Lime for clubroot management
Fall application of Zero Grind limestone performed similar to spring applied hydrated lime. Generally, both products applied at 10 t/ha increased soil pH, reduced disease severity, increased yield and reduced spore density of P. brassicae in the soil.
Clubroot, caused by Plasmodiophora brassicae Wor., is well established across canola growing areas of western Canada. It can cause severe losses under high disease pressure – up to complete yield loss. Acidic soils favour the development of clubroot, but research has found that pH above 7.2 can reduced disease severity. In vegetable Brassica crops, adding lime as a soil amendment has proven to be effective in controlling clubroot.
Previous research found varying levels of clubroot disease reduction occurs with different forms of lime such as limestone (calcium carbonate), hydrated lime (HL; calcium hydroxide), dolomitic lime (containing calcium and magnesium carbonate), and quicklime (calcium oxide).
This research study was conducted with the objectives to assess the efficacy of fall and spring applications of limestone, and spring applications of hydrated lime for clubroot management. The results help to understand how less expensive but slower acting products like limestone can help farmers more economically control clubroot.
Field and greenhouse experiment were conducted at the Crop Diversification Centre North, Alberta Agriculture and Irrigation, Edmonton, AB, Canada, in 2019 to 2020 and 2021 to 2022. The field experiment was conducted at two sites with naturally infested clubroot soils at CDC North. In both experiments, pulverized limestone (Zero Grind; CaCO3) and hydrated lime (HL; Ca[OH]2) were compared at different timings. Lime was immediately incorporated to 4 inches (10 cm) after application using a rotary tiller.
Field rate/timing treatments included:
- ZG applied at rates of 5.0 and 10 t/ha in either fall or spring,
- HL spring application at 5.0 and 10 t/ha,
- Fall ZG application plus spring HL application at 2.5 or 5.0 t/ha each,
- Untreated control.
Field lime treatments were applied in the fall on 24 October 2019 and 16 October 2021, and spring liming was conducted on 25 May 2020 and 16 May 2022. One week after the spring lime treatment, the clubroot-susceptible hybrid, 45H31, was seeded.
Soil pH and resting spore density was measured from 4 inches (10 cm) deep soil samples collected before lime application and 10 days after seeding. Plant height, weight, aboveground biomass and root tissue were sampled 8 weeks after seeding, when clubroot disease severity was also assessed. Plots were taken to yield.
Increased pH and reduced disease severity
In field trials, all lime treatments increased soil pH compared to the untreated control. Soil pH levels at site 1 prior to treatment were 5.36 in 2019 and 5.24 in 2020. The highest pH after lime application at site 1 was 7.49 in 2020 and 7.08 in 2022. Site 2 had similar pH increases.
Application rates of 5 t/ha increased soil pH by 0.87 to 1.20 regardless of product type or timing of application. The largest increases were associated with spring hydrated lime (1.03 to 1.20 pH increase) and fall Zero Grind plus spring hydrated lime (1.06 to 1.20 pH increase).
Higher pH increases were observed with the 10 t/ha application rate, ranging from 1.56 to 1.98 pH increase. Similar to the lower rate, spring hydrated lime (1.91 to 1.98 pH increase) and fall Zero Grind plus spring hydrated lime (1.89 to 1.95 pH increase) had the highest increases. Fall Zero Grind at 10 t/ha increased pH by 1.76 to 1.95. The lowest increase came from spring Zero Grind at 5 t/ha with pH increases from 0.87 to 0.95.
All lime treatments provided significant disease severity reductions with the exception of spring Zero Grind at 5 t/ha at one site-year. For all lime treatments at a total rate of 10 t/ha, disease severity was lower than treatments at 5 t/ha. Fall Zero Grind at 10 t/ha and fall Zero Grind plus spring hydrated lime at 10 t/ha combined had the most significant reduction in disease severity. Fall Zero Grind reduced disease severity by 54.9 to 69.6 per cent compared to the untreated control. The fall Zero Grind plus spring hydrated lime reduced disease severity by 54.7 to 69.6 per cent. Spring hydrated lime at 10 t/ha reduced disease severity by 48.6 to 60.1 per cent.
Increased yield with lime application
All lime treatments at 10 t/ha significantly increased yield in field experiments. The highest yield increases in 2020 were from the fall Zero Grind plus spring hydrated lime (total 10t/ha) with yield increase of 80.6 per cent at site 1 and 584 per cent at site 2. The second highest yield increase was fall Zero Grind 10 t/ha with a yield increase of 70.5 at site 1, and 571 per cent at site 2.
In the 2022 harvest year, the fall Zero Grind at 10 t/ha increased yield by 296 per cent at site 1 and 220 per cent at site 2. The second highest yield measured in 2022 was with the combination of fall Zero Grind plus spring hydrated lime (total 10 t/ha) with a 275 per cent increase at site 1 and 191 per cent increase at site two.
The very large yield increases were due to poor yields of the untreated control at Site 2 in 2020 and both sites in 2022 due to heavy clubroot disease. For example, in 2022, the untreated control at site 1 yielded seven bu/ac (0.388 t/ha) compared to the fall Zero Grind 10 t/ha treatment at 27 bu/ac (1.537 t/ha).
In almost all cases over the two years of field trials, application of lime significantly reduced the number of resting spores compared to the untreated controls. For example, in 2021 to 2022, resting spores at site one was 1.3 x 106. After treatment, the number of resting spores ranged from around 2.5 x 105 to 8 x 105.
Greenhouse trends similar for susceptible variety
In greenhouse experiments, the susceptible 45H31 and moderately resistant CS2000 canola hybrids were sown into a soil/growth mixture in 38 L tubs. The soil mixture had been inoculated with P. brassicae pathotypes 3D or 3H on the Canadian Clubroot Differential set at a concentration of 1 x 106 resting spores per gram. A uninoculated control using soil free from P. brassicae spores was included for comparison.
The greenhouse rate/lime treatments were the same as the field experiment. After fall lime application to the tubs in October, the tubs were placed outside to overwinter. Spring lime applications were made in May, and then moved into the greenhouse where the susceptible and moderately resistant hybrids were seeded.
The average pH in the greenhouse potting mix was higher than the field trials at 6.03 in 2019 and 5.95 in 2020. Lime applications at 5 t/ha raised pH values by 0.92 to 1.10 and pH increase after 10t/ha applications were 1.82 to 2.00.
In the greenhouse trials, disease severity in the un-limed control was very high for the susceptible variety at around 80 per cent, but was lower for the moderately resistant CS2000 hybrid at between 40 to 50 percent. All lime treatments significantly reduced disease severity for both the susceptible and moderately susceptible varieties.
For the susceptible variety, the lime treatments generally reduced disease severity down to between 10 to 40 per cent depending on rate and timing. For the moderately susceptible variety, lime applications reduced disease severity to less than 15 per cent for all treatments.
Greenhouse yields were numerically higher for all lime treatments in 2020, but only three treatments were significantly higher for the susceptible variety. These were fall Zero Grind at 10 t/ha, spring Zero Grind at 10 t/ha, or the combination of fall Zero Grind and spring hydrated lime at 2.5 t/ha + 2.5 t/ha each. For the moderately susceptible variety, the only significant yield increase in 2020 was fall Zero Grind at 5 t/ha + spring hydrated lime at 5 t/ha.
In 2021 the highest greenhouse yields for both hybrids were with the combination of fall Zero Grind at 5 t/ha + spring hydrated lime at 5 t/ha, and the spring hydrated lime at 10 t/ha. For the susceptible variety, yield increases were 126 to 184 per cent, and 161 to 211 per cent on the moderately susceptible variety compared to the control.
Overall, the lime treatments significantly increased soil pH, reduced disease severity, increased yield and reduced spore density of P. brassicae in the soil. Fall application of Zero Grind limestone performed similar to a split application of fall Zero Grind plus spring hydrated lime applications, and a spring hydrated lime application. With a lower cost of fall application of Zero Grind limestone compared to spring hydrated lime, limestone products such as Zero Grind could be considered as a clubroot management tool on acid soils on the Prairies.
This study was supported by Alberta Canola, SaskCanola, and the Manitoba Canola Growers Association through the Canola Agronomic Research Program (CARP) and by Agriculture and Agri-Food Canada and the Canola Council of Canada through the Canadian Agriculture Research (CAP) Partnership (project no. CCC Activity 5.9 P1 Cluster).
Yu, Z., Hwang, S.F., Strelkov, S.E. 2025. Evaluation of Fall and Spring Lime Applications for the Management of Clubroot of Canola in Western Canada Plant Dis. 109:5, 1051-1062
OPEN ACCESS: https://doi.org/10.1094/PDIS-04-24-0806-RE
Photo courtesy Stephen Strelkov:
Roots collected from the field experiments, showing clubroot symptom development across the treatments from left to right: Control, Fall ZG (10 t/ha), and Fall (5 t/ha) + Spring (5 t/ha).
