Alfalfa fields in southern and central Indiana need to be inspected immediately for weevil tip feeding and skeletonization of leaves. Three southern Indiana Purdue Agricultural Centers have reported 12, 32, and 87% tip feeding this past week in fields being regularly monitored (thanks to Don Biehle, Richard Huntrolds, and Frankie Lam). Betsy Smith, Grower’s Co-op, out of Terre Haute informed us that 8 out of 10 fields being scouted in her area have reached the treatment threshold.
Last fall and winter’s relatively mild temperatures have obviously allowed for successful egg and adult over-wintering. The high variability in numbers and size of larvae seen this spring, also indicate that egg laying occurred over an extended period, and that larvae will be present longer than normal. Unfortunately, unless beneficial organisms (both parasitic wasps and pathogenic fungi) begin to really kick into gear, some fields may need to be treated more than once. Should this be the case, be certain to adhere to rate-use and pre-harvest restrictions.
Our management guidelines at this time for southern Indiana suggest that fields be treated when there are 3 or more larvae per stem and tip feeding is at least about 50%. Most insecticide labels suggest using higher rates for increased residual control, we concur with this recommendation. Refer to management guidelines and recommended insecticides for alfalfa weevil in last week’s Pest&Crop.
Most of our dutiful trapping cooperators throughout the state captured black cutworm moths this past week, including three intensive captures (9 or more moths caught over 2-nights). This recent flush of moths is attributed to warm wind currents from the Gulf Coast States and southwestern portions of the country.
Significant moth captures at this time, along with the use of heat units to predict the beginning of larval activity, gives us an indication of potential severity of the problem and locations of concern. Thus, we are able to predict with some degree of accuracy when and where crop damage is likely to occur based on these data. Refer to the “Weather Update” in future issues of the Pest&Crop as we track heat unit accumulations and predicted damage in your area.
Should one treat for black cutworm before or at planting? Because of the sporadic outbreak nature of this pest, the tried, true, and economic approach to black cutworm management is to scout fields, determine infestation and damage levels, and use a rescue treatment, if needed. Producers using insecticide-treated seed may have a false sense of security concerning black cutworm control. Certainly the systemic activity of these newer insecticides during the seedling stage should help suppress small larvae feeding on plants. However, fields attracting egg-laying moths during multiple flights will likely experience significant damage and stand losses.
Armyworm pheromone traps in Kentucky and two black light traps in Indiana (thanks Todd Hutson and Frankie Lam) have confirmed that this pest from the “memorable” 2001 season has made its annual arrival into the Midwest, albeit numbers are low at this time. Watch for our black light trap catch reports in future Pest&Crop newsletters.
The US Environmental Protection Agency has granted a Section 18 Specific Exemption for use of Folicur 3.6 F fungicide for suppression of Fusarium head blight (scab) of wheat. This Section 18 registration allows use of Folicur on wheat up to the beginning of flowering (Feekes growth stage 10.51), and is valid for the period April 15 through June 30, 2004. Users of this fungicide must have a copy of the label in their possession at the time of pesticide application. Copies of the label can be obtained at County Extension Offices.
Fusarium head blight harms wheat in three ways. It reduces yield and test weight. Also, the fungus that causes scab (primarily Fusarium graminearum in the Corn Belt) produces a toxin in grain. This toxin, variously termed deoxynivalenol, DON, or vomitoxin, is chemically stable and persists through milling and baking. The FDA recommends that finished wheat products contain no more than 1 ppm of DON. Last year, wheat from many fields in southern Indiana and Illinois and down into the southeastern US contained DON levels of 8 to 10 ppm. Folicur may help reduce these problems in the future.
Researchers have been conducting uniform fungicide trials for control of Fusarium head blight in wheat and barley for several years. While Folicur does not provide complete control, it has been among the best of the materials tested for suppression of head blight and reduction of DON in grain. Results from many uniform trials indicate that proper use of this fungicide can reduce symptom severity and DON levels by about 30% in winter wheat.
Development of Fusarium head blight depends on weather. Basically, wet conditions from about a week before flowering through the end of flowering are conducive for disease development. So, if a grower has wet, humid conditions for the week before flowering, use of Folicur may be justified. Another predisposing condition for development of Fusarium head blight is corn residue in a wheat field. Fusarium graminearum is a common stalk rot fungus in corn. Even though a previous crop of corn may not have had a problem with stalk rot, the fungus will be present in the stalks. When weather is favorable in the spring, the fungus produces spores on the corn residue and these can infect wheat. Spores of F. graminearum can also infect wheat planted into ground that has no corn residue because they are airborne and can travel from field to field. However, the risk of disease is greater if the source of the spores (corn residue) is within the wheat field. There is now available a weather-based disease risk model for Fusarium head blight (see companion article). Growers can use this to help decide whether to use a fungicide.
Folicur can be applied by ground or aerial sprayers. The label provides information on spray volume, use of surfactants, pre-harvest interval restrictions, and restrictions on use of straw for bedding. To achieve the best performance with Folicur, it is important to get as much fungicide as possible on the wheat head (spraying wheat before heads have emerged will provide no control of Fusarium head blight). Most spray systems are designed to place material on foliage, which is essentially a horizontal surface. Spraying heads requires depositing fungicide on a vertical surface. Based on results from the uniform fungicide trials, the most effective nozzle configuration is to use a pair of XR8001 flat fan nozzles at each nozzle body location on the boom, one oriented forward and the other backward, at about 45∞ from vertical. As the sprayer moves through the field this directs spray to the front and back of each head relative to sprayer travel direction. Single TwinJet TJ8002 nozzles will also deliver spray effectively to heads.
Dr. Don Hershman, an extension plant pathologist at the University of Kentucky, states that for aerial application of Folicur, nozzles should be angled to direct spray 90° to the direction of travel. Spray droplets should be from 300 to 400 micrometers (microns) in diameter. The Folicur label specifies that spray volume for aerial application should be at least 5 gal/A. Spraying in early morning, when dew is still on plants, will promote better fungicide coverage on heads.
Results from the uniform fungicide tests indicate that spraying wheat at the beginning of flowering (Feekes 10.51) provides the best suppression of head blight. The beginning of flowering is when wheat first becomes vulnerable to infection. This stage of growth can be recognized by the presence of anthers just above the middle of the head. Anthers are pale yellow, tubular structures that contain pollen. If application is delayed, infection may have already occurred and disease suppression will not be satisfactory.
For the past 4 years, plant pathologists in North Dakota, South Dakota, Indiana, Ohio, and Pennsylvania have conducted research on the epidemiology of Fusarium head blight (scab) of wheat with the goal of developing a weather-based forecast for the disease. Dr. Erick De Wolf, a plant pathologist at The Pennsylvania State University, his graduate students, and colleagues in other states (particularly Ohio) have crunched a lot of data and developed a risk model for Fusarium head blight. Much like a weather forecast, it predicts the probability of an event, in this case an epidemic of head blight. An epidemic is defined as a severity of at least 10% in the field. This means that 10% of the heads are completely blighted, or that 20% of the heads are 50% blighted, and so forth.
As briefly discussed in the companion article about Folicur 3.6 F fungicide for suppression of head blight, weather has a strong influence on development of this disease. Even though the fungus that causes head blight (Fusarium graminearum) is ubiquitous in corn residue, if weather is unfavorable for spore production or infection there will be little or no disease. Conversely, if weather is favorable, a major outbreak of head blight will occur, often over a large area. Last year, head blight was common in southern Indiana and Illinois, and extending down to the southeastern US.
The model can be accessed via the Web. The url is www.wheatscab.psu.edu/.
From this homepage, a user can find some background information about Fusarium head blight and the model. The model itself is easy to use. Select the Risk Map Tool and answer a couple of questions. The model uses real time weather data for 7 days prior to wheat flowering. The user specifies the flowering date and then chooses the model for spring wheat or winter wheat. Indiana users would select winter wheat. When winter wheat is selected, an additional choice is offered: whether or not there is corn residue. This question refers to whether there is corn residue in the wheat field itself. In developing the model it became clear that for winter wheat it made a difference in the accuracy of prediction whether corn residue was present or not, so this factor was included in the model. For given weather conditions, the risk of head blight is greater for wheat planted into corn residue than for wheat on clean ground on in residue of some other crop.
The next screen displays a US map. The model can be used for states depicted in green. When the user chooses a state, a closer view is displayed on the screen, and there are purple dots that designate the locations of hourly weather stations (the risk model uses hourly data rather than the standard daily weather data that is collected at many sites around the country). The user can choose a weather station nearest his or her wheat field. Clicking on this dot will show the risk for that location, and will show a color-coded risk map for the states displayed on the screen. Red, yellow, and green indicate areas of high, moderate, and low risk, respectively. As I have been working with the model, I find that some stations within a “red” area show a low risk, so it’s a good idea for a user to look both at the map and the risk assessment for a station near his or her field.
Because the model uses real time weather data, not predicted weather data, it can only be run for the current day or some earlier day. For example, if a user’s wheat field flowered today, the model could be run for today’s date to indicate the risk of head blight in that field. However, this would be the same day that Folicur should be applied if a grower were using the model to help determine whether a fungicide would be warranted. This can present some timing problems—running the model the same day that one would want to schedule treatment. A user could start using the model once wheat has headed (See Pest & Crop Issue No. 2 for a detailed discussion of wheat growth stages). Normally, wheat begins flowering a day or two after heads are fully emerged. Using the date of full head emergence as the “flowering date” for the model would give some idea as to the favorability of weather for head blight. If the model predicted a high risk for a “flowering date” that was actually 1 or perhaps 2 days before actual flowering, it is likely that the risk would not change dramatically by the time wheat actually flowered.
This model has just been launched and it is not foolproof. It is expected to have a prediction accuracy of about 80%. Additional research and the experience from making the model generally available this year will permit refinement in the future. The risk model will be a helpful tool in deciding whether to use Folicur fungicide (see companion article). Whether or not a fungicide is used, the model may also help in making marketing decisions. Grain buyers and processors should find it useful for indicating whether there may be problems in finding sources of sound wheat.
Dry Topsoil Concerns Some Corn Growers – (Bob Nielsen)
Nielsen, R.L. (Bob). 2002. Requirements for Uniform Germination and Emergence of Corn. Corny News Network, Purdue Univ. Available online at www.kingcorn.org/news/articles.02/Germ&Emerg_Req-0520.html. (Verified 4/20/04).
Sawyer, John. 2000. Anhydrous application and dry soils. Integrated Crop Mgmt. Newsletter, Iowa State Univ. Available online at www.ipm.iastate.edu/ipm/icm/2000/10-23-2000/anhydrous.html. (Verified 4/19/04).
Don’t forget, this and other timely information about corn can be viewed at the Chat ‘n Chew CafÈ on the Web at www.kingcorn.org/cafe. For other information about corn, take a look at the Corn Growers’ Guidebook on the Web at www.kingcorn.org.