Crop isolation, not extended rotation length, reduces blackleg (Leptosphaeria maculans) severity of canola (Brassica napus) in south-eastern Australia

Abstract

Due to the large increase of canola production in Australia, current blackleg cultural control recommendations (extended rotation length and isolation distance from canola stubble) are not adhered to by farmers in many canola-producing regions. Canola crops are increasingly being sown in short rotation and, in many instances, adjacent to paddocks containing canola stubble. In this study, the level of disease in commercial canola crops was determined for different rotations and distances from canola stubble. There was a strong relationship between the presence of canola stubble from the previous year (6-month-old stubble) and distance to current canola crops, but no relationship between the presence of older (18–42 month old) stubble and distance to current canola crops. Blackleg severity was highest where canola crops had been sown adjacent to 6-month-old canola stubble, with the level of blackleg severity decreasing markedly in the first 100 m. Disease severity then generally declined up to 500 m. Plants 500–1000 m from 6-month-old stubble had similar levels of blackleg infection. Blackleg severity was similar between canola crops sown into 18-month-old canola stubble (short rotation) and crops sown into paddocks that had no history of canola for at least the previous 3 years (long rotation). Based on these findings, we recommend that canola crops should be sown at distances greater than 100 m and preferably 500 m from last season's canola stubble, rather than extending rotation length between crops.

Insights into fighting against blackleg disease of Brassica napus in Canada

“…Ascospores are mainly dispersed by wind thus can travel long distances, whereas pycnidiospores can only travel short distances by rain-splash. Therefore, in Australia, the recommended distance between canola fields is more than 400 m as canola plants grown within 400 m are in higher risk of infection than that of more than 400 m (Marcroft et al 2004). In western Canada, however, the recommended distance between canola fields is at least 50-100 m to reduce the impact of inoculum movement (Guo and Fernando 2005).…”

“…In Australia, the major inoculum of blackleg is ascospores, in combination with pycnidiospores (Barbetti 1976;Marcroft et al 2004). In western Canada, pycnidiospores are the most important sources of inoculum in infection and disease development (Petrie 1995;Guo and Ghanbarnia et al 2011;Dilmaghani et al 2013).…”

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

“…Reducing ascospore loads is desirable, because in field experiments over three cropping seasons, canola pod number and seed yield declined linearly as blackleg severity increased (Hwang et al., 2016). Benefits can thus be obtained by better management of stubble loads to reduce transmission of inoculum between cropping seasons, which depends jointly on distance between fields and the size of the source population (Marcroft et al, 2004). Transmission between fields can be predicted from spore dispersal (Bousset et al., 2015; Marcroft et al, 2004; Savage et al., 2013).…”

“…Benefits can thus be obtained by better management of stubble loads to reduce transmission of inoculum between cropping seasons, which depends jointly on distance between fields and the size of the source population (Marcroft et al, 2004). Transmission between fields can be predicted from spore dispersal (Bousset et al., 2015; Marcroft et al, 2004; Savage et al., 2013). Thus, spatially explicit models can be used to study and ultimately design combinations of landscapes, cultivar choice and tillage practices promoting resistance durability against blackleg (Hossard et al., 2015; Lô‐Pelzer et al., 2010).…”

“…Additional experiments would be needed to relate disease control choices in one season to epidemiological consequences for the following season. Of particular interest in this context, would be to test for a host genotype effect on spore production, for example, genotypes with reduced spore production for high canker severity (Marcroft et al, 2004). …”