Agronomy Update
Hydrated lime can help reduce clubroot disease

Clubroot prefers acidic soils, and the application of lime to increase soil pH may reduce disease. Limestone (CaCO₃) is the main source for agricultural application to neutralize soil pH and improve plant growth. High-calcium hydrated lime is a dry powder produced by combining quicklime (CaO) with water, resulting in the product Ca(OH)₂, containing approximately 75 per cent CaO and 25 per cent water. Once mixed with water, hydrated lime quickly dissolves, resulting in a highly alkaline solution.

Research trials were conducted by University of Alberta graduate student Nicole Fox and her supervisors, Stephen Strelkov and Sheau-Fang Hwang, to: evaluate the potential of hydrated lime for reducing clubroot disease development under field conditions; compare the efficacy of varying rates of hydrated lime and limestone for clubroot control at different inoculum levels under greenhouse conditions; and measure the effect of different lime treatments on P. brassicae proliferation in canola host roots. 

Replicated field trials were conducted in 2017 and 2018 at two sites at Alberta Agriculture and Forestry’s Crop Diversification Centre North in Edmonton. In 2017, hydrated lime rates were calculated based on the targeted pH relative to the pre-treatment pH of the soil in the plots. The pre-treatment pH at site one was 6.3, and 5.1 at site two.

To increase pH by 0.5, a rate of 3.36 tonnes of lime per hectare (t lime/ha) was used for the calculations. At site one, the target pH values were 7.0 (4.7 t lime/ha required), 7.5 (8.0 t lime/ha) and 8.0 (11.4 t lime/ha). At site two, the target pH values were 6.0 (6.0 t lime/ha), 6.5 (9.4 t lime/ha) and 7.0 (12.7 t lime/ha). To maintain consistency, the same amounts of lime were applied to the trials in 2018, and the plots were placed adjacent to the previous year’s plots.

The lime treatments were broadcast and incorporated immediately afterwards to a depth of three to four inches (8 to 10 cm). The trials were seeded to a clubroot-susceptible canola seven to eight days after lime application and incorporation. 

In 2017, moderate to high rates of hydrated lime reduced the Index of Disease (ID) by 35 to 91 per cent in the susceptible canola cultivar eight weeks after seeding. The average ID for the control treatment (pH 6.3) at site one was 47 per cent. The high rate of lime reduced ID to four per cent, the moderate rate ID was 6.7 per cent, and the lowest rate of lime was 37.5 per cent. 

In 2018, due to several environmental factors, no effect of lime treatment was observed in the field trials. 

In addition to the field trials, a greenhouse study in 2017 and 2018 compared the efficacy of hydrated lime and limestone in reducing ID in susceptible and resistant canola cultivars at different application rates and inoculum concentrations. 

Canola was planted into a potting medium with an initial pH of 5.3. The potting medium was inoculated with P. brassicae resting spores to target concentrations of 1 × 10³, 1 × 10⁴, 1 × 10⁵, and 1 × 10⁶ resting spores per gram of potting medium.

“Zero Grind” limestone or hydrated lime were applied and incorporated into the soil at rates equivalent to 4.7, 8.1, 11.4 or 14.8 t lime/ha, to adjust the pH to 6.0, 6.5, 7.0 or 7.5. 

In 2017, ID was very high in treatments that did not receive lime, at 92 to 100 per cent in the susceptible canola, and low in the resistant canola at 9 to 13 per cent. The application of hydrated lime at all rates eliminated visible symptoms in both cultivars at all inoculum concentrations, with the exception of the rate 8.1 t lime/ha at 1 × 10⁶ spores/g medium, which developed an ID of 18 per cent. 

Limestone was less effective in reducing ID. On the susceptible canola, significant reductions in ID were observed only at the two lower resting spore concentrations, and limestone treatment at any rate had no effect on ID at the two highest spore concentrations. Similar trends were observed with the resistant cultivar.

Overall, the trends in 2018 were consistent with those observed in 2017.

Root tissues from the greenhouse trials were analyzed by quantitative PCR to measure the amount of P. brassicae DNA present in the root tissue. These results followed a similar trend as observed with ID symptoms, where hydrated lime was much more effective than limestone, especially at a higher spore load. 

While hydrated lime appears to provide superior clubroot control, at a cost of about $320 per tonne, it is considerably more expensive than limestone at $54 per tonne. In fields that are infested only mildly with P. brassicae, the application of limestone may be sufficient as a tool to manage the pathogen. Liming an entire field may never be economical, but the application of lime to P. brassicae-infested patches in a field, such as hot spots and field entrances, could be an important strategy for clubroot management in canola.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.