Neal Evans

Frequency of avirulence alleles in field populations of Leptosphaeria maculans in Europe

This paper describes the first large-scale Europe-wide survey of avirulence alleles and races of Leptosphaeria maculans. Isolates were collected from the spring rape cultivar Drakkar, with no known genes for resistance against L. maculans, at six experimental sites across the main oilseed rape growing regions of Europe, including the UK, Germany, Sweden and Poland. Additionally in Poland isolates were collected from cv. Darmor, which has resistance gene, Rlm9. In total, 603 isolates were collected during autumn in 2002 (287 isolates from Germany and the UK) and 2003 (316 isolates from Poland and Sweden). The identity of alleles at eight avirulence loci was determined for these isolates. No isolates had the virulence allele avrLm6 and three virulence alleles (avrLm2, avrLm3 and avrLm9) were present in all isolates. The isolates were polymorphic for AvrLm1, AvrLm4, AvrLm5 and AvrLm7 alleles, with virulence alleles at AvrLm1 and AvrLm4 loci and avirulence alleles at AvrLm7 and AvrLm5 loci predominant in populations. Virulent avrLm7 isolates were found at only one site in Sweden. Approximately 90% of all isolates belonged to one of two races (combinations of avirulence alleles), Av5-6-7 (77% of isolates) or Av6-7 (12%). Eight races were identified, with four races at frequencies less than 1%. The study suggested that Rlm6 and Rlm7 are still effective sources of resistance against L. maculans in oilseed rape in Europe. The results are comparable to those of a similar survey done in France in autumn 2000 and 2001.

Fitness cost of virulence differs between the AvrLm1 and AvrLm4 loci in Leptosphaeria maculans (phoma stem canker of oilseed rape)

To investigate whether the reported fitness cost of virulence at the AvrLm4 locus in Leptosphaeria maculans is common to other loci, near-isogenic (NI) isolates differing at AvrLm1 locus were produced in vitro. Fitness of virulent (avrLm1) or avirulent (AvrLm1) isolates on Brassica napus without the corresponding R (resistance) gene Rlm1 was investigated in controlled environment (CE) and field experiments. Results indicate that there is a measurable fitness cost for avrLm1 compared to AvrLm1 isolates in terms of number of lesions, size of lesions, distance grown through leaf tissue towards the petiole in CE experiments and systemic growth from leaf lesions to stems in field experiments. There were differences in fitness cost between the AvrLm1 and AvrLm4 loci. There was a cultivar effect on fitness cost of virulence at the AvrLm1 locus but not at the AvrLm4 locus. In CE experiments, the optimal temperature for leaf infection was greater for AvrLm4 isolates than for AvrLm1 isolates. Field experiment results suggest that on the same host AvrLm4 isolates are more fit than AvrLm1 isolates in warmer seasons. The fitness cost at the AvrLm4 locus was generally greater than at the AvrLm1 locus, suggesting that the corresponding R gene Rlm4 may be more suitable than Rlm1 for redeployment in commercial cultivars after an interval of a few years.

Adaptation to increasing severity of phoma stem canker on winter oilseed rape in the UK under climate change

Various adaptation strategies are available that will minimize or negate predicted climate change-related increases in yield loss from phoma stem canker in UK winter oilseed rape (OSR) production. A number of forecasts for OSR yield, national production and subsequent economic values are presented, providing estimates of impacts on both yield and value for different levels of adaptation. Under future climate change scenarios, there will be increasing pressure to maintain yields at current levels. Losses can be minimized in the short term (up to the 2020s) with a ‘low’-adaptation strategy, which essentially requires some farmer-led changes towards best management practices. However, the predicted impacts of climate change can be negated and, in most cases, improved upon, with ‘high’-adaptation strategies. This requires increased funding from both the public and private sectors and more directed efforts at adaptation from the producer. Most literature on adaptation to climate change has had a conceptual focus with little quantification of impacts. It is argued that quantifying the impacts of adaptation is essential to provide clearer information to guide policy and industry approaches to future climate change risk.

Dissemination of information about management strategies and changes in farming practices for the exploitation of resistance to Leptosphaeria maculans (phoma stem canker) in oilseed rape cultivars

The management of phoma stem canker (blackleg disease, caused by Leptosphaeria maculans) is an integral component of oilseed rape production. In this paper, we discuss the information about management strategies that is disseminated in Europe and Australia. New cultivars have been introduced with improved resistance to disease, but sometimes this resistance has been overcome as new races of the pathogen have emerged. When cultivars with single major gene resistance have been introduced into areas with high inoculum concentrations, significant economic damage has been caused by new races of L. maculans within 2–3 years. Quantitative or polygenic resistance has also been used successfully against stem canker and offers more durable disease resistance if plant breeders and farmers deploy this resistance more effectively. Strategies to improve the durability of resistance need to be developed and tested in practice. New information on the occurrence of virulence and avirulence genes in populations of Leptosphaeria maculans and modelling of the durability of resistance provide opportunities for plant breeders, specialist technical organisations, cooperatives, advisory services and farmers to collaborate and better exploit cultivar resistance. Changing economic and environmental factors influence cropping practices and, if to be considered successful, management strategies must show clear financial benefits. Technology transfer will need to address all aspects of managing stem canker and other diseases of oilseed rape and using effective written, verbal and electronic methods of communication.

Resistance to infection by stealth: Brassica napus (winter oilseed rape) and Pyrenopeziza brassicae (light leaf spot)

Light leaf spot (Pyrenopeziza brassicae) is an important disease on winter oilseed rape crops (Brassica napus) in northern Europe. In regions where economically damaging epidemics occur, resistance to P. brassicae in commercial cultivars is generally insufficient to control the disease without the use of fungicides. Two major genes for resistance have been identified in seedling experiments, which may operate by decreasing colonisation of B. napus leaf tissues and P. brassicae sporulation. Much of the resistance present in current commercial cultivars is thought to be minor gene-mediated and, in crops, disease escape and tolerance also operate. The subtle strategy of the pathogen means that early colonisation of host tissues is asymptomatic, so a range of techniques and molecular tools is required to investigate mechanisms of resistance. Whilst resistance of new cultivars needs to be assessed in field experiments where they are exposed to populations of P. brassicae under natural conditions, such experiments provide little insight into components of resistance. Genetic components are best assessed in controlled environment experiments with single spore (genetically fixed) P. brassicae isolates. Data for cultivars used in the UK Recommended List trials over several seasons demonstrate how the efficacy of cultivar resistance can be reduced when they are deployed on a widespread scale. There is a need to improve understanding of the components of resistance to P. brassicae to guide the development of breeding and deployment strategies for sustainable management of resistance to P. brassicae in Europe.

Geographic variation in severity of phoma stem canker and Leptosphaeria maculans/ L. biglobosa populations on UK winter oilseed rape ( Brassica napus )

Phoma stem canker, caused by Leptosphaeria maculans and L. biglobosa, is the most important disease of oilseed rape in Europe. Differences between L. maculans and L. biglobosa in their life-cycles enable the two species to co-exist on oilseed rape crops over a cropping season. This review considers the factors affecting geographic variation in the severity of phoma stem canker epidemics and in the structure of the population of the pathogens in the UK, where the most severe epidemics occur in the south of England and cankers do not develop in Scotland. It is concluded that this variation is directly related to differences in climate, since weather-based models show that stem canker severity increases with increasing winter/spring temperature and temperatures are greater in the south of the UK. It may also be related to differences in pathogen populations, since surveys showed that the proportion of the more damaging L. maculans in stem cankers was greatest in southern England, with most L. biglobosa in northern England. Regional variation in agronomic practices such as cultivar choice and fungicide use may also indirectly influence phoma stem canker severity. Differences in cultivar choice result in differences in L. maculans race structure, which may influence the severity of epidemics. Differences in fungicide use may also influence pathogen populations, since L. maculans and L. biglobosa differ in their sensitivities to different azole fungicides. These factors are discussed in relation to strategies for sustainable production of oilseed rape by adaptation to threats posed by climate change.

Sustainable strategies for managing Brassica napus (oilseed rape) resistance to Leptosphaeria maculans (phoma stem canker)

Sustainable strategies for managing Brassica napus (oilseed rape) resistance to Leptosphaeria maculans (phoma stem canker) , Sustainable strategies for managing Brassica napus (oilseed rape) resistance to Leptosphaeria macula... , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Erratum: The impact of climate change on disease constraints on production of oilseed rape (vol 2, pg 143, 2010)

The authors would like to correct a series of errors printed in the above paper. Due to a mistake in a table of official figures with respect to land use in the UK, the figure used for the acreage of oilseed rape produced for Scotland should have been ‘35 780 ha’ and not ‘398 720 ha’ (Table 4). Thus data quoted in the “Results” section of this paper that include this erroneous data (i.e. any “Scottish data” or “total UK” data) are also erroneous by an order of magnitude. Data in the last three lines of the following tables should now read as follows. Overall, this does not affect the main conclusions of the paper. However, the “Abstract” should now state that the value of the crop (if stem canker and light leaf spot were effectively controlled) was predicted to increase by £2.5 M in Scotland (rather than £28 M) by 2050 under a high emissions scenario and that under the same scenario, UK disease losses were predicted to increase by £30 M (rather than £50 M).