Factors affecting production of inoculum of the blackleg fungus (Leptosphaeria maculans) in south-eastern Australia

Abstract

The production of windborne ascospore inoculum of the blackleg fungus (Leptosphaeria maculans) was determined during 2000 and 2001 in 3 environments (Birchip, low rainfall; Wonwondah, medium rainfall; Lake Bolac, high rainfall) in Victoria. The weight of canola stubble (kg/ha) remaining on the soil surface in paddocks was estimated 6, 18, 30 and 42 months after harvest of the original canola crop. In all 3 environments only small amounts of stubble were present 18 months after harvest. Eighty percent of the 6-month-old stubble comprised stems and branches, with the remaining 20% being root material, while 42-month-old stubble consisted only of root material. Paddocks subjected to raking and burning contained only half the weight of stubble compared with paddocks that were harrowed. Where canola was harvested in January, even when no management strategy was used, 80% of subsequent stubble was no longer on the soil surface by July of that year. Pseudothecia from 6-month-old stubble from the high rainfall environment discharged significantly more ascospores than stubble of the same age from the medium rainfall environment, which in turn discharged more than stubble from the low rainfall environment. In all environments, paddocks containing 6-month-old canola stubble discharged 30-fold as many ascospores per hectare as older stubble paddocks.

Maturation of pseudothecia and discharge of ascospores of Leptosphaeria maculans on oilseed rape stubble

“…Further research, in which the duration of wetness after each spray is estimated, would provide an indication of whether the total duration of wetness or the distribution of wetness periods influenced the rate of pseudothecium maturation. Nevertheless, the findings in the current study that interrupted wetness reduced the production of pseudothecia on stubble agree with earlier findings that the quantity of rainfall affected the quantity of ascospores released from stubble in Australia (Marcroft et al 2003). Marcroft et al (2003) reported that large numbers of ascospores were released from 6 month-old stubble collected from high rainfall areas whereas stubble from medium to low rainfall areas continued to release fewer ascospores for longer periods.…”

“…Nevertheless, the findings in the current study that interrupted wetness reduced the production of pseudothecia on stubble agree with earlier findings that the quantity of rainfall affected the quantity of ascospores released from stubble in Australia (Marcroft et al 2003). Marcroft et al (2003) reported that large numbers of ascospores were released from 6 month-old stubble collected from high rainfall areas whereas stubble from medium to low rainfall areas continued to release fewer ascospores for longer periods. Similarly, in Saskatchewan, Canada, sporulation occurred earlier in the field as the frequency of moistening infested stubble between April and June increased (Petrie 1994).…”

“…The time taken for pseudothecia of L. maculans to mature on naturally infested stubble decreased nearly linearly when temperature increased from 5 to 20°C in continuous wetness in a controlled environment but was longer in natural conditions, particularly in dry weather in the UK (Toscano-Underwood et al 2003). In south-eastern Australia (Marcroft et al 2003) and Western Australia (Salam et al 2003), more ascospores were discharged earlier in regions with higher rainfall than those with less.…”

Major Gene and Polygenic Resistance to Leptosphaeria maculans in Oilseed Rape (Brassica napus)

“…L. maculans can survive and reproduce on even the most resistant lines (Marcroft et al, 2004). As quantitative resistance is partial, when L. maculans inoculum concentrations are high, it may not prevent large yield losses (Salisbury et al, 1995;Khangura and Barbetti, 2001;Marcroft et al, 2003).…”

Population diversity and genomics of the fungal pathogen of canola Leptosphaeria maculans

“…Current methods to conduct this include the use of fungicides (Khangura & Barbetti 2004), burning of infected stubble and rotating cultivars containing different sources of resistance (Salisbury et al 1995). Marcroft et al (2003) recommend that new canola cultivars should be sown at a distance of at least 500 m from old stubble. The best method to prevent crop losses, however, has been the sowing of disease-resistant cultivars, which is based on the interaction of effector and resistance proteins of the pathogen and plant (See section 1.3.1).…”