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4/13/2009 - 4/19/2009

A 2009 Prediction for Stewart’s Disease of Corn

By Forrest W. Nutter, Jr. and Alison Robertson, Department of Plant Pathology; Jon Tollefson, Department of Entomology; Rich Pope, Corn and Soybean Initiative 

December, January and February temperatures were all colder than average across Iowa. As a result, the risk for Stewart’s disease of corn in 2009 is low to negligible throughout all of Iowa, based on two predictive models.

Stewart’s disease, also known as Stewart’s wilt, is caused by the bacterium Pantoea stewartii. An insect vector, the corn flea beetle, plays a critical role in the plant-to-plant spread of this causal microorganism.

The bacterium survives winter within the gut of hibernating corn flea beetles. If winter temperatures are mild enough for the corn flea beetle to survive, the pathogen will also survive. In the spring, surviving beetles infested with the bacterium will emerge from grassy areas near corn fields and begin to transmit the pathogen to corn seedlings as they feed.

Stewart's disease can occur at any stage of plant development, but symptoms are almost always associated with flea beetle feeding. Corn seedlings can wilt rapidly from systemic infection, and seedling death is common, especially in seed corn and sweet corn fields. Plants that do survive will be stunted and will serve as a source for future generations of corn flea beetle to acquire and spread the pathogen.

Plant-to-plant spread by overwintering corn flea beetles will continue until late May, when corn flea beetles lay their eggs at the base of corn plants. The overwintering generation of adult corn flea beetles then dies. This begins a “beetle-free” period lasting 2 to 3 weeks (early to late June), which ends as the next (known as the first) generation of adult corn flea beetles begin to emerge. During this “beetle-free” period, foliar insecticide sprays are not effective.

The first summer generation of adult corn flea beetles emerges in late June, and feeds on infected corn plants. The beetles can acquire the bacterium, and facilitate the further spread of the bacterium to healthy corn plants. Later in the growing season, usually after pollination, the leaf blight stage may occur. Diseased plants at this phase exhibit long, wavy streaks (lesions) that are initially water soaked, and then turn yellow and die. Corn flea beetle feeding scars are usually visible within the lesions. If the disease is severe, whole leaves may wilt and die.

A second summer generation of corn flea beetles will emerge about mid-August. It is this generation that can harbor the bacterium that will overwinter for next season.
   
Mild winters during the past decade have resulted in an increased occurrence of Stewart's disease in Iowa. Two disease prediction models are available to predict the seasonal and county-level risk of Stewart’s disease. These models are: the Stevens-Boewe Index Model, and the Iowa State Mean Monthly Temperature Model. Both models use the monthly mean winter temperatures that occur during December, January and February to predict the degree to which the corn flea beetle population survived the winter.

The Stevens-Boewe Index predicts the severity (how much of the corn leaf tissue is infected) of the leaf-blight stage of Stewart's disease in the late summer. The risk is calculated by summing the average monthly temperatures for December, January and February. A sum below 80 indicates a negligible risk, 80 to 85 is considered low, 85 to 90 indicates moderate risk, and greater than 90 is considered a severe risk. Stewart’s disease predictions based upon the Stevens-Boewe Index, for the late leaf blight phase of Stewart's disease throughout Iowa in 2009 are presented in the map below.  
steven bowes map

 

The Iowa State University Model predicts the prevalence of Stewart's disease at the county-level. A high prevalence of Stewart's disease is predicted if the mean monthly air temperatures for December, January and February are each above 24 degrees F. The mean monthly temperatures for January and February 2009 in the northern three tiers of counties and in central Iowa were all well below 24 degrees F, and suggests that survival of large corn flea beetle populations were unlikely this winter.

However, February temperatures in west central, east central and all of southern Iowa did average above 24 degrees F, which does favor slightly better flea beetle survival.  The month-by-month average temperatures are shown in Figure 2.

Continuous snow cover in parts of Iowa from early December to February could have functioned as an insulation blanket to protect beetles from subfreezing temperatures. Thus, corn flea beetle populations may be slightly higher than we project because of better-than-anticipated beetle survival in areas with persistent snow cover. The corn flea beetle populations in the fall of 2008 were extremely low and spotty so the anticipated risk of damage due to Stewart’s wilt is still likely negligible statewide. 
 
2009 Iowa ISU model

Insect Economic Thresholds
Stewart’s disease can be controlled on susceptible corn by controlling the corn flea beetle with a foliar-applied insecticide, but timing of the application is critical. Seed treatments also may provide a better approach to control. A 2000 study at the University of Illinois demonstrated that two insecticides, imidacloprid (Gaucho®) and thiamethoxam (Cruiser®), applied to sweet corn seed, reduced the incidence of Stewart's wilt by 50 to 85 percent under field conditions with naturally occurring populations of corn flea beetles.

According to the researchers, these seed-treatment insecticides controlled Stewart's wilt during the very early growth of corn plants when applications of conventional, foliar insecticides were ineffective. The full article, Control of Stewart’s Wilt in Sweet Corn with Seed Treatment Insecticides, is available online. Use the following thresholds for rescue treatments in corn:

  • Field corn--prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
  • Seed corn--on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.

Labeled insecticides include, but are not limited to, Asana XL, Capture 2 EC, Lorsban 4 E, Pounce 3.2 EC, and Warrior. See manufacturer's labels for use rates and restrictions.

 

Forrest W. Nutter, Jr. is a professor in the Department of Plant Pathology working on disease risk models for improved disease management. Alison Robertson is an assistant professor in the Department of Plant Pathology with extension and research responsibilities. Jon Tollefson is a professor in the Department of Entomology with extension and research responsibilities. Rich Pope is an extension program specialist working in the Corn and Soybean Initiative.

April 13 Crops and Weather Report

On April 13, Doug Cooper , Extension communications specialist, interviewed Iowa State University Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope and corn agronomist Roger Elmore for the weekly crops and weather update.

Extension climatologist Elwynn Taylor tells Cooper that La Niña appears to be making a comeback, maybe. The Southern Oscillation Index (SOI) has gained back some of its strength the past week or so.

Rich Pope says winter annual weeds need to be scouted in some parts of the state.

Extension corn agronomist Roger Elmore reports that farmers are starting to express concerns that wet weather may delay spring planting again this year. Last year’s yields were surprisingly good – thanks to a major improvement in weather conditions in the middle of August.

Consultants Now Required for Aerial Pesticide Applicators

By Kristine J. P. Schaefer, Pest Management and the Environment

There continues to be changes regarding pesticide application in Iowa. The April 5 ICM News article discussed changes in pesticide record-keeping requirements and the Iowa Bee Rule. Today’s article addresses an additional requirement concerning aerial application of pesticides.

All of these changes were initiated by the Iowa Department of Agriculture and Land Stewardship (IDALS) and went through a public comment period before becoming part of the Iowa Administrative Code. IDALS is responsible for writing and enforcing all amendments to the Iowa Administrative Code. The role of the Pest Management and the Environment program at Iowa State University is to communicate changes and updates to pesticide applicators throughout the state. The new aerial applicator changes are described below.

An aerial applicator, applying pesticides to agricultural land, can now only operate in Iowa in cooperation with an “aerial applicator consultant.” An aerial applicator consultant coordinates the commercial application of pesticides by aerial applicators. The consultant must be a resident of Iowa; hold a certification in Category 11, Aerial Application; and either a valid commercial applicator license or pesticide dealer license.   

An aerial applicator consultant has a variety of responsibilities and requirements they must comply with. A partial list of the responsibilities includes:

• Meet with aerial applicators prior to pesticide applications to verify compliance  with  Iowa’s pesticide rules, the requirements of the Federal Aviation Administration, and the Iowa department of transportation
• Provide detailed aerial maps of application location
• Maintain daily communication with aerial applicator during pesticide  applications and daily oversight of pesticide handlers' activities
• Provide information regarding sensitive areas and apiaries
• Provide instructions for proper emergency response procedures in the case of a pesticide spill or accident

This change is part of amendments to Chapter 45, “Pesticides,” of the Iowa Administrative Code. The amendments, effective Feb. 1, 2009, outline the requirements, qualifications and duties of the aerial applicator consultant; the procedures for aerial application; and the license, certification, and continuing instruction requirements for aerial applicators operating in Iowa.

A full description of the amendments can be found in Chapter 45, pages 13-15, of the Iowa Administrative Code. IDALS has prepared checklists for aerial applicator consultants and for aerial application jobs.  Copies of the checklists are available at the IDALS website. Please contact IDALS, Pesticide Bureau, at 515 281-8591 for additional information.

 

Kristine Schaefer is a program specialist in the Department of Entomology serving on the Pesticide Management and the Environment team. Schaefer can be reached by email at schaefer@iastate.edu or by phone at (515) 294-4286.

Soybean Seed Treatment

X.B. Yang, Department of Plant Pathology

Seed treatment was not a major production issue ten years ago because less than 3 percent of soybean planted in Iowa used seed treatment. Now the number is more than 50 percent, according to a survey. Such changes are associated with changes in soybean production. Increased cost of seeds and early planting in spring may be driving forces for such changes.  

It is no question that seed treatments can increase yield in fields where risk of seedling diseases are high, for example, growers in Ohio routinely use seed treatment to prevent Phytophthora damping off. Despite the greater demand for seed treatment today, it is unknown if the majority (50% or more) of Iowa soybean fields will see an economic return from treatments. This is because of lower seedling disease risk in Iowa compared to Ohio and because soybean plants have a greater ability to grow over a large gap. Assessing the risk of seedling diseases in particular fields before use of a seed treatment can provide the producer needed protection while reducing production cost. 

Why seed treatment  
In Iowa, as well as elsewhere in the North Central Region, seed treatments are mainly to protect seedling from damping off by Phytophthora and Pythium. In some years Rhizoctonia and Fusarium can be production problems to a few growers, both neither create major problems.  

When to treat seeds or effectiveness of seed treatments
Because in most seasons seedling diseases are not a general problem and are unique problems to individual Iowa farmers, producers should selectively use seed treatments in order to reduce production cost. Below are specific cases where one should use treated seeds.

1. When seed quality is poor, such as last year. Seed treatment will not improve germination rate, but will protect further stand loss. Generally speaking, this year the seed quality has been good. Seed quality has not been an issue.

2. Your fields have a history of severe damping off from Phytophtora or Pythium and the coming spring is wet. Phytophthora can cause damping off for some Iowa soybean producers, especially in southern Iowa. However, if spring is not wet, the disease will not be a problem. This also applies to Pythium when soybeans are planted early. Pythium is a cool temperature disease and is not a concern when soybean is planted later in May or after.

3. Replanting.  If replanting is needed, seed treatment is insurance for a good stand.  The lack of stand establishment during germination is a sign of seedling disease in your field. Using seed treatment is a must. However, insects such as seed corn maggots sometimes cause seed rot. Make sure the lack of soybean stand is from disease, not insect when you replant.

4. Early planting. Early planting, planting before May, is not a reason in itself for seed treatment. When soybean is planted earlier, the soil is cool and seedling diseases Pythium and Fusarium occur in cool soils. Past surveys of these diseases indicate Fusarium only accounts for about 10 percent of seedling disease problems in Iowa.  

Finally, many seed treatment has multiple packaging which includes insecticide such as Cruiser. Seed treatment with insecticide will not help aphid control, but it may reduce first generation of bean leaf beetles if the insect is a concern in your production.
 

 

XB Yang is a professor of plant pathology with responsibility in research and extension. Yang can be contacted by email at xbyang@iastate.edu or by phone at (515) 294-8826.



This article was published originally on 4/20/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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