Foliar Wheat Disease Management with Fungicides – Oklahoma 2024

Categories: Fungicides

Meriem Aoun, Small Grains Pathologist

Department of Entomology & Plant Pathology Oklahoma State University

COOPERATORS: Dr. Meriem Aoun¹, Brian Olson¹, Tyler Pierson², and Dr. Amanda de Silva Oliveira²

¹Departments of Entomology & Plant Pathology, Oklahoma State University, Stillwater, OK; ²Plant & Soil Science, Oklahoma State University, Stillwater, OK.

PURPOSE:  To determine the efficacy of fungicides for controlling foliar diseases when applied to hard red winter wheat in Oklahoma.

LOCATION:  Entomology & Plant Pathology Research Farm; Stillwater, Oklahoma (Google Maps coordinates:  36.1266690, -97.1047885)

VARIETY: ‘OK Bullet’ hard red winter wheat                    

SOIL TYPE:  Norge loam

PLANTING DATE & RATE:  10-NOV-2023 at 90 lb/ac

FUNGICIDE APPLICATION DATES:

Green up application at Feekes’ growth stage 6 (GS6: 1^st node detectable at base of main tiller): 20-MAR-2024

Flag leaf application at Feekes’ growth stage 10.1 (GS 10.1: heads (awns) just starting to emerge from the boot): 17-APR-2024.

TARGET ORGANISMS (DISEASES): P. striiformis (stripe rust), Puccinia triticina (wheat leaf rust), Blumaria graminis f. sp. tritici (wheat powdery mildew), and Zymoseptoria tritici (Septoria leaf blotch)

PLOT DESIGN: Plots were 49 in. (7 rows; 7 in. spacing) by 20 ft, replicated four times in a randomized block design. Data were analyzed using R version 4.1.3.  Mean separation (LSD) test was conducted only if ‘F’ test was significant.

CULTURAL PRACTICES:

Fertilizer: Based on a soil test with a 50 bu/ac yield goal, 150 lbs/ac of 46-0-0 (Urea) and 150 lbs/ac 18-46-0 (Diammonium Phosphate/DAP) dry fertilizer were applied on September 9, 2023. On Feb 7, 2024, a top dress with 50 lbs/ac of 46-0-0 (Urea) was applied.

Herbicide:  None applied

Insecticide:  Gaucho 600 (2.4 fl oz/cwt) applied as a seed treatment to limit barley yellow dwarf

Irrigation:  No irrigation provided

METHOD OF APPLICATION:  Fungicides were applied with a CO₂ wheelbarrow sprayer as a broadcast foliar application using flat fan nozzles (8003EVS) spaced 18 inches apart. The sprayer was calibrated to deliver fungicides at a rate of 15 GPA.

DISEASE ASSESSMENT: When sufficient foliar disease was present, plots were evaluated for type of disease and rated for percent infection using modified keys based on, “A Manual of Assessment Keys for Plant Disease. 1971. Clive James. Canada Department of Agriculture Publication No. 1458.” In this rating system, we took the percentage of leaf surface area covered by a disease.

HARVEST METHOD: Plots were harvested with a Wintersteiger Delta small plot combine. Grain weight, test weight, and moisture content were collected from each plot. Grain moisture at harvest was ~ 8% for all plots. The grain yield of the plots was adjusted to 12% moisture.

GENERAL OBSERVATIONS: 

Fall and winter rains (accumulated rainfall OCT-FEB was 10.9 inches) in Stillwater provided adequate moisture for good wheat growth and development. The accumulated rainfall in spring of 2023 (March – May) was 11.6 inches with the highest levels of moisture recorded in May 2024 (7.5 inch). By early April, the wheat was ahead of schedule, with the flag leaf appearing around two weeks earlier than normal. Low levels of barley yellow dwarf (BYD) infections were present but scattered randomly across the trial. Although BYD may have slightly impacted yield it was not a major concern. 

Stripe rust was severe in April and early May. Stripe rust was first observed on March 31 in Stillwater. On April 17, stripe rust severity reached 20% on average on the untreated check. Stripe rust severity was rated in this trial on April 17, just before the flag leaf application. On May 1^st, 2024 (2 weeks after the flag leaf application), stripe rust was rated again and the untreated check showed an average severity of 27.5%. Leaf rust appeared in this trial around the last week of April – early May. On May 1 and May 9, leaf rust ratings were collected with the untreated check showing an average severity of 32.5% and 90.0%, respectively. Powdery mildew was observed in April and May but at low levels (average severity on the untreated check was 1% on April 17 and 10 % on May 1^st). The foliage senesced quickly after May 9 and no more ratings could be made. No indication of phytotoxicity by any fungicide treatment was observed. The rainfall in May delayed harvest by a couple of weeks. The plots were harvested on June 19, 2024.

SUMMARY: 

There were 21 fungicide treatments in this commercial fungicide trial and one untreated check (Table 1). Results from this trial demonstrated the efficacy of most fungicide treatments in managing foliar fungal diseases (primarily stripe rust and leaf rust) (Table1). Stripe rust and leaf rust were significantly lower for most fungicide treatments in comparison to the untreated check. Consequently, most fungicide treatments resulted in higher yield and test weight. Yield (adjusted to 12% moisture) ranged from 51.7 bu/ac to 87.0 bu/ac. Test weight ranged from 56.2 to 62.9 lb/bu. The untreated check had the lowest yield and test weight (Table 2).

Fungicide treatments that were applied at green up only (treatments 2, 3, and 15) were not efficient in controlling rust diseases. Treatments # 4, 12, 13, 11, 18, 8, 6, 10, 21, 17, 9, and 5 were associated with the lowest levels of rust diseases and the highest yields and test weight (Table 1, 2).

Table 1. Fungicide treatments in the commercial fungicide trial at Stillwater, Oklahoma in 2024.

Table 2. Summary of foliar wheat disease management at Stillwater, Oklahoma 2024.