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May 12-19, 2003
C.O.R.N. 2003-13
In This Issue:
A) Early Season Soybean Diseases
B) Applying Preemergence Herbicides to Emerged Corn
C) Wet Soils and Potential for Corn Seedling Blights
D) Wheat Development and Potential for Leaf Diseases
E) Wheat Head Scab Disease Risk
F) Aphids in Wheat
G) Cutworm and Alfalfa Weevil Update
H) Sawfly Larvae in Grasses
I) Slug Update
Pythium and Phytophthora sojae are two water molds that can attack seeds and seedlings when the soils are saturated. The soil temperatures this past week were close to optimum for both Pythium and P. sojae. While some areas in Ohio may have been cooler, P. sojae is still active, it just takes a little longer. Symptoms on the plants for both of these organisms are brown to light tan discoloration, sunken, and soft tissue of the roots and hypocotyl. Pythium seed and seedling diseases can be managed with a seed treatment (Allegiance or Apron XL) along with increased soil drainage. There are no soybean varieties that are resistant to Pythium. Phytophthora is best managed with resistance, in combination with seed treatments (at the highest labeled rates) and improving soil drainage.
Typically a resistance gene (Rps gene) will provide good protection of soybean seedlings to Phytophthora. But over time, populations of Phytophthora shift, in that they are able to cause disease on the Rps genes used in that field. When this happens, producers need to change to varieties with different Rps genes or look for varieties with combinations of Rps genes or rely on varieties with partial resistance. If a producer is faced with a replant situation, do not put the same variety back in the same field. Also do not plant varieties with the same Rps gene. If the variety that was wiped out has Rps-1a, switch to varieties with Rps-1c, Rps-1k, or Rps-3a. If the variety that was wiped out has Rps-1k or Rps-3a, switch to varieties with gene combinations. In both cases choose varieties with high levels of partial resistance. READ the fine print on the tables, scores for partial resistance range from 1 to 10. For some companies 1 indicates high levels of partial resistance and for other companies 10 indicates high levels of partial resistance. For the Ohio performance trials, scores of 3.5 to 4.0 indicates the highest levels of partial resistance and should yield even under Phytophthora pressure. For partial resistance ratings of soybean varieties in Ohio - consult the website for the scores - http://www.oardc.ohio-state.edu/soy2002/
As more and more fields in Ohio develop P. sojae populations which can cause disease on varieties with the different Rps genes, it becomes critical that producers keep track of which Rps genes they have used.
Rhizoctonia can also be a problem. Both seeds and seedlings can be attacked.
Symptoms of Rhizoctonia are depressed areas that are brick red to dark brown
in color on the roots and/or stem at the soil line. Plants that do emerge will
have an off-yellow color. There are a number of seed treatment compounds which
are effective for managing Rhizoctonia.
Where soil was tilled prior to planting, two options exist in fields with emerged corn, since weeds and corn will be emerging together. One option is to stay with the full rate of an atrazine premix product, which will control small, emerged grass and broadleaf weeds and also provide residual control. In OSU research, early postemergence application of atrazine premix products, when the first flush of weeds has emerged but is less than about an inch tall, has been consistently effective. Labels vary somewhat, but most atrazine premix products specify application when weeds are less than 1 ½ inches tall and corn is no more than 5 to 12 inches tall, depending upon the product. Additional atrazine can be included where weeds are larger, and we suggest atrazine rates of 2 lbs active ingredient per acre where grasses are more than 1/2 inch tall. Mixtures containing Callisto (such as Lumax), Hornet, 2,4-D, dicamba, or other herbicides can improve control of emerged weeds where necessary. Where grasses are more than 1 1/2 inches tall, the addition of a reduced rate of a postemergence grass herbicide (Accent, Steadfast, Option, etc) is suggested. The addition of nonionic surfactant or crop oil concentrate will generally be necessary where weeds have emerged, but check labels for specific recommendations on tank-mixtures and adjuvant use to avoid crop injury.
The other option in tilled fields is to apply a broad-spectrum postemergence herbicide treatment in several weeks when weeds are approximately 4 to 6 inches tall. This will become more appealing if wet weather continues to prevent fields from being suitable for traffic. As weed size (and especially grass size) starts to exceed 1 to 2 inches, the effectiveness of atrazine premix products will decrease, and the need for a full-season residual herbicide program will decrease. In other words, there comes a point when adapting a preemergence herbicide program to an early postemergence application timing will make less sense than applying a postemergence herbicide program that handles weeds of larger size. A number of effective postemergence programs are available, and mixtures containing three to four different active ingredients are not uncommon. The window of application for most postemergence treatments tends to be defined by grass size, since Steadfast, Option, Basis Gold, and other postemergence grass herbicides must be applied before annual grasses exceed 3 to 4 inches in height.
No-till corn that has emerged without any herbicide treatment can be much more problematic, since these fields are likely to contain any number of perennial and winter annual weeds in addition to the summer annual weeds that will emerge with the corn. Higher rates of atrazine will be extremely beneficial for control of many no-till weeds. Best option for no-till corn may be to apply an atrazine premix product (Bicep, Guardsman, etc) as soon as field conditions allow, and include additional atrazine (up to 2 lbs/A). The addition of 2,4-D, dicamba, Hornet, or Callisto can greatly improve control of no-till weeds, and may be necessary in many fields. The addition of nonionic surfactant or crop oil concentrate will generally be necessary. Another option for those willing to use a two-application program - apply atrazine plus 2,4-D as soon as possible and follow with a postemergence treatment in several weeks to control any later-emerging weeds.
A few notes about the risk of corn injury. Small corn is generally fairly tolerant of postemergence herbicides, but we have some concerns relative to current weather conditions. Cold weather this week will reduce the ability of emerged corn to metabolize herbicides, possibly increasing the risk of injury. We suggest delaying application of herbicide until warmer weather returns, to minimize the risk of injury and maximize herbicide activity in weeds. Growth of weeds slows during cold weather, so there is probably little disadvantage to waiting.
Ohio farmers have nearly completed corn planting and the corn has either emerged or is emerging in fields. Most are reporting emergence has been good and stands appear even in the earliest planted fields. Heavy rains over the past week have saturated soils throughout the state and has increased the risk of stand losses due to seedling blight diseases. In some areas with poor drainage, fields are flooded or portions of fields are under water. The risk of stand losses due to disease depends on several factors including the duration soils remain saturated, the growth stage of the corn, the soil temperature and the seed treatment fungicide used.
Corn seedling diseases are most severe under wet, cold conditions. Essentially all corn is susceptible to seedling diseases caused by the soil borne pathogen Pythium since no reasonable level of resistance has been identified in corn. Soil temperatures below 60 F are considered too cold for rapid corn growth. Since corn has no resistance to Pythium the plant is dependent on rapid growth and the development of new roots to minimize the effect of disease. When corn growth is restricted by cold soil temperatures and wet soils favoring Pythium infection, seedling diseases can be severe. Current soil temperatures across the state range from 62 to 66 degrees F. If the soil can maintain temperatures above 60 F during the wet period, seedlings have a better chance for growth and survival as soils begin to dry. Of course, if soils remain saturated for a longer period of time due to continued rain, the potential for damage from disease could increase. So if it stops raining soon and the temperatures warm, corn fields should have minimal problems.
All seed corn is now treated with either Apron XL (mefenoxam) or Allegiance (metalaxyl) seed treatments. These materials are the best available for control seedling blights caused by Pythium. They should provide protection for a couple of weeks after planting or until the plants emerge. They will not provide extended protection if soils remain wet for weeks.
Wheat in southern Ohio is beginning to head (Feekes growth stage 10.1. Central Ohio growers are reporting wheat in boot stage (growth stage 10) and in northern Ohio the wheat growth stage ranges from flag leaf emergence (growth stage 8) through boot stage.
We have received several reports of powdery mildew in a few fields throughout the state. It is likely that these fields are planted to varieties that are susceptible to this disease. It would be important to scout fields for disease problems especially if you know the variety planted is susceptible to powdery mildew. Threshold levels for powdery mildew and for other diseases is when the disease becomes detected on the leaf below the flag leaf. If disease is found throughout the field at this level then a fungicide application may be warranted. Tilt at 4.0 fl. oz. per A is the better fungicide for mildew control of the materials available.
The recent wet, warmer weather and severe winds will likely spread the spores of the fungus causing Stagonospora leaf and glume blotch. This fungus has been present on the lower leaves of plants in many fields in Ohio. Spores are spread by splashing rain. Symptoms of leaf blotch may not become evident for a couple of weeks, however scout fields and look for small chocolate brown spots on leaf surfaces. As time progresses the lesions become pale or tan in color, but the center remain chocolate brown. Evaluate how far up the plant the disease is progressing. If it reaches the leaf below the flag leaf before flowering, a fungicide application may be warranted. Recommendations for using foliar fungicides on wheat can be obtained on the OSU web site Ohioline at http://ohioline.osu.edu/b785/b785_9.html
Wheat is beginning to head in southern Ohio and these fields will be flowering over the next few days. Wet, warm weather during the flowering period of the wheat crop is always a cause for worry because of the potential destructiveness of Fusarium head blight or scab. The last major statewide epidemic of head scab was in 1996, but it has been important on a local basis somewhere in the state nearly each year. For the past several years we have been attempting to forecast the development of head scab. We have been using two different models to predict the risk of head scab. One model predicts the risk of scab at flowering by examining the weather conditions (temperature and rainfall) during the seven days prior to flowering. This model has been particularly valuable in predicting when an epidemic will not occur. The second model looks at the weather conditions during the time period from seven days prior to flowering to ten days after flowering. This model has been about 80% accurate in predicting when an epidemic will occur. We are now beginning to use these models to assess the weather conditions for this year.
At this time we are assuming that only the wheat in southern Ohio is in heading
growth stage and only those fields that started to head late last week could
begin the flowering stage. Flowering generally occurs 4 to 5 days after the
field begins to head and can be recognized when 50% of the tillers in the field
have anthers visible on the heads. We have run the Pre-flowering Scab Risk Prediction
model using weather data from several locations in southern Ohio. Results have
indicated a high risk probability for these areas. This means that the weather
conditions for development and spread of the scab fungus spores has been very
favorable. We will continue to monitor the weather information and the progress
of the crop and provide disease risk information in this newsletter over the
next few weeks. Visit the Ohio Field Crop Disease web site for updated information
on Forecasting Fusarium Head Blight on Wheat (http://www.oardc.ohio-state.edu/ohiofieldcropdisease).
We have received a couple of reports of small numbers of aphids being detected in wheat during the past week. There are several species of aphids that may be found in wheat including English grain aphid, bird cherry-oat aphid, corn leaf aphid and greenbug (not found on wheat in Ohio). Aphids are capable of causing problems on wheat either by the movement of a plant virus, Barley Yellow Dwarf Virus (BYDV), http://www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/byd.htm, or by the aphids feeding on the plants. The virus for Barley Yellow Dwarf is generally transmitted to the wheat in the fall or early spring before Feekes growth stage 4. Insecticides are not recommended for prevention of BYDV after Feekes growth stage 4.
Aphids can under certain conditions continue to build in number and damage wheat either by feeding on the plant during seedling to boot stage or by feeding on the wheat heads later in the season. At this time, the number of aphids being found in most fields is below the treatment threshold of 100 aphids per foot of row. I inspected some wheat at the Western Branch (WB) of OARDC last Thursday and found < 10 aphids per foot of row.
Aphids that I found in wheat at the WB http://entomology.osu.edu/ag/waphid.htm (see photo) resemble the bird cherry-oat aphid. According to the literature, this is one of the first aphids to appear in wheat in the spring and is also a vector of BYDV. This aphid is olive green with a red-orange band across the back of the insect. The red-orange area surrounds the cornicles ("tail pipes") that protrude from the back of the insect and are black. As aphid numbers build, we also begin to see a number of predators and parasites in the wheat. There were a couple of lady beetle species in the WB field and they averaged 2 adults per foot of row. I did see some mummified aphids http://entomology.osu.edu/ag/amummy.htm (see photo) which is an indication of parasite activity against the aphids. The predators and parasites in many cases help keep the aphid numbers below economic levels.
Cutworm leaf feeding and cutting was observed in 2-3 leaf corn at the Western Branch of OARDC last week. The number of plants damaged was below the economic level and no cutworms were found. However, it does indicate that cutworms are active and as corn emerges, corn fields will need to be checked for cutworm.
Only a little alfalfa weevil feeding damage is being reported throughout the
state. The worms are still small and not very numerous. It looks like we might
be able to escape damage from alfalfa weevil this year.
This past week caterpillar-looking insect larvae were brought into the office that had been collected off grasses from an alfalfa-grass mixed field. At first glance they resembled armyworm based on general appearance, feeding injury, and time of year. However, they were identified as a sawfly species, which are wasps (Hymenoptera), not moths or butterflies (Lepidoptera). The main characteristic that indicated a sawfly was 6-7 prolegs, a pair on each body segment. Although sawflies are rarely seen in crop fields in Ohio, we have obtained additional reports of a sawfly showing up in other grassy situations in the state, and have a report from wheat in a surrounding state. Currently, these are the only reports we have of them. There have not been any reports of this pest in wheat in Ohio at this time.
Whether sawflies will be found in any other fields or in wheat is unknown.
However, one species, the grass sawfly, is known as an infrequent pest of wheat.
Thus, wheat growers should be aware of the potential for these insects to be
in their fields, however, unlikely. If these sawflies are found in your wheat
field, please contact us by email at hammond.5@osu.edu
or eisley.1@osu.edu.
We have received reports of slug injury in fields in states south of Ohio. In our state, reports are being obtained of egg hatch. Thus, growers should be aware of possible problems over the next 3-5 weeks. Fields that were planted a few weeks ago appear to be emerging fairly well, and any slug injury to them will be leaf feeding. Growers should watch for heavy leaf feeding, especially if the weather stays cool. The biggest problem is in those fields that are not planted, and probably will not be for a while because of the wet conditions. As slugs continue to hatch out and grow, and fields are planting later in May and early June, the concern with slugs will become greater. Thus, growers should monitor the slug situation in their fields closely.
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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.
State Specialists: Pat Lipps, Anne Dorrance & Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Bruce Eisley (IPM) , Peter Thomison (Corn Production , and Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Bruce Clevenger (Defiance), Clark Hudson (Seneca), Howard Siegrist (Licking), Dusty Sonnenberg (Henry), Todd Mangen (Mercer), Steve Foster (Darke), Ray Wells (Ross), Greg Labarge (Fulton), Gary Wilson (Hancock), Harold Watters (Miami), Mark Koenig (Sandusky), Glenn Arnold (Putnam), Steve Prochaska (Crawford) and Roger Bender (Shelby).Editor: Steve Prochaska Web Editor: Nathan Watermeier
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