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its life cycle. An ‘ascospore shower’ method was developed to study the fitness of near isogenic isolates of L. maculans. Fitness of avirulent isolates with avirulent gene AvrLm1 (corresponding to the major resistance gene Rlm1) was compared with that of virulent isolates with virulent allele avrLm1 under controlled conditions (5, 10, 15, 20 and 25°C). Plants of Eurol and Darmor without the corresponding resistance gene were inoculated with ascospores of AvrLm1 or avrLm1 produced in vitro using the ascospore ‘shower’ inoculation method. On both Eurol and Darmor, plants inoculated with AvrLm1 ascospores developed more lesions than plants inoculated with avrLm1 ascospores. The diameter of lesions caused by AvrLm1 was bigger than that of lesions caused by avrLm1. The results indicate that avrLm1 may have a fitness deficit due to loss of positive genes linked to the avirulence gene.

Major gene resistance in Australian canola overcome by Leptosphaeria maculans (blackleg, phoma) within 3 years

S.J. Sprague1,2 and S.J. Marcroft3

1CSIRO Plant Industry, Canberra, ACT 2601, Australia, 2School of Botany, Melbourne University, Vic. 3010, Australia, 3Department of Primary Industries, Horsham, Vic. 3401, Australia.

Severe blackleg epidemics, caused by the fungus Leptosphaeria maculans, in Australia in the early 1970s devastated the developing canola industry. The subsequent expansion of the industry has been a direct result of the release of canola cultivars with good polygenic blackleg resistance. This resistance has remained stable, however, there is anecdotal evidence to suggest that resistance erosion has occurred in older varieties.  In 2000, the first canola variety incorporating a major gene for blackleg resistance from Brassica rapa subspecies sylvestris was released onto the Australian market. Varieties with this ‘sylvestris’ resistance have been widely adopted by growers as the gene provided almost complete protection against L. maculans. Following a report of stem canker in these varieties, a survey of over 100 commercial canola crops was undertaken in south eastern Australia in 2003 to determine whether the resistance was breaking down. The isolates capable of overcoming the sylvestris resistance were widespread with infected plants identified in 95% of paddocks surveyed.  In three isolated regions (up to 50,000ha), crop yields were reduced by up to 90%. While the severity of blackleg differed between regions, the extensive area of canola stubble from infected crops in 2003 creates a high risk scenario across the entire area of south eastern Australia for crops with the sylvestris resistance grown in 2004.

Development of new fungicides: The art of finding a compromise which overcomes contradictory requirements

Dr. Karl Heinz Kuck, Bayer CropScience, Research, D 40789 Monheim, Germany

Beside toxicological aspects, some decades ago the level and the spectrum of fungicidal activity were the most important driving forces during fungicide discovery and development. Meanwhile, a multitude of other requirements have become of equal importance. Provided an interesting biological activity has been detected during the discovery process in a cluster of chemically related molecules, development candidates are, nowadays, subjected to a cascade of additional considerations and decisions. In the frame of the given variability it has to be decided early what kind of biological profile offers the best compromise between several possibilities. Examples for these contradictory requirements are: - Physico-chemical and/or metabolic stability of the fungicide. On one hand it is wanted to have a long-lasting activity in order to reduce the necessary number of applications. On the other hand the resulting residue level in plants and in the environment should ideally fall below the analytical detection level as soon as possible. - Systemic distribution of the active ingredient in the plant shows different optima in different crops. Whereas in lignified plants a high mobility of the fungicide is required, a high mobility is often leading to an unwanted acropetal accumulation in non-lignified plant parts such as leaves. - Pronounced curative and/or eradicative activities are an important part of the progress that modern fungicides offer to the farmer as it facilitates the timing of application considerably. On the other hand, a regular curative and eradicative use is increasing considerably the resistance risk. - The same types of inherent conflict is given regarding the optimal dose rate. High dosages, bearing some efficacy reserves are beneficial for resistance management and a solid control of the pathogen. For environmental and cost reasons, a low dosage is preferred. - The commercialisation of solo products offers benefits in view of costs per application and the flexibility to be used either alone or in alternation or tank mix schedules. Compared with this, the commercialisation of ready-mix products with a suited partner fungicide reduces greatly the potential for misuse in regard of a safe resistance management but normally cause higher costs.

Crop loss assessment for Sclerotinia stem rot in winter oilseed rape

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