CORN
Crop Observation and Recommendation Network

April 10-15, 2001
C.O.R.N. 2001-8

In This Issue:

A) Predictions For Stewart's Disease Of Corn
B) Soybean Seed Treatments - What They Can And Cannot Do For You
C) Will We Have Slugs In 2001?
D) New Plant Pathology Resources On The Web
E) Plant Pest Diagnostic Clinic Update

A) Predictions For Stewart's Disease Of Corn - Pat Lipps CORN Questions

Stewart's bacterial leaf blight was extremely common throughout Ohio over the past 4 seasons. The bacterium causing this disease is carried and spread by the Flea Beetle. During years with mild winter temperatures the flea beetle adults overwinter in large numbers. In the spring, as corn emerges from the ground, the flea beetles feed on the young plants and spread the bacterium which in turn causes seedling wilt and leaf blight. The incidence of Stewart' disease is related to the ability of the flea beetle to overwinter.

Flea beetle adults become active in the spring when soil temperatures reach 65 F. Adults are most active on sunny, warm windless days. They hide in cracks in the soil during windy, cool or cloudy periods. After feeding and mating, adult females lay eggs at the base of the corn plants. Larvae feed on corn roots and are full grown in about two weeks. There are at least two generations per year in Ohio. The beetle overwinters as an adult in the soil near corn fields. It prefers bluegrass sod, but may be found in fencerows, roadsides, and woods.

The occurrence of Stewart's bacterial disease is totally dependent on the level of flea beetle survival over winter. For may years, winter temperatures have been used to predict the severity of Stewart's disease because higher populations of the flea beetle survive during mild winters than during cold winters. The 'flea beetle index' is calculated as the sum of the average temperatures (Fahrenheit) of December, January and February. Index values of less than 90 indicate negligible disease threat, 90-95 indicates low to moderate levels, 95-100 indicated moderate to severe and values over 100 predict severe disease.

The flea beetle index and predicted incidence levels for OARDC Branch locations in Ohio are:
Hoytville, Wood Co. = 76.0; Negligible
Wooster, Wayne Co. = 82.8; Negligible
South Charleston, Clark Co. = 82.3; Negligible
Jackson, Jackson Co. = 92.9; Low

Dr. Celeste Welty, OSU Entomology Extension, has calculated the flea beetle index for predicting Stewart's disease at 14 locations in Ohio. To check the index value of a location near you visit the OSU IPM web site at http://www.ag.ohio-state.edu/~ipm/traps/fbeetle.htm.

These numbers indicate that flea beetle populations should be very low in the northern two thirds of Ohio and that Stewart's bacterial disease should be negligible in these areas. The index for southern Ohio is marginal, indicating that flea beetles may have overwintered at reduced levels in the south. Overall, the level of Stewart's leaf blight and seedling wilt should be low to nonexistent this year. Time will tell if this prediction is correct. The flea beetle index has been a relatively good predictor over the years.

B) Soybean Seed Treatments - What They Can And Cannot Do For You - Anne Dorrance CORN Questions

Soybeans can be attacked early in the season from a number of soil-borne pathogens including Pythium, Phytophthora sojae, Rhizoctonia and Fusarium. In addition, some seed may carry pathogens such as Phomopsis that will kill seeds after planting. There isn't one fungicide that will take care of all of these problems. Some products are good for controlling Rhizoctonia, Fusarium and Phomopsis. Two products, metalaxyl and mefenoxam will control the water molds, Pythium and Phytophthora. These two pathogens are different from the other fungi in several ways; they produce a swimming spore (zoospore), have cell walls made of glucans and have a "body" that is comprised of streaming cytoplasm (a mini pathology/biology lesson).

Fungicide seed treatments will only provide protection for a limited period of time after planting. Fields in continuous soybeans where soybean pathogen populations increase in number and reduced tillage systems provide for a more favorable environment with higher soil moisture levels. In addition, Phytophthora can only attack plants when soils are totally saturated and when the Rps genes in the soybean plant can no longer detect the Phytophthora.

The bottom line is that seed treatments will only work to protect seeds for the first few weeks after planting, when the environmental conditions are favored for disease development. For example, if Ohio gets heavy rains in late June or July, and Phytophthora populations are high and the varieties are susceptible, this will wipe out all the benefits of the seed treatment. It is also true if rains continue through the whole season and the root rots contribute to yield loss, seed treatments will be ineffective for these problems.

We cannot predict, or is it likely that we will ever be able to tell, when the environment, the pathogen population and the condition of the soybean plant favor disease development similar to last spring. What we can recommend is that in most cases - the application of seed treatment fungicide will save the cost of replanting and help the plants get established early in the production season. I was discussing seed treatments with my colleagues a few weeks ago and told them that I thought that the use of seed treatments was similar to going shopping and buying things on sale. They assured me that it wasn't even close to the same thing.... But to use a seed treatment to save the cost of replanting and potentially improve a stand seems pretty close to me! I wonder if their wives would agree with me?

C) Will We Have Slugs In 2001? - Ron Hammond, Bruce Eisley CORN Questions

Slug population densities were large in the fall of 2000, suggesting that spring populations in 2001 will also be high. Thus, we believe the potential for slug problems appears to be great at this time. Corn and soybean growers who have had problems with slugs in the pasts should plan on monitoring their fields closely for slug populations and injury.

Because slug damage to soybeans often occurs at germination and emergence resulting in significant stand loss, growers with high slug populations and a history of poor soybean stands might want to consider an at-planting time application of a molluscicide bait. In most years, egg hatch occurs in mid to late May so that later planted fields have the greatest potential for stand problems. In 2000, the egg hatch occurred much earlier though, and even soybeans planted in late April and early May had infestations that caused stand loss.

Growers are advised to determine if slugs are present and active at the time of planting in order to make the appropriate decision. This is best done by searching at the soil level underneath the crop residue for the presence of slugs and/or slug eggs. If mostly eggs are present at planting, treatment is probably not necessary at that time. If slugs have hatched and are active, treatment might be warranted.

Observations taken last year in replanting situations indicates the value of an at-planting time application when slugs are present and actively feeding. We do not have economic thresholds available. A grower decision should be based on field history and the relative presence of a large population.

When treating, the most commonly used molluscicide is Deadline MPs, which should be broadcast at 10 lbs per acre at or following planting. If past history suggests that only certain areas of a field have had a problem, only those areas need to be treated.

Farmer observation about the efficacy of these at-planting time applications for slugs can be directed to Ron Hammond. Please send information concerning the location, date of planting and application, method of application, level of success, to Ron at hammond.5@osu.edu. Check areas, approximately 50 by 50 ft, should be left to determine the treatment's efficacy. We will continue to update the slug situation in the CORN newsletter throughout the spring.

D) New Plant Pathology Resources On The Web - Pat Lipps, Mills CORN Questions

The Plant Pathology "field crops disease" web site recently went on line. Symptoms, pictures and management tools are available for diseases of corn, soybeans, wheat and alfalfa, along with links to many other sites including C.O.R.N. The site can be found at www.oardc.ohio-state.edu/ohiofieldcropdisease

Recently 10 corn fact sheets have been revised and a new one created. They are available on the web at the following addresses:

AC-16 Corn Smut http://ohioline.ag.ohio-state.edu/ac-fact/0016.html

AC-21 Eyespot Disease of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0021.html

AC-22 Anthracnose Leaf Blight and Stalk Rot of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0022.html

AC-24 Maize Dwarf Mosaic Maize Chlorotic Dwarf Diseases of Corn
http://ohioline.ag.ohio-state.edu/ac-fact/0024.html

AC-29 Northern Leaf Spot of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0029.html

AC-31 Common Corn Rust http://ohioline.ag.ohio-state.edu/ac-fact/0031.html

AC-33 Gibberella Stalk Rot of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0033.html

AC-34 Crazy Top of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0034.html

AC-37 Stewart's Bacterial Wilt and Leaf Blight of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0037.html

AC-38 Gray Leaf Spot on Corn http://ohioline.ag.ohio-state.edu/ac-fact/0038.html

AC-46 Diplodia Ear Rot of Corn http://ohioline.ag.ohio-state.edu/ac-fact/0046.html

E) Plant Pest Diagnostic Clinic Update - Greg La Barge CORN Questions

Do you need identification of insects, plant diseases, weeds and other plant problems as a part of your daily efforts? If so, you'll want to check out the newly revised web site for the C. Wayne Ellett Plant and Pest Diagnostic Clinic at:

http://www.ag.ohio-state.edu/~plantdoc/cweppdc/cweppdc.html

The site has been completely revised to include information on how to take and send samples, fees and forms, and additional background information about the Clinic. Using this site will help us provide you with the best identification and fastest turnaround time.

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.

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.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

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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