Canola crops were monitored throughout the Western Australian wheatbelt during
1996–99 to determine the incidence and severity of crown cankers caused
by the blackleg fungus (Leptosphaeria maculans). All
crops surveyed had blackleg. The incidence of crown canker was
48–100%, 15–100%, 9–94% and
48–100% during 1996, 1997, 1998 and 1999, respectively. The mean
incidence of crown cankers statewide was 85, 63, 55 and 85% in 1996,
1997, 1998 and 1999, respectively. The severity of crown canker (expressed as
percentage disease index) ranged between 30 and 96%, 3 and 94%,
5 and 78% and 21 and 96% during 1996, 1997, 1998 and 1999,
respectively. These high levels of blackleg can possibly be attributed to the
accumulation of large amounts of infested canola residues. In 1999, there were
effects of variety, application of the fungicide Impact, distance to last
year’s canola residues and rainfall on the incidence and severity of
blackleg. However, there were no effects of sowing date or region on the
disease incidence or severity once the other factor effects listed above had
been considered. In 1995, an additional survey of 19 sites in the central
wheatbelt of Western Australia assessed the survival of the blackleg fungus on
residues from crops grown in 1992–94. The residues at all sites carried
blackleg. However, the extent of infection at any particular site varied from
12 to 100% of stems with the percentage of stems carrying pseudothecia
containing ascospores varying between 7 and 96%. The high levels of
blackleg disease found in commercial crops are indicative of significant
losses in seed yields, making it imperative that management of blackleg be
improved if canola is to remain a viable long-term cropping option in Western
Australia.
Fungicide sensitivity and resistance in the blackleg fungus, Leptosphaeria maculans, across canola growing regions in Australia
