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Issue No. 3, Article 7/April 22, 2011

Managing Fusarium Head Blight (Scab) of Wheat

Fusarium head blight (aka scab) can be one of the most devastating diseases of wheat when conditions are favorable for its development. Fusarium head blight can cause both yield and quality losses. Quality losses can be due to lower test weights and contamination of grain by toxins (deoxynivelanol, or DON) produced by the fungus that causes Fusarium head blight; both can be a serious problem for producers and millers. Because the fungal pathogen that causes Fusarium head blight (Fusarium graminearum, also known as Gibberella zeae) can affect corn as well, causing Gibberella stalk and ear rot, the pathogen is already present throughout Illinois in many fields.

Weather is generally the driving factor in the development of Fusarium head blight. Because wheat is susceptible during flowering, weather conditions during flowering through kernel development play a key role in the incidence and severity of the disease. Moderate temperatures (75 to 85°F), prolonged periods of high humidity, and prolonged wet periods favor development.

A disease forecasting system based on weather conditions known as the Fusarium Head Blight Risk Assessment Tool is available online. A "risk map" (Figure 3) lets you see the risk of Fusarium head blight throughout Illinois (and other states). The forecasting system was developed through collaboration among many university plant pathologists and funded through the U.S. Wheat and Barley Scab Initiative.


Figure 3. Screen capture of the Fusarium Head Blight Risk Assessment Tool, April 20.

By the time symptoms of Fusarium head blight appear on wheat heads, which show up as "bleached" or with both green and bleached areas, it is too late to manage the disease. Successful management requires an integrated approach and begins before planting, when you are deciding which varieties to plant and which fields to plant into wheat.


Symptoms of Fusarium head blight on wheat ("bleached" heads).

Foliar fungicides are the only "in-season" option for control of Fusarium head blight. Although fungicides are a good control option, losses will still occur on a highly susceptible variety sprayed with a fungicide in an environment favorable for disease. Only three fungicides are available that provide a respectable level of scab control: Caramba (BASF), Prosaro (Bayer CropScience), and Folicur (Bayer CropScience; many other products also contain the same active ingredient as Folicur--tebuconazole--since Folicur is now off-patent). A summary of University of Illinois trials from 2008 to 2010 has shown that Caramba and Prosaro are more effective than Folicur in providing control of scab and the associated mycotoxin DON (Table 5). Even then, only about 50% control of scab and 40% control of DON are achievable.

Table 5. Effectiveness of foliar fungi-cides for control of deoxynivalenol (DON) and head scab in wheat.

Product

DON (% control)*

Scab (% control)*

Caramba, 13.5 fl oz/A

40-45

40-50

Prosaro, 6.5 fl oz/A

40-42

45-53

Folicur, 4 fl oz/A

22-28

30-40

Data are summarized from University of Illinois and Southern Illinois University fungicide trials conducted from 2008 to 2010 at Monmouth, Urbana, Brownstown, Dixon Springs, and Carbondale.
*Percent control is relative to the nontreated control plots for each location.

Timing for applying foliar fungicides for scab and DON is critical. Target your applications for early flowering (Feeke's growth stage 10.5.1--when anthers are just beginning to extrude from the middle part of the wheat head). It is also important to use nozzles oriented to spray forward, which improves coverage of the wheat head.

I reiterate that product choice is critical. Caramba, Prosaro, and Folicur and other tebuconazole products are the only ones with efficacy against scab. Applying a strobilurin fungicide (Headline, Evito, Quadris, Stratego, or Quilt) at the late growth stages (boot stage or heading) could actually increase the level of DON in harvested grain rather than decrease it.

Achieving the best control of scab and the associated mycotoxin DON requires an integrated approach of management practices: planting location, variety choice, and foliar fungicides.

Because the fungus that causes scab of wheat is the same one that causes Gibberella stalk and ear rot of corn, planting wheat into corn stubble increases the risk of scab. Although that strategy sometimes fits better into cropping systems in parts of southern Illinois from a practical point of view (corn is harvested first, which allows timely planting of wheat behind corn as opposed to soybean), farmers must take into account this increased risk when choosing varieties and making fungicide decisions.

No wheat variety is completely resistant to scab, but differences in susceptibility do exist. Dr. Fred Kolb at the University of Illinois conducts an annual trial to evaluate wheat varieties for resistance to scab. The results are published on-line (Adobe PDF).

Field research trials at three Illinois locations in 2010 evaluated the effect of previous crop (corn or soybean), variety (six varieties differing in susceptibility to scab), and foliar fungicide (Prosaro vs. no treatment) on control of scab and DON. Averaged across the locations, nearly 90% control of DON was achieved (relative to planting a susceptible variety after corn without a fungicide) when a moderately resistant variety was planted after soybean and sprayed with Prosaro fungicide (Figure 4). This demonstrates that integrated management truly is needed to achieve the highest levels of control of DON and scab.


Figure 4. Summary of 2010 University of Illinois integrated management trials for scab and DON. Results indicatedt that an integrated management system combining variety choice (planting moderately resistant [Res] vs. susceptible [Susc] varieties), field choice (planting into soybean stubble [Soy] vs. corn stubble [Corn]), and fungicide treatment (Prosaro [Fung] vs. no treatment [No fung]) is needed to achieve the highest level of scab and DON control. Research was funded by the U.S. Wheat and Barley Scab Initiative.

--Carl A. Bradley

Author:
Carl A. Bradley

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