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Investigating lentil seeding rate and fungicide interactions

Disease should be managed independent of plant stand.

March 24, 2024  By Bruce Barker


A foliar fungicide application decision should be made independent of plant stand and based on an assessment of disease potential. Photo courtesy of Chris Holzapfel.

The theory is that denser plant stands resulting from higher seeding rates can create a more ideal environment for lentil diseases such as ascochyta blight and anthracnose. Conversely, higher seeding rates can lead to better stand establishment and potentially higher yields under favourable growing conditions. To explore the interaction between seeding rate, plant stand, disease development and fungicide application, a research project was conducted in 2021 and 2022 at three locations in Saskatchewan.

“Although previous work also included a weed control component, for this project, we chose to focus on seeding rates and fungicides to simplify field trial management and data collection,” says principal investigator Chris Holzapfel, research manager with the Indian Head Agricultural Research Foundation at Indian Head, Sask.

The research was conducted at IHARF and two other Agri-Arm sites: the Western Applied Research Corporation at Scott and the Wheatland Conservation Area Inc. at Swift Current. It was funded by the Saskatchewan Pulse Growers.

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The objective was to demonstrate the effects of lentil seeding rates and subsequent plant densities on competition with weeds, disease, yield and grain quality, in addition to the agronomic response to foliar fungicide applications. A second objective was to demonstrate the most profitable combinations of seeding rates and foliar fungicide application strategies for lentils under a range of Saskatchewan growing conditions.

This project was initiated as a follow-up to a recently concluded ADF/SPG/WGRF research project led by Jessica Enns (Weber) in collaboration with multiple Agri-ARM sites and the University of Saskatchewan. That study found that using a pre-seed residual herbicide reduced early-season annual weed populations by 66 per cent compared to the traditional pre-seed burn-off using only glyphosate. It also found that the seeding rate with the highest yield and net returns was 19 seeds per square foot (190 seeds/m2) compared to rates of 13 or 26 seeds/ft2 (130 or 260 seeds/m2). Weed density was also highest at the lowest seeding rate. Disease severity also increased with increasing seeding rates, and a fungicide application was more likely required when lentils were seeded with a higher seeding rate.

In Holzapfel’s research, small red lentils were seeded at three rates of 13, 19 and 25 seeds/ft2. Foliar fungicide treatments included no foliar fungicide, a single fungicide application of Dyax applied at early bloom at a rate of 160 ml per acre (395 ml/ha), and a dual fungicide application of Dyax at early bloom followed by Lance WDG applied approximately 14 days later at 170 g per acre (420 g/ha).

Data collection included plant densities, disease ratings, weediness ratings, seed yield, test weight and seed size. Data from Scott in 2021 were not included in the final analyses due to extreme variability caused by drought.

As expected, plant stand establishment increased linearly with increasing seeding rate.

A pre-seed burn-down and in-crop herbicides were applied, but no residual herbicides were used. While a detailed analysis of weed counts and biomass was not part of the research, a visual assessment rating found that higher weed pressure occurred at lower seeding rates. Overall, the plots were relatively clean, and the overall weediness rating was 1.7/10 at 13 seeds/ft2 and 1.4/10 for the 19 and 25 seeds/ft2 seeding rates.

Low disease pressure
The first disease rating was taken at the start of flowering, just before the first fungicide application. At that time, disease pressure was negligible at all sites, ranging from 0.1 to 1.2 per cent of stem and leaf area infected. Indian Head-22 had the highest disease pressure, and Swift Current-21 had the lowest – consistent with the wetter environmental conditions at Indian Head.

“Based specifically on these observations along with the weather and environmental conditions in late June and early July, it would have been difficult to recommend a fungicide application at all sites except IH-22, where disease pressure was low but symptoms were present and the weather/soil conditions were wetter than normal,” says Holzapfel.

Final disease ratings were conducted about seven days after the second fungicide treatment was applied. Except for Swift Current-21, where the disease rating decreased to negligible, the disease rating at the other four sites increased, although disease pressure was still very low. At Indian Head-22, overall disease pressure remained quite low even though wet weather would have favoured disease development. Here, the disease ratings averaged 2.3 per cent at 13 seeds/ft2, 3.5 per cent at 19 seeds/ft2 and 4.2 per cent at 25 seeds/ft2. Holzapfel says that averaged across seeding rates, fungicide, not unexpectedly, did not appear to have much impact on disease at the sites with the lowest pressure.

Impact on yield
Yields were highest at Indian Head-22 at 3,113 lbs./ac. (3,498 kg/ha), followed by Scott-22 at 2,158 lbs./ac. (2,425 kg/ha), Indian Head-21 at 1,979 lbs./ac. (2,221 kg/ha), Swift Current-22 at 1,662 lbs./ac. (1,868 kg/ha) and Swift Current-21 at 1,333 lbs./ac. (1,498 kg/ha). At Indian Head-21, the yield was marginally higher with a higher seeding rate. In the remaining cases, yield was unaffected by seeding rate at two sites and declined with a higher seeding rate at two sites.

At Indian Head-21, the 25 seeds/ft2 seeding rate was most profitable, netting $35.22 per acre more ($87/ha) than at 13 seeds/ft2. At the other sites, the increased cost of higher seeding rates meant that relative marginal profits were neutral or lower with higher seeding rates.

Fungicide application did not result in a significant increase in yield, except at Indian Head-22, where a single or dual application yielded statistically higher at three to four per cent more than the control. Additionally, the interaction of seeding rate and fungicide did not generally affect yield. This suggested that the response to fungicide, or lack thereof, was consistent within individual sites regardless of seeding rate.

Because of the low disease pressure and lack of yield response, the relative profit margin was lower at all sites with a fungicide application. Even at Indian Head-22, where yield was higher with a single fungicide application, the single fungicide application had the same marginal profit as the control when product and application costs were accounted for. The dual fungicide application resulted in a net loss in marginal profit of $21.05/ac. ($52/ha) compared to the control.

At the other sites, marginal profits with a single application were negative at $0.40 to $18.62/ac. ($1/ha to $46/ha) less than the control, and from $31.58 to $58.70/ac. ($78 to $145/ha) less with the dual application.

“While we know that lentils can be quite susceptible to disease and the impacts can be severe if they are not managed, these results reinforce the need to base fungicide applications on actual disease pressure and probability of response as much as possible to maximize profits,” says Holzapfel.

Overall, Holzapfel says the research indicates that seeding rates of 13 to 19 seeds/ft2 could be considered low risk and optimal from an economic point of view. If high seedling mortality is expected, a higher rate could be beneficial. The research also found that disease should be assessed outside of plant populations and that a fungicide application should be based on an assessment of disease potential, using a fungicide decision support checklist that was developed by Agriculture and Agri-Food Canada and can be found on Saskatchewan Pulse Growers’ website. 

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