if possible, when dandelions are more likely to be actively growing. The activity of herbicides on dandelions may be reduced if applied under our current dry conditions when dandelions have relatively little foliar growth.

C) Late- Season Corn Leaf Disease Diagnosis - Pat Lipps and Dennis Mills CORN Questions

Due to the poor condition of much of the corn crop in Ohio, people are having difficulty diagnosing some of the disease problems in fields of drought stressed corn. Additionally, the plants in fields have begun to die prematurely. Drought stress, associated nutrient deficiency symptoms and insect feeding injury also makes the symptoms of certain diseases more difficult to recognize. Probably the best way to evaluate leaf diseases in the crop at this time is to look at the greener leaves above the ear since most of the lower leaves on the plants are already dead or wilted.

Most fields are showing extensive flea beetle injury on the leaves, particularly near the leaf tips. This injury looks like thin white lines etched into the leaf surface. When sufficient injury occurs on the leaves, the tissue may die and look like a leaf disease. To help identify flea beetle injury symptoms hold the affected leaf up so the sunlight shines through the tissue so that the flea beetle feeding injury becomes very evident even when the leaf tissue has been killed.

Stewart's bacterial leaf blight is seen on the same plants that have flea beetle feeding. This is because the flea beetle carries and transmits the bacterium that causes Stewart's leaf blight. It is more difficult to identify the disease on heavily diseased leaves than on leaves that have a lot of green area left, so right now look at the upper leaves of the plants. Stewart's lesions begin as small pale green water-soaked areas associated with flea beetle feeding tracks. The lesions rapidly enlarge as the bacterium multiplies in the leaf veins killing the green chlorophyll in the leaf cells. The advancing edges of the lesions along the veins are pale green and as the tissue dies the diseased area turns brown. The advancing edge of the lesions have a very wavy outline which is characteristic of Stewart's leaf disease.

Gray leaf spot is also common in fields, especially those in continuous corn. The gray leaf spot fungus lives on corn residues over the winter and spores are spread to the healthy crop in mid summer. Generally, gray leaf spot increases in severity as the season progresses and the morning foggy weather we frequently have in Ohio in late August and September favors infection of leaves. The gray leaf spot lesions are typically tan and rectangular in shape. However, many of the newer hybrids have lesions that are more yellow to orange in color and the rectangular shape is most evident in the older and larger lesions. When attempting to diagnose gray leaf spot, look at leaves where the lesions are well separated by green tissue. Look for lesions with a rectangular shape where the lesions appear to be limited to between the leaf veins.

Northern leaf blight is not as common in Ohio this year as it was last year, however affected plants can be found in certain fields planted to susceptible hybrids. These lesions are quite unique and relatively large. Northern leaf blight lesions are cigar shaped, one half inch to over one inch wide and six inches or more long. The lesions are tan, but frequently the spores of the fungus may make the center of the lesions appear pale olive green color. It does not take many of these large lesions to destroy a leaf. Apparently the dry growing conditions have limited the spread of the disease in most fields.

Although these leaf diseases have been common in Ohio corn fields this year, the drought conditions have been much more important to the productivity of the corn crop than leaf diseases. The presence of leaf diseases have put additional stress on the plants and may predispose plants already stressed by drought to stalk rot diseases. For more information on the importance of stalk rot diseases in Ohio's corn crop please read the articles in the two previous C.O.R. N newsletters on the web at http://corn.osu.edu/archive/. For more information on corn diseases their diagnosis and control visit our Ohio Field Crop Disease web page at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.

D) Premature Plant Death in Corn and Effects on Yield - Peter Thomison CORN Questions

Premature plant death is occurring in many corn fields across Ohio largely as the result of continuing dry weather. Plants that have died or are dying prematurely have lost or are rapidly loosing their green tissue, and have turned sort of a yellowish brown, tan, or brown color. Another symptom of premature corn death are ears that are drooping (ear tips pointing downward) because the shanks connecting the ear to the stalk have collapsed. Normally ears will remain erect until about black layer (physiological maturity) after which they will start to droop downward as the ear shank tissue collapses. However, genetic background also influences ear position as ears of some hybrids remain upright longer than others (possibly due to differences in shank length). Premature ear drooping can also occur as the result of other prematurity stresses - such as European corn borers tunneling into the shanks, foliar and stalk rot diseases, late season hail damage, early frosts, etc.

Yield losses in corn plants that have died prematurely vary depending on when death occurs. In studies conducted in Minnesota back in the early 1980's, effects of premature death on yield were simulated by defoliating plants at different kernel development stages. Losses were estimated at 39 to 42% when plants were harvested at full dent following defoliation; and at 11-12% when plants were harvested at late dent following defoliation. Stalk rots are often associated with corn that dies prematurely - check out Pat Lipp's recent article for more on monitoring stalk rots and stalk quality in corn as harvest nears (specifically, read "Premature Death in Corn and Stalk Quality", C.O.R.N. #27, 2002). Premature plant death increases the likelihood of stalk lodging, and fields with high percentage of affected plants should probably be marked for early harvest.

E) Minimizing Harvest Losses in Drought Damaged Corn Fields - Peter Thomison CORN Questions

This year's widespread drought has resulted in smaller than normal ears, and much greater percentage of "nubbin" ears in many fields. In addition, plants are shorter than normal with reduced ear heights. As a result of these conditions, some combine and harvesting adjustments may be necessary. The following are management suggestions from ag engineers and equipment specialists on harvesting drought damaged crops.

1. Review the operator's manual for suggestions on harvesting a "light crop".
   
2. With short or lodged corn, run the gathering snouts and chains low. Watch for stones, and be sure stone protective devises are working.
   
3. Drive carefully and at normal speeds to avoid excessive harvest loss and machine damage from stones.
   
4. For small ears, set stalk rolls and snapping plates closer than normal to snap off a higher percentage of ears. Do not attempt to snap off barren cobs.
   
5. If clean shelling is a problem, increase cylinder speed slightly, and if necessary, decrease concave clearance. With a rotary machine, check rotary concave clearance. Avoid excessive damage to kernels from good ears.
   
6. If cleaning losses are high, open the chaffer and chaffer extension slightly.
   
7. Initially decrease the amount of air from the cleaning fan. If cleaning becomes a problem, increase the fan blast, and close the lower sieve slightly.
   
8. Be alert to changes in weather and crop conditions, and make adjustments as necessary.
   

F) 2002 Farm Science Review - Ray Wells CORN Questions

 September 17-19 are the dates for this year's OSU Farm Science Review at the Molly Caren Agricultural Center near London. The OSU Extension Agronomic Crops Team and the OSU Extension Precision Agriculture Team will have exhibits located at the corner of the Alumni Park on Friday Avenue, near the center of the main exhibit area. Anyone who stops in and completes a brief survey will receive a copy of the new and improved edition of Bulletin 827, the Corn, Soybean, Wheat, and Alfalfa Field Guide. See You there!!

G) Tri-County Agronomy Day - Howard Siegrist CORN Questions

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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.

Editor: Andy Kleinschmidt        Web Editor: Tom Rosati

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