Fusarium head blight (FHB) is one of the most important diseases of wheat in Canada. Even a small per hectare yield loss due to FHB results in the loss of tens of thousands of dollars per grower and millions of dollars collectively. Multiple control strategies are required to control the disease because each strategy has a moderate impact. The most important of these are: selection of wheat varieties with genetic resistance; cultural practices, such as crop rotation with non-host crops, possibly supplemented with tillage and other residue management tools; and the last line of defense, fungicide application during the flowering stage of the crop.

One of the most challenging issues for wheat growers is to determine the correct timing of a fungicide application. The optimum timing in spring and winter wheat is reported to be just a few days after emergence of the wheat spike from the boot when the anthers become visible; however, not all spikes emerge at the same time. This variability is due to the differences in emergence among the main stem spike and those of the tillers, variability in the length of the flowering period and variability in the time required for each floret to open. Researchers from Japan reported that disease severity and toxin content of infected wheat could be reduced by a very late stage application of fungicide, approximately 20 days after anthesis, which is at the late milk stage (Yoshida et al. 2012). Thus, there was a need to test fungicide efficacy in western Canada, particularly in durum, because there is no resistant durum germplasm and because there has been no fungicide timing trials in durum wheat. Durum wheat is used almost completely for human consumption; therefore, reducing the amount of fusarium damaged kernels and toxin accumulation in the grain is extremely important.

Field trials were carried out from 2016 – 2018 at Saskatoon, Melfort, Scott, Outlook and Indian Head to determine the importance of fungicide timing and seeding rate for the control of FHB symptoms and DON accumulation. Eight treatments of metconazole fungicide ‘Caramba®’ were applied to two seeding rate treatments: 400 seeds/m² and 75 seeds/m². The fungicide treatments consisted of an untreated check (no fungicide), a treated check (fungicide application at all stages), and applications at: BBCH 59 (heading), BBCH 61 (early anthesis), BBCH 65 (50% anthesis), BBCH 69 (late anthesis), BBCH 73 (soft dough) and a treatment with two applications: BBCH 61 followed by BBCH 73. Parameters evaluated were: FHB index, percent Fusarium-damaged kernels (% FDK), deoxynivalenol (DON) content, protein content, and yield.

The seeding rate treatment had a variable effect on parameters evaluated across all location years. The optimum timing of application was the same for both seeding rates.  The high seeding rate increased yield, but there was no interaction with fungicide application timing. Year-to-year weather differences led to variability in disease levels on the susceptible durum variety (CDC Desire) tested in 2016, 2017 and 2018. Under extended wet conditions, when the potential of severe FHB occurs, as was the case in 2016, all anthesis applications starting at BBCH 61 to BBCH 69 had a similar effect on FHB index, FDK, DON accumulation and yield. Fungicide application at these growth stages would benefit Saskatchewan durum growers by reducing the impact of the disease and toxin accumulation in the grain. In years with moderate disease pressure (2017 and 2018), the BBCH 65 application (full flowering: 50% of anthers mature) appeared to be optimum for FHB mitigation. Application of fungicide at the late stage (BBCH 73) generally did not reduce DON concentration more than a single application at BBCH 65 to 73. The results of the dual application (BBCH 61 + BBCH 73) treatment for disease control, FDK level, and toxin accumulation were similar to the BBCH 65 application at all location years.

Another key to success in disease control is understanding the distribution and prevalence of the pathogen species in the area. The predominant species causing FHB is Fusarium graminearum, but several other species may cause the disease in wheat in Canada. Species may differ from each other in terms of the severity of symptoms and the amount of toxin produced. Thus, variability in the causal Fusarium spp. across the province may be a reason for the variable results obtained from a fungicide application. Differences in Fusarium spp. and the toxins they produce may even influence the appropriate fungicide timing.

Fusarium spp., the chemotype diversity, and the mycotoxin levels were determined in 132 wheat samples collected across Saskatchewan from 2014 – 2016. Five Fusarium spp.: F. graminearum, F. culmorum, F. avenaceum, F. poae, and F. sporotrichioides were identified. Fusarium graminearum was the most frequent species identified and quantified followed by F. avenaceum. The chemotypes associated with the Fusarium spp. were 3-ADON and 15-ADON. The NIV chemotype was not detected among the infected wheat samples collected. Chemotype 3-ADON was detected more frequently and in greater amount than 15-ADON.  Mycotoxins quantified were DON, 3-ADON, 15-ADON, T2 and HT2 toxins.

This information is of value to wheat growers, particularly durum wheat growers, as it indicates that under FHB conducive environmental conditions growers can widen their window of application.  Our results suggest improved FHB control within the BBCH61-69 growth stages, which is a wider window than currently recommended (BBCH61-65). Additional information on FHB management in durum in more environments would be beneficial, but further experiments on fungicide timing in durum wheat may not be warranted at this time.

Reference

Yoshida M, Nakajima T, Tomimura K, Suzuki F, Arai M and Miyasaka A. 2012. Effect of the timing of fungicide application on Fusarium head blight and mycotoxin accumulation in wheat. Plant Disease 96: 845-851.