Timing of Fungicide Application in Wheat

Introduction

Diseases caused by fungal organisms can infect wheat at various stages of growth. The development and severity of these diseases is largely influenced by the amount of inoculum present in the field and environmental conditions, primarily temperature and moisture. The impact of the disease(s) on wheat production and return on investment (ROI) can depend on the growth stage of the wheat crop when the disease develops. Therefore, the timing of foliar fungicide applications may depend on a number of factors including the current and forecasted weather conditions, susceptibility of the wheat product, severity of the infection and growth stage.

Wheat growth stages

The Feekes scale is a numerical scale that describes the stages of growth of a wheat plant from emergence (1.0) to maturity (11.4). Table 1 summarizes the stages of growth associated with the Feekes scale.

Table 1. Feekes Scale of Wheat Growth and Development.¹

Fungal diseases of wheat

Early season leaf diseases

Powdery Mildew

Powdery mildew is caused by the fungus Blumeria graminis f. sp. tritici and can be found in almost any area that wheat is grown. Colonies of mycelium, resembling cotton, are the primary symptom and are easily seen on the upper surface of the lower leaves (Figure 1). It is most prevalent in the joint to flag-leaf stage (Feekes 6 to 9) of development. The white patches can appear as early as mid-April in the Midwest. The optimum temperature range for development of powdery mildew is 59 to 70 °F (15 to 21 °C), while temperatures above 77 °F (25 °C) can often lead to a deterioration of the mycelium. A dense population of wheat along with high fertilizer rates can increase severity of the disease. Severe infection of the flag leaf may result in the greatest yield loss (up to 45% has been reported) by reducing kernel size, kernel number and test weight.²

Tan Spot

Tan spot is caused by the fungus Pyrenophora tritici-repentis and is one of the more damaging diseases of wheat in the Midwest. Because the fungus survives on wheat residue, tan spot is usually the most severe in fields where large amounts of infested residue remain on the soil surface. Symptoms can begin as early as late March in the Midwest, and early June in the Northen Plains appearing initially as small, dark oval to diamond shaped spots that become tan with a yellow halo. Lesions can eventually grow and merge together resulting in large sections of infected tissue (Figure 2). Spores are dispersed by wind. Infection and development of the disease is favored by temperatures ranging from 68 to 82 °F (20 to 28 °C) with frequent rain or dew keeping the leaf surfaces wet. Crop rotation is considered the best control for tan spot.^3,4

Septoria Blotch

Septoria blotch (also known as Septoria tritici or speckled leaf spot) is caused by Septoria tritici and infects primarily leaves (Figure 3). Symptoms are commonly found on lower leaves in the fall or early spring. Lesions are initially yellow turning gray or ash-white as they mature with small black specks known as pycnidia which produce the fungus. Disease development is most aggressive between 50 and 68 °F (10 to 20 °C) with extended periods of leaf wetness. Wet and windy weather conditions increase the likelihood of spreading the disease. It can overwinter on wheat residue.^4,5

Rust diseases

Three of the more common and economically important rust diseases that can attack wheat are stripe, leaf and stem rusts. Table 2 summarizes and compares the specific pathogen, the plant parts that can be infected, descriptions of the lesions produced by each disease as well as the optimum temperature range and hours of leaf wetness needed for development of each disease. Note that there is a fairly wide range of optimum temperatures indicating that, for example, stripe rust can develop earlier in the season compared to stem rust.

Table 2. Comparison of three rust diseases in wheat.⁶

Stripe Rust

Leaf Rust

Stem Rust

Fusarium Head Blight (Scab)

Fusarium head blight (FHB), also known as head scab, is caused primarily by Fusarium graminearum although several other species of Fusarium fungi may also produce symptoms. The mycotoxin deoxynivalenol (DON), also called vomitoxin, is produced by this disease. Visible symptoms of FHB, which is mainly bleaching of the heads, first show up between flowering and grain fill (Figure 7). Kernels that are produced within bleached areas of the head may appear as shriveled and chalky. Periods of high humidity (>70%) for 36 hours or more along with temperatures in the range of 68 to 77 °F (20 to 25 °C) provide ideal conditions for development of the disease.⁷

Management options for controlling fungal diseases in wheat

Farmers have multiple options in managing fungal diseases including foliar applied fungicides, seed treatments, wheat seed product selection, crop rotation, and tillage. Seed applied fungicide treatments, such as Raxil^® PRO MD Seed Treatment, Raxil^® PRO Shield Seed Treatment or EverGol^® Energy Seed Treatment fungicide can help provide consistent, early season protection. The fungal diseases discussed in this article can survive on infested wheat residue and can benefit from a crop rotation in which wheat is grown in one out of three years to manage the impact of these diseases. Tillage can help remove volunteer wheat plants and provide increased soil contact with infested residue. This may result in increased microbial action than can aid in the decomposition of residue, while decreasing the amount of inoculum remaining on the residue.⁴

Foliar Fungicide Application and Timing

Several foliar fungicide products are available for early-season and late-season applications for leaf diseases. The three classes of fungicides primarily used in wheat are triazoles (FRAC Group 3), SDHI (FRAC 7) and strobilurins (FRAC Group 11). Group 3 fungicides include triazole and triazolinthione(a sub family which are part of a large chemical group known as demethylation inhibitors, or DMI). They are broad spectrum and generally considered to be preventative and curative compounds. Prothioconazole, an active ingredient in Prosaro^® 421 SC Fungicide can provide both preventative and curative functions. Fungicides in Group 7 and 11 are preventative only, providing little or no curative activity. Absolute^® MAXX Fungicide contains tebuconazole and trifloxystrobin, Group 3 and 11, respectively, providing both preventative and curative properties. The amount of systemic movement of a fungicide within the wheat plant will vary with fungicide products, so growers should take that into consideration when choosing a product. For more information on fungicide modes of action, please refer to https://www.cropscience.bayer.us/articles/bayer/fungicide-modes-action.

To achieve the optimum fungicide application timing, it is important to scout fields regular basis. To maximize fungicide effectiveness, it essential to time the application according to the label instructions. Fungicide applications are often timed according to crop growth stage. Scouting can help determine if an infection is beginning to occur and if environmental conditions are forecasted to be favorable for disease development.

For early-season diseases, such as powdery mildew, septoria leaf blotch and tan spot, it may be beneficial to apply a fungicide if one or more of the following conditions are met:

• Environmental conditions currently are, and are forecasted to be, favorable for the disease to develop.
• High levels of fungal inoculum exist due to the previous crop being infested.
• Continuous wheat planting.
• Field history of the disease(s).
• The seed product being planted is susceptible.
• Yield potential is high
• High commodity price is favorable to ROI

The optimal timing for an early fungicide application to control leaf or foliar diseases is when the wheat has one or two joints present (Feekes 6 to Feekes 7). Early fungicide applications can often be tank mixed with herbicide or fertilizer treatments which can help reduce the cost of application. However, these early-season diseases typically affect the lower canopy and does not contribute significantly to yield potential.^8,9 Early fungicide applications will not protect newer leaves from rust diseases that can develop in later growth stages.

The most common use of fungicides in wheat is to help protect the crop from later-season leaf diseases like rusts. Perhaps the most important goal in fungicide applications to wheat is to protect the flag leaf. The flag leaf is the primary driver of photosynthesis that supports head and grain development. The penultimate leaf (the first leaf below the flag leaf) and the flag leaf combined account for about 70 to 90 percent of photosynthates produced by the plant and used for grain fill.¹ Preventative fungicides should be applied during Feekes 8 (flag leaf emergence) to Feekes 9 (ligule of the flag leaf is visible) growth stages if leaf diseases are present in the lower canopy. If disease symptoms are already present on the flag leaf, it is too late to apply a preventative fungicide.

The late-season applications designed to protect the flag leaf are considered to have the best chances of a positive return on investment (ROI) compared to fungicide use earlier in the season.

If Fusarium head blight is the most threatening disease to your wheat crop, the timing of a fungicide application is critical and should be applied at Feekes Growth Stage 10.5.1 (flower initiation).

Absolute^® MAXX Fungicide is labeled in the United States for several diseases including septoria leaf blotch, tan spot, powdery mildew and rusts. Labeled applications for early season suppression of these diseases is 3.0 to 4.0 fl oz/acre. Otherwise, the labeled rate for control is 5.0 fl oz/acre applied when conditions are favorable for disease development.

Prosaro^® 421 SC Fungicide and Prosaro^® PRO 400 Fungicide are labeled for control of Fusarium head blight (FHB), powdery mildew, septoria leaf blotch, tan spot, rusts and several other diseases. The labeled application rates range from 6.5 to 8.2 fl oz/acre for Prosaro^® 421 SC Fungicide and 10.3 to 13.6 fl oz/acre for Prosaro^® PRO 400 Fungicide. Each of these products should be applied at Feekes 10.5.1 growth stage to control FHB. For leaf and stem diseases, they should be applied when disease symptoms first appear.

Sources:

¹Lollato, R. 2018. Kansas State University Research and Extension. MF3300. https://bookstore.ksre.ksu.edu/download/wheat-growth-and-development-poster-20x30_MF3300

²Salgado, J.D., and Paul, P.A. 2016. Powdery mildew of wheat. Ohioline. Ohio State University Extension. https://ohioline.osu.edu/factsheet/plpath-cer-11

³De Wolf, E. 2017. Tan spot. Kansas State University Research and Extension. EP134. https://bookstore.ksre.ksu.edu/download/tan-spot_EP134

⁴Friskop, A. and Liu, Z. 2021. Fungal leaf spot diseases of wheat: tan spot, septoria/stagonospora nodorum blotch and septoria tritici blotch. North Dakota State University. https://www.ndsu.edu/agriculture/extension/publications/fungal-leaf-spot-diseases-wheat-tan-spot-septoria/stagonospora-nodorum#:~:text=Three%20important%20fungal%20leaf%20spot%20diseases%2C%20tan%20spot%2C,reduce%20test%20weight%20and%20yield%20by%2050%20percent

⁵Salgado, J.D., and Paul, P.A. 2016. Leaf blotch diseases of wheat – septoria tritici blotch, stagonospora nodorum blotch and tan spot. Ohioline. Ohio State University Extension. https://ohioline.osu.edu/factsheet/plpath-cer-07

⁶De Wolf, E., Murray, T., Paul, P., et al. Multi-state extension and research committee for small grains diseases. Kansas State University. Publication MF-2919. https://bookstore.ksre.ksu.edu/download/identifying-rust-diseases-of-wheat-and-barley_MF2919

⁷2019. Fusarium head blight of wheat. Crop Protection Network. https://cropprotectionnetwork.org/encyclopedia/fusarium-head-blight-of-wheat

⁸University of Nebraska-Lincoln Extension. 2008 Application timing and preharvest intervals for wheat fungicides. Cropwatch. https://cropwatch.unl.edu/application-timing-and-preharvest-intervals-wheat-fungicides

⁹University of Nebraska-Lincoln Extension. 2008. Timing of wheat fungicide applications Cropwatch. https://cropwatch.unl.edu/timing-wheat-fungicide-applications

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