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November 12-25, 2002
C.O.R.N. 2002-38
In This Issue:
A) What About Corn Flea Beetle In 2003?
B) When Is It Too Late To Control Dandelions?
C) Efficacy Of Seed Treatment Fungicides For Agronomic Crops
In Ohio
D) Managing Field Crop Diseases With Host Resistance - 4 Key
Points
The corn flea beetle (CFB) is a pest of corn in Ohio primarily because of its potential to transmit Stewart's Wilt, a bacterial disease that causes wilting and death of young plants and leaf blight on older plants. The beetle will occasionally built up in large enough numbers on young seedlings early in the season to seriously damage corn to the point that stand may be reduced but this is rare.
This pest overwinters as an adult in leaf litter and other protected places. The adults emerge in the spring and move to corn. There may be 2 to 3 generations per year in Ohio. During very cold winters beetle mortality is great and the threat from CFB is reduced. However, when winters are mild, such as was the case in Ohio in the winter of 2001-2002, then beetles survive in large numbers and can be a threat to corn.
A method has been devised (flea beetle index) that can be used to estimate winter survival of the CFB. The index is calculated as the sum of the average temperatures of December, January and February. Index values of less than 90 indicate minimum damage potential while an index value of 100 or more indicates severe damage potential.
The flea beetle index calculated during the winter of 2001-2002 was greater than 100 for all weather stations reporting in Ohio. Large numbers CFB emerged in the spring of 2002 and migrated to corn. Cool and wet growing conditions in the spring of 2002 resulted in less than ideal conditions for corn growth. Some fields were infested with CFB to the point that a foliar insecticide had to be applied to prevent stand loss.
Ohio normally only has problems with the overwintering beetles on corn and the later generations of CFB do not cause a problem. However, in 2002, the later generations continued to build and due to less than ideal growing conditions because of drought, CFB feeding was evident on the leaf edges and leaf tips of corn plants in June and later. These later generations spread Stewart's leaf blight to severe levels in many fields.
What about CFB in 2003? Large numbers of CFB adults are entering overwintering sites this fall. A mild winter (flea beetle index > 100) will result in high levels of beetle survival and the potential for serious problems in the spring. Personnel at OSU Extension will monitor weather conditions in December 2002 and January & February 2003 to determine the average temperatures during this period. If weather conditions remain mild during this period then the threat of overwintering flea beetle numbers and the potential for Stewart's wilt remains high. There are a couple of strategies that could be used to help deal with high beetle numbers in 2003.
The first strategy would involve the use of a systemic insecticide for flea beetle applied at planting time. There are 2 seed treatments, Cruiser and Gaucho, with systemic activity that are labeled for flea beetles. Cruiser is labeled on field, pop, sweet and seed corn while Gaucho is labeled on field and sweet corn. Other systemic insecticides applied at planting that have flea beetles on the label are Counter and Furadan 4F. Both are labeled on field, pop and sweet corn. This strategy might be used if the hybrid being planted has very little resistance to Stewart's wilt.
The second strategy is to begin scouting corn fields as soon as they emerge and then use a foliar insecticide as a rescue treatment to control the flea beetles if economic thresholds are reached. This strategy might be use if the hybrid being planted has high resistance to Stewart's wilt. Both strategies require fields be thoroughly scouted on a regular schedule.
Insect infestations are very difficult to predict and this is true for corn flea beetle. The important thing for growers in Ohio in 2003 is to pay attention to the winter weather and if weather conditions remain mild then anticipate the possible need for flea beetle control. Additionally, when buying corn seed for 2003, select hybrids with higher levels of resistance to Stewart's wilt and leaf blight.
Several past C.O.R.N. newsletter articles have covered our herbicide recommendations for dandelion control. What we really do not have a good handle on is how late in the fall herbicide treatments can be applied and still maintain their effectiveness on established dandelions.
We applied our 2001 fall treatments in mid-November during a week of warm weather, and the weather had been relatively warm up until that time. This year is somewhat more of a challenge, since last week's cold weather appears to have affected dandelion growth, and wet soil conditions will make application difficult in many fields through the next week or so.
The small dandelions at Western Branch in South Charleston have turned purple, yet the large, well-established plants are still green and healthy. We assume that herbicide treatments applied over the next month or so will maintain much of their effectiveness (and will certainly be more effective than spring treatments) as long as the ground is not frozen and herbicide can be applied during a few days of relatively warm weather. We would expect herbicide activity on dandelion to decline, however, if we experience a prolonged period of very cold weather.
Our research over the past three years indicates that herbicides can be applied through December for control of winter annual weeds, even after they have been subjected to an extended period of cold conditions.
Seed treatment fungicides are useful tools to manage seed and soil borne pathogens. Seed treatments are highly recommended in Ohio on seeds of corn, soybeans, wheat and alfalfa for fields with poor drainage and a history of stand establishment problems. It is important that agronomic crop producers know what the pathogens are in specific fields in order to choose the best fungicide or combination of fungicides for that field. In addition, the correct choice of fungicide will also limit losses due to seed-borne pathogens. Refer to Extension Bulletin 639 "Seed Treatment for Agronomic Crops" http://ohioline.osu.edu/b639/index.html) for more detailed information concerning seed and seedling diseases affecting field crops in Ohio. Seed treatment companies have made available several new products in recent years. The current seed treatment fungicide recommendations can be found on the Ohio Field Crop Disease web site (http://www.oardc.ohio-state.edu/ohiofieldcropdisease) in the section - Seed Treatment Efficacy.
The most effective and economic method of disease control is to plant disease resistant varieties and hybrids on your farm. Here are four points you need to know and understand to get the most out of resistant vairities.
1. SCOUT, SCOUT, SCOUT your fields. As we well know, disease does not occur every year. Years when environmental conditions are favorable for disease are the best years to evaluate variety performance. Scouting can alert you to disease problems early so appropriate action can be taken, like applying a fungicide, at the proper time to achieve maximum return on dollars spent.
2. Limit inoculum build-up. Partial resistance can be overcome when high levels of the pathogen are present in a field and the environment is especially favorable for the pathogen. Both crop rotation and tillage work on reducing the numbers of pathogens in a field.
3. Limit environmental effects - Many pathogens require long hours of moisture either as free water, or high humidity. Tillage and tiling are effective in reducing the time soils remain saturated. Avoiding excessive fertilization by using a balanced fertilizer program. Avoid excessive plant populations that promote lush canopy development and plant-to-plant competition for nutrients and water. Choose production practices that work to promote crop health without favoring pathogens.
4. Keep good records. This is necessary to record the varieties used in each field and keep records of those "good" and "bad" disease years. Keep records on how much disease developed in various fields and how well the so called resistant varieties performed. Use this information to make variety selection in the future.
The final point, begin your record keeping now! It is important to make written notes while your memory is fresh and you are still bemoaning the poor yields we had in Ohio during 2002. The drought had a major impact on yields. To assist you in your records, we have provided some of our observations on field crop diseases for 2002.
SOYBEANS
Phytophthora root and stem rot - was present in several fields, surprisingly even with the drought. However, soils were saturated for most of the month of May, which favored the development and germination of the oospores. This is a thick-walled spore that stays in the soil for years. Even limited rains in late May and early June were enough to saturate the soils that favor infection of the soybean roots. Following the late rains in August, stem rot was evident. In two field research plots in Northwest Ohio we had 70% kill of the most susceptible variety.
Soybean Cyst Nematode - This beast did quite well in 2002. Several soil samples arrived at the clinic with alarmingly high numbers of nematodes. We also had several plant samples that arrived around the fourth of July - these were loaded with cyst females and these females already had made eggs. This indicates that these populations were high early in the growing season and were most likely at advanced stages of development. In addition, samples with cyst also had other root rots. Two root pathogens and a drought is an awful lot to ask of a plant.
Charcoal rot - this root and stem pathogen was present in Southern Ohio at economically damaging levels for the first time in many years. Soybeans in these fields died prematurely, both from charcoal rot and a number of other root pathogens. This is a pathogen we will begin to monitor in this region. The concern will be if this pathogen will increase during these consecutive seasons of dry growing conditions.
Bean Pod Mottle Virus - a few plants reported from all areas of the state that had classic green stem symptoms as well as up-turned malformed pods. The preliminary assays are coming back positive for the presence of BPMV in these plants as well as in a few samples of Bean leaf beetle.
Sclerotinia stem rot, brown stem rot, stem canker and SDS were all very limited this year. There were a few fields with problems but levels were much lower than in the past. In fact, one field where we are testing a biocontrol product for Sclerotinia stem canker had 1 plant in the whole field. In years where the environment is favorable for this pathogen, 30 to 40% of the plants with Sclerotinia is not uncommon. The pathogens which cause these diseases all require moist conditions during the mid to late vegetative growth stages. This would have been at the beginning of the drought.
CORN
Seedling blights caused by soil fungi like Pythium and Fusarium were very common in many fields throughout Ohio. Wet, cold soils that remained saturated for several weeks favored infection by these fungi. Poorly drained soils, especially no-till fields had problems with stand establishment. Although there is little we can do about the weather, improving drainage is important. Additionally, check to make sure an effective seed treatment is used on the seed.
Stewart's bacterial leaf blight was the most evident corn leaf disease during 2002. The mild winter of 2001-2002 permitted high populations of the corn flea beetle to survive and begin to feed on young corn plants. As the beetles acquired the bacterium from infected plants they spread it rapidly from one plant to the next throughout the summer. By late summer significant damage to the leaves in many fields was evident. It was easy to detect those fields that were planted to hybrids with resistance to this disease and those that were not. You can expect an epidemic of this disease again in 2003 if the winter weather is mild and flea beetles again survive in large numbers.
Stalk rot diseases were severe in many fields, especially those that suffered from drought stress, but still were able to produce and ear with reasonable yield. The drought stress combined with the stress of making grain on minimal nutrition and water greatly increased the stalk rot diseases. Fusarium and Gibberella stalk rots were particularly severe causing lodging and lost grain in the field. Always buy hybrids with the best standability. Ohio is notorious for stalk quality problems.
Fungal leaf blights were not as common in Ohio as past years because of the persistent drought conditions. It just was not wet enough for the fungi to produce spores or infect leaves.
WHEAT
Barley yellow dwarf was more common and yield limiting in 2002 than in past years. This was due to the extended mild weather Ohio experienced in the fall that favored the activity of aphids that carry this virus disease to young wheat plants. Additionally, some wheat producers planted wheat before the acceptable planting date (Hessian Fly Safe Date) and because the weather was warmer in mid September, these early planted fields were heavily visited by aphids and in some cases over 80% of the plants in the field had symptoms of barley yellow dwarf.
Fusarium head scab occurred at only low to moderately low levels in Ohio in 2002. Colder than normal temperatures limited the development of the Fusarium fungus and prevented high levels of infection. Dry conditions during grain fill in mid to late June limited the accumulation of the mycotoxin, Vomitoxin, in those heads that were infected.
Tan spot, a leaf disease usually not common to Ohio, was present in many fields throughout the state. Apparently the early May wet conditions followed by drier condition in June favored disease development. We do not know much about the resistance in our wheat varieties to this disease because of its relatively rare occurrence.
Powdery mildew and Stagonospora leaf blotch were not major problems for most
farmers this year because it was too wet in May for powdery mildew and too cold
for Stagonospora leaf blotch. Unfortunately, the cold temperatures in mid May
caused extensive freezing injury to many fields resulting in significantly reduced
yields.
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.
State Specialists: Pat Lipps, Anne Dorrance, & Dennis Mills (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Bruce Eisley (IPM) and Ron Hammond (Entomology) District Specialists: Ed Lentz (Agronomy) Extension Agents: Woody Joslin (Shelby), Greg La Barge (Fulton), Howard Siegrist (Licking), and Harold Waters (Miami)Editor: Greg LaBarge 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.
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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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