Management of Fusarium Head Blight and Fungicide Timing Tips

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Bleaching symptoms caused by Fusarium head blight. (Source: GROWMARK, Inc.)

• The pathogen that causes Fusarium head blight (FHB) can infect wheat and barley heads and result in significant yield losses and reduced grain quality.
• Grain loads may be “docked” or rejected due to the presence of DON, a toxin (vomitoxin) produced by the fungus that causes FHB.
• FHB is best managed by integrating resistant varieties, cultural practices and timely fungicide applications.

Now is the time to be thinking about Fusarium head blight in areas where wheat and other small grains are produced, as wheat has started to flower in the more Southern parts of the region, whereas we still are at least 2-3 weeks away in higher latitudes. This article will go over the basics of FHB and management.

The pathogens that cause FHB
Fusarium head blight (FHB), also known as head scab or ear blight, is most often caused by the fungal pathogen Fusarium graminearum;  Fusarium culmorum and other species can be involved. The pathogens grow on dead and decaying plant tissue and do particularly well on corn and small grain residue. When conditions are wet and warm in the spring, (≈70°F) the fungus will break dormancy and begin to produce spores. Some of these spores (conidia) travel locally (feet to yards) from residue with driving rains, whereas other spores (ascospores) are launched into the atmosphere and can travel many miles. At night when conditions are calm, these spores are deposited from the atmosphere onto fields. Spores landing on exposed wheat heads infect via the glumes/spikelet, and may eventually grow into the the rachis, choking off water and nutrient movement above the point of infection. After infection the fungi can also produce mycotoxins that can accumulate in grains. Heads are more susceptible to infection during early flowering (Feekes 10.5.1) through the soft dough stage (Feekes 11.2). FHB infection is favored by extended periods of high humidity (>90%) and temperatures of 59°F - 86°F. The combination of reduced grain fill due to infection of the rachis and spikelet and quality issues related to reduced test weights and vomitoxin contamination, can cause significant crop losses. 

Symptoms and mycotoxins (DON)
Symptoms of FHB include bleaching of some or all the spikelets while unaffected portions of the head remain green (Image 1). The fungus may  infect the rachis causing the neighboring spikelets to become bleached. During warm humid weather, pink to orange spore masses may be observed on infected spikelets (Image 2). Infected kernels may be shriveled and chalky white or may be plump with pink/reddish ends. DON contamination is possible in both types of infection but typically are present at much greater concentrations in shriveled, white grains.  The best time to start assessing FHB in a field is between 18-24 days after the start of FGS 10.5.1. when heads are still green, but the fungus has had time to infect and produce symptoms.

Management
FHB is best managed by integrating different management tactics. These include using moderately resistant varieties, cultural practices and chemical controls. Research has shown that in some cases integration of practices can reduce FHB by nearly 90% compared to not using any management practice. 

Resistance to FHB is not complete, meaning that disease still occurs. However, research has shown that the use of a moderately resistant variety often reduces infection and DON accumulation by approximately 30-60% when compared to susceptible varieties. Information on varieties and resistance ratings can be found for various wheat classes and locations at the Scabsmart website here. These evaluations are made using misted nurseries, and therefore are more reliable and accurate than variety trials relying solely on local weather patterns. 

Chemical control: Timely fungicide applications can aid in FHB management, suppressing DON by approximately 45% and FHB by approximately 50%. A few fungicides in the triazole family and most recently some SDHI + triazole premix products are available for FHB management and are most effective when applied from early flowering (Feekes 10.5.1) through about 6 days after the start of flowering. Fungicides need to contact heads to be efficacious, and the variability in heading and flowering is a major reason why fungicides provide suppression of FHB, not control such as with foliar diseases.  Apply fungicides following label instructions.  Fungicides for FHB will also provide protection against glume blotch and foliar diseases.  Keep in mind that the supply of products this season will be tight.  Last minute applications may require shifting to different products.  You can sleep well knowing that there are several products to choose from.  Products that have the best efficacy for FHB (based on replicated university trials) include (in alphabetical order), Caramba, Miravis Ace, Proline, Prosaro, Prosaro Pro, and Sphaerex. 

Timing a Fungicide for FHB.  
We want to target our fungicides to hit a window between the onset of flowering (Feekes growth stage (FGS) 10.5.1 and 6 days after this point in time.  We define FGS 10.5.1 as the point in time when 50% of the MAIN tillers in a field have started to flower. This means that 50% of the wheat heads are exposed but not flowering, and secondary tillers may be between boot and early heading. It can be very hard to achieve a perfect application for FHB, as subtle differences in field conditions can result in a lack of uniformity in flowering, and secondary tillers will start to flower several days after main tillers and continue to flower for at approximately 1-2 weeks, depending on the temperature.  Although secondary tillers may not contribute much to overall yields, they certainly can contribute to DON contamination and dockage at the elevators.  One of the reasons we only achieve suppression of FHB/DON is due to this lack of uniformity and extended flowering window in small grains.   So how do we time our fungicides to get the most suppression of FHB?  A lot has to do with scouting before flowering and assessing the forecasted temperatures.  

When your field is at the boot stage, make sure you are checking your fields every 1-2 days.  Once about 50% of the heads on the main tillers have cleared the ligule, expect flowering to begin within 3-4 days under moderate temperatures (70’s-80’s°F).  Development will be slower at cooler temperatures, and more rapid at elevated temperatures. You will have 6 days from the start of flowering to make an application of your FHB labeled fungicide to ensure maximum efficacy.  Knowing this, you should be able to line up your sprays while heads are emerging, or ensure orders are put in to allow the applicators ample time to get to the field and hit the fungicide application window.  
    
  Fungicides in the strobilurin group (QoI FRAC code 11) should not be used past flag leaf emergence (FGS 8) as they have been shown to increase DON levels in grain if FHB occurs.   

Cultural practices: F. graminearum and F. culmorum grow well on corn and small grain residue, but not as well on soybean residue. Planting wheat behind soybeans may reduce the amount of local FHB inoculum and therefore disease potential. If crop rotation is not an option, tillage can help bury the residue and favor decomposition thus reducing the amount of local inoculum. Remember that a portion of FHB spores will be deposited from the atmosphere and therefore if conditions are conducive, disease will still occur. 

For more information on FHB management options contact your local crop specialist. 
 

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Image 2. Fusarium head blight spore masses on infected heads. (Source: GROWMARK, Inc.)

Sources:

http://extension.udel.edu/factsheets/fusarium-head-blight-management-in-wheat/
http://cropdisease.cropsciences.illinois.edu/?p=908
https://scabsmart.org/