Bob Mulrooney, Extension Plant Pathologist; email@example.com
With all the corn that was planted last season much of our wheat crop was planted in corn ground with varying levels of tillage: no-till, minimum-till, and mold board plowed. The one down side of planting into corn residue if it is exposed on the soil suface is that one of the stalk rot fungi, Fusarium graminearum, is also the principal pathogen that causes Fusarium head blight or scab of wheat. Vulnerability of wheat is increased when the fungus is present in the field versus having the fungus blow in from another site.
Fusarium head blight (FHB) or scab of wheat and the accumulation of the mycotoxin deoxynivalenol (DON) in harvested grain, are periodically problems in Delaware. High DON levels will end up in rejection of wheat loads. Fortunately in recent years we have not had a severe outbreak of scab in Delaware.
Strobilurin fungicides (e.g., Quadris, Headline) or fungicide containing a strobilurin (e.g., Quilt, Stratego) are not recommended for scab control because they may result in elevated DON levels compared to untreated wheat. The only class of fungicides that have not had this increase in mycotoxins is the triazole class. The only registered triazoles for wheat and barley in Delaware are Tilt and Proline. There has been a much higher rate of success at suppressing scab with Proline than with Tilt in work conducted by Dr. Arv Grybauskas at the University of Maryland.
The proper use of Proline will help suppress FHB and DON when used with other FHB/DON management tactics. However, Proline is not a “silver bullet” for managing FHB/DON. In other words, do not expect Proline to provide the same level of FHB/DON control as you have come to expect when fungicides are used to control other wheat diseases. The key is to think in terms of disease suppression, not control. Nevertheless, a 40% reduction in FHB and DON can have a significant economic impact locally, state-wide, and regionally if FHB is moderate to severe in 2008. But, be advised that significant losses due to FHB and/or DON can still occur even where Proline is applied if FHB is severe.
For FHB/DON suppression the Proline 480SC label indicates a use rate of 4.3 to 5.7 fl oz/A applied to wheat “within a time period from when at least 75% of the wheat heads on the main stem are fully emerged (~Feekes stage 10.4) to when 50% of the heads on the main stem are in flower (~Feekes stage 10.52)”. Applications cannot be made within 30 days of harvest. Although the Proline label allows for some flexibility in terms of timing of application, most of the efficacy data for Proline in suppressing FHB/DON are based on application at early flowering (Feekes stage 10.51).
Excellent fungicide coverage on wheat heads is crucial to achieve the greatest possible FHB/DON suppression. This is no small challenge since most spray systems used in wheat were developed to deliver pesticides to foliage (horizontal structures). In order to maximize coverage on heads (vertical targets), significant changes may need to be made to the sprayer boom system. Also, discipline must be exercised to ensure that proper sprayer pressure and volumes are used. The Proline label gives some suggestions on how to achieve acceptable spray coverage.
Making Appropriate Fungicide Spray Decisions
One desire we all have is for fungicides to be used only when needed. Regular field scouting for foliar fungal diseases has been successfully used by growers for many years to determine if and when to spray fungicides. However, this is not possible with FHB since once symptoms are present it is TOO LATE to spray. Note: Proline is also effective on glume blotch, rusts, and tan spot.
Go to http://www.cdms.net/ to access the Proline label.
Below are some general guidelines to help you determine if you should spray Proline for FHB/DON suppression.
During period leading up to, during and immediately after head emergence:
*Soil moisture has been good for the past month (relates to spore production, dispersal of Fusarium graminearum spores, and crop infection)
*Crop has good yield potential (relates to economics and crop density, which increases canopy humidity and may increase spore production, facilitate spore dispersal, and encourage crop infection)
*Temperatures 68-86°F (relates to spore production and crop infection)
*Humidity is high (80% day or night) and/or free water (such as dew) is present on the heads during this period (relates to spore production, dispersal, and crop infection)
If most or all of the above conditions exist when the crop is just beginning to flower, consider spraying as soon as possible.
New Web-Based FHB Prediction Tool
In addition to the above general guidelines, an exciting new tool can also be used to help determine the FHB risk and need to spray. This tool is a web-based, disease forecasting model made available by Penn State University, The Ohio State University, Kansas State University, and the U.S. Wheat and Barley Scab Initiative. This forecasting model utilizes real-time weather data from numerous National Weather Service stations within each state. Go to http://www.wheatscab.psu.edu/ and click on “Risk map tool”.
You will be asked if you are growing winter or spring wheat. At this point you will come to a U.S. map and are asked to click on the state of interest. The FHB Risk Management Tool page will have a map that shows where the weather data are being retrieved. To the upper left corner of the page is a calendar section labeled “Assessment Date”. This section needs a bit of explaining. You will note right away that the tool will only let you click on the current date and the preceding 7 days. So, if you estimate your crop will begin to flower (the beginning of FHB susceptibility) on May 7, but it is only May 3, the best you will be able to do is to determine if the weather on May 3 (or the previous 7 days) is favorable for FHB. My advice is to begin determining the FHB risk using this model 1-2 weeks out from crop flowering. Keep checking your wheat and keep checking the model every 1-2 days. By the time your crop reaches early flowering, you should have a good feel for the FHB risk in your area. If the forecast model says the FHB risk is high (medium if you are not a risk taker), and the forecast matches your local weather and crop reality, then you might consider spraying as soon as possible.
Once you actually see it and play around with it, what I have said above will make much more sense. The model does have several practical limitations in predicting final FHB levels; these are clearly discussed within the Prediction Center website. Perhaps the greatest limitation of the model is that it does not account for weather conditions during flowering and grain fill. Specifically, disease-favorable weather occurring during late flowering and grain fill can greatly impact final FHB/DON levels. The bottom line is that final FHB/DON levels may not always be reflected by the model’s risk output. The authors of the model discuss this limitation under “Reality Check” in the “Model Details” section of the Prediction Center.
We all hope that FHB is non-existent this spring. However, if this is not the case, wheat producers now have an additional tool to use to minimize FHB and DON development this spring.
Adapted for Delaware from “FUNGICIDAL CONTROL OF FUSARIUM HEAD BLIGHT (HEAD SCAB) AND DEOXYNIVALENOL (DON) IN WHEAT” By Don Hershman in the April 14 issue of the Kentucky Pest News. http://www.uky.edu/Ag/kpn/kpn_08/pn080414.htm#whefun
If you have read all this you may be wondering what this is all about. In the past we have not had a fungicide for scab control to consider. The added management decision is whether to wait and use a new fungicide at flowering that would give some level of scab suppression and rust and glume blotch control if the weather turns out to be favorable for scab or take your chances that it does not show up and take advantage of the disease protection that the strobilurins or strobilurin/triazole combos provide, when applied at head emergence. Unfortunately we do not have any current data with Proline and its control of other diseases besides scab, since we have not had weather for diseases in the last several years that provided the needed disease control information.