Canola cultivar mixtures and rotations do not mitigate the negative impacts of continuous canola

Abstract

Harker, K. N., O'Donovan, J. T., Turkington, T. K., Blackshaw, R. E., Lupwayi, N. Z., Smith, E. G., Dosdall, L. M., Hall, L. M., Kutcher, H. R., Willenborg, C. J., Peng, G., Irvine, R. B. and Mohr, R. 2015. Canola cultivar mixtures and rotations do not mitigate the negative impacts of continuous canola. Can. J. Plant Sci. 95: 1085–1099. High-frequency canola (Brassica napus L.) rotations increase canola production risks. From 2008 to 2013, direct-seeded experiments involving several variations of continuous canola were compared with wheat (Triticum aestivum L.) and field pea (Pisum sativum L.) rotated with canola at five western Canada locations. Continuous canola rotations involved sequences of different herbicide-resistant canola and two-cultivar mixtures of herbicide-resistant canola from different sources in the same year. Fertilizers, herbicides, and insecticides were applied as required for optimal production of all crops. Rotating herbicide-resistant canola types over years or mixing two cultivars of the same herbicide-resistant type provided no pest management, yield or seed quality advantages compared with planting the same herbicide-resistant cultivar type each year. In 2013, weed biomass was lower in canola preceded by other crops than most continuous canola treatments. Compared with continuous canola, when 1 or 2 yr of wheat or field pea and wheat were inserted into 3-yr rotation cycles, 2010 root maggot damage was reduced 6% and 2013 blackleg [Leptosphaeria maculans (Desmaz.) Ces. & De Not.] incidence and severity were reduced 53 and 54%, respectively. Furthermore, yields were 22% higher when canola was grown only once in 3 yr compared with continuous canola and the wheat–canola–canola rotation. The most important mitigation strategy to ensure long-term sustainable canola production is to rotate canola with other crops.

Attempts to rescue yield loss in continuous canola with agronomic inputs

“…Even though all varieties were blackleg-resistant, the genetic basis of the resistance could vary, and the pathogen population could be different at individual sites (Liban et al 2016;Zhang et al 2016); this may have affected disease responses to fungicides between sites and years. A previous experiment (Harker et al 2015a) did not find an effect of rotating different hybrid canola varieties on blackleg disease. The authors are not aware of any evidence that hybrid canola varieties used in this study would differ in response to increased fertilizer or tillage treatments.…”

“…Experiments were conducted in western Canada from 2014 to 2016 at Lacombe (52.5°N, 113.7°W) and Beaverlodge (55.2°N, 119.4°W), AB, and Brandon, MB (50.0°N, 99.9°W), on 6-yr continuous canola established from a previous study (Harker et al 2015a). At Beaverlodge, the soil was a grey Luvisol loam soil with a pH of 5.8 and 7.7% organic matter.…”

“…Additional data collection included crop density 2 wk after emergence, total in-crop post-spray weed density (including canola volunteers) at least 2 wk after herbicide application, and blackleg incidence and severity. Blackleg assessments were conducted using 50 stem base/root samples randomly collected from each plot shortly after swathing using the methods described by Harker et al (2015a).…”

“…resistance genes reduced blackleg disease and associated yield losses (Marcroft et al 2012;Van de Wouw et al 2014). However, in western Canada, where blackleg resistance backgrounds are not publically available, neither mixing cultivars within nor over years improved disease or yield outcomes in continuous canola (Harker et al 2015a).…”

Spatio‐temporal connectivity and host resistance influence evolutionary and epidemiological dynamics of the canola pathogen Leptosphaeria maculans

“…It has been proposed that selectively reducing the contribution of pre-adapted pathogen populations, by rotation or cultural practices, could slow evolution (Bousset & Chèvre, 2013). Currently, stubble from the previous year is the primary source for L. maculans inoculum (Marcroft et al, 2004) although changes in tillage practices (McCredden et al, 2017) or shorter rotations with more frequent return of canola on the same fields (Kutcher et al, 2013;Harker et al, 2015) might alter this situation. Given that the survival of Leptosphaeria maculans decreases over the first year following harvest, options for selectively reducing contributions of different inoculum sources could include increasing the distance to a spore source (Bousset, Jumel, Garreta, Picault, & Soubeyrand, 2015;Marcroft et al, 2004;Savage, Barbetti, MacLeod, Salam, & Renton, 2013), stubble management by burial (Huang, Fitt, & Hall, 2003;Marcroft et al, 2004;Naseri, Davidson, & Scott, 2008;Thürwächter, Garbe, & Hoppe, 1999), flooding (Cai et al, 2015) or chemical application (Wherrett, Sivasithamparam, & Barbetti, 2004).…”