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Flag leaf timing or head timing fungicide on wheat: Which is the better choice?

Disease pressure

Leaf disease is a common occurrence in Alberta wheat production. Tan spot and septoria can cause yield and quality impacts on wheat. Under high disease pressure, there can be significant reductions in the photosynthetic area of yield-bearing leaves.

Additionally, Fusarium head blight (FHB) has been expanding in Alberta (Harding et al. 2018). FHB infection can occur anytime the wheat head has emerged until the start of senescence (personal communication with Dr. Kelly Turkington, pathologist at Agriculture and Agri-food Canada, Lacombe). In combination with a multi-pronged management approach (see managefhb.ca), FHB impacts on yield and quality can be reduced through fungicide application at head timing (MacLean et al. 2018; Wiersma and Motteberg 2005).

Challenge of timing

This begs the question, should we be targeting flag leaf timing or head timing for fungicide application? If the crop is at risk of foliar disease and FHB, will head timing fungicide provide adequate control or is a dual fungicide application needed?

The goal of fungicides is to protect the plant’s photosynthetic area, to reduce impacts of FHB, or a combination of both. Foliar leaf disease pressure at flag leaf timing may warrant a flag leaf fungicide application. While both FHB risk and foliar leaf disease at head timing may warrant a fungicide application at anthesis. However, just because disease is present, does not mean a fungicide application will provide an economic return.

Within typical Alberta growing conditions, it is not always easy to decide if flag or head timings of fungicides are preferable. Additionally, even in high disease pressure years, the economic returns from applying dual fungicide at both flag leaf and head timing are unlikely (personal communication with Dr. Sheri Strydhorst, agronomy research specialist at Alberta Wheat and Alberta Barley Commissions).

Data by collected by Asif et al. in a 2018 and 2019 research trial (unpublished) indicates that although dual application can increase yields and decrease leaf disease, additional economic return was unlikely. The trial compared a single flag leaf fungicide application to dual fungicide applications made at flag leaf timing (GS 39) and head timing (GS 61-63). Based on product costs and market prices at the time of the trial, a dual application cost $52.66/ac ($130.08/ha) and required a 4.0-4.6 bu/ac (0.27-0.31 tonnes/ha) yield increase over a single application to break even. One site-year (Bon Accord 2019) did see a yield increase of 4.2 bu/ac (0.28 tonne/ha) over the single flag leaf fungicide application indicating economic breakeven, but no additional economic benefits were observed over a single fungicide application. Producers are left to decide between protecting their crop at flag leaf or head timing.

Under typical Alberta conditions favourable for foliar leaf disease development, crop rotation, variety selection (based on genetic resistance to diseases), disease presence, and environment will all impact the severity of leaf disease. High humidity and adequate rainfall provide the environment for foliar leaf spot disease to establish and spread through the crop canopy. However, these same conditions also increase Fusarium damage kernels (FDK) and Deoxynivalenol (DON) risk, assuming Fusarium graminearum (Fg) is present. This means a producer at risk of leaf disease may also be at risk of FDK impacts and DON quality reductions associated with Fg With the increased occurrence of FHB across Alberta, this is the case for more farmers every year.

Alberta and Saskatchewan research shows that often a head timing (foliar leaf disease and FHB protection) fungicide provides the same, if not better, yield benefits as compared to a flag leaf (foliar leaf disease protection) timing fungicide. This means that a farmer may not have to choose between a flag leaf timing fungicide or a head timing fungicide. If FHB is a risk, a farmer can potentially get a dual benefit by spraying fungicide at head timing to protect their yield from both leaf spotting disease and FHB.

For more information on FHB management, see managefhb.ca.

Important note: this is not the case for stripe rust. If stripe rust is present in the field, fungicide applications should be applied at the 5% leaf coverage threshold to avoid significant economic losses. Farmers should not wait until flag leaf or head timing to protect their crop from stripe rust.

Research

Recent research by Asif et al. (2021) investigated fungicide timing of two commonly grown Canadian Western Red Spring varieties, AAC Brandon and AAC Viewfield. The trials were conducted in 2018 and 2019 in Lethbridge, Red Deer, Bon Accord and Barrhead. Fungicides were applied flag leaf timing (Trivapro at BBCH 39-45), head timing (Prosaro XTR at BBCH 61-63) and an untreated check.

The Asif et al. (2021) research also included fungicide treatments at herbicide timing (BBCH 22-23) and plant growth regulator (PGR) timing (BBCH 30-32). These early timings did not increase yields. For detailed yield results of the earlier fungicide applications at herbicide and PGR timings, see here.

Disease, Yield and Economics Results

Averaged over responsivesite-years both flag leaf and head timing applications displayed significantly lower leaf disease than the untreated check. At responsive sites, there was no significant difference in yield between flag leaf and head timing fungicide timing. Both flag leaf and head timing fungicides were significantly higher yielding than the untreated control by 10% and 11%, respectively (Table 1). The results show that at responsive sites, 7.2-7.6 bu/ac and 6.4-6.8 bu/ac profit were reported from the flag leaf and head timing fungicide applications, respectively. These results are similar to results seen in Saskatchewan (MacLean et al., 2018) and in Minnesota (Wiersma and Motteberg, 2005) where fungicide at head timing provided equal or better yield results than a flag leaf timing fungicide.

†Responsive site-years were characterized by higher relative humidity (65.4 - 74.0%) and an average of 273 mm of precipitation. Responsive sites were Barrhead 2019, Bon Accord 2019, Red Deer 2018 and Red Deer 2019. Unresponsive site-years were characterized by lower relative humidity (57.7 - 63.7%) and an average of 175 mm of precipitation. Unresponsive sites were Barrhead 2018, Bon Accord 2018, Lethbridge 2018 and Lethbridge 2019.

Table 1: Leaf disease levels, yield increases at responsive sites and economics of flag timing (Trivapro) and head timing fungicide (Prosaro XTR) application compared to untreated check for the CWRS varieties AAC Brandon and AAC Viewfield. Trials were conducted over 2018 and 2019 in Lethbridge, Red Deer, Bon Accord and Barrhead.

Recommendations

The most economically beneficial practices from the Asif et al. (2021) study were spring wheat fungicide applications at flag leaf timing (BBCH 39-45) or FHB timing (BBCH 61-63) when environmental conditions [higher relative humidity (65.4-74.0%) and an average 273 mm of precipitation] were conducive for disease development.

For producers deciding whether to apply fungicide at flag leaf or head timing on their wheat:

  • Continue to scout for leaf disease early and often to be aware of disease risk.
  • Short rotations and varieties susceptible to leaf disease increase the risk of foliar disease and FHB impacting yield and quality.
  • Fields with shorter rotations and susceptible varieties may be at greater risk of yield loss due to foliar disease if fungicide is delayed.
  • Fungicide applications should not be seen as a silver bullet for disease control and should always be implemented in combination with sound agronomic management.
  • If FHB is a risk, delaying fungicide applications to head timing may not have a negative impact on yield and quality as compared to flag leaf timing fungicide.
  • If disease risk is low at and before flag leaf timing, continue checking for disease pressure. If disease pressure increases as head timing approaches, a fungicide at head timing may still provide yield benefits by protecting the yield-bearing leaves.
  • Spray for stripe rust at the recommended application timing of 5% leaf coverage. Famers should not wait until flag or head timing to protect their crop from stripe rust.

References:

Asif M., Strydhorst S., Strelkov S.E., Terry A., Harding M.W., Feng J., Yang RC. 2021. Evaluation of disease, yield and economics associated with fungicide timing in Canadian Western Red Spring Wheat. Can. J. Plant Sci. https://doi.org/10.1139/CJPS-2020-0318

Harding, M.W., Howard, R.J., Feng, J., Laflamme, P., Turkington, T.K., Gräfenhan, T., and Daniels, G.C. 2018. Monitoring Fusarium graminearum in Alberta: looking back 20 years. Can J. Plant Pathol. 40: 141.

MacLean, D. E., Lobo, J. M., Coles, K., Harding, M. W., May, W. E., Peng, G., Turkington, T. K., and Kutcher, H. R. 2018. Fungicide application at anthesis of wheat provides effective control of leaf spotting diseases in western Canada. Crop Protection. 112: 343- 349.

Wiersma, J. J. and Motteberg, C.D. 2005. Evaluation of five fungicide application timings for control of leaf-spot diseases and fusarium head blight in hard red spring wheat. Can J. Plant Pathol. 27: 25-37