Brassica Populations Research Articles

Toxicity and efficacy of azadirachtin, and oxymatrine-based biopesticides against cabbage aphid and their impacts on predator insects

The aphidicidal activity and efficacy of azadirachtin- and oxymatrine-based biopesticides on the cabbage aphid Brevicoryne brassicae (Linnaeus, 1758) (Hemiptera: Aphididae), and their impacts on the main insect predators of this aphid were assessed under laboratory, semi-field, or field conditions. Laboratory results showed that azadirachtin- and oxymatrine-based biopesticides provided high acute toxicity against B. brassicae nymphs and adult females, in a manner dependent on the concentration, developmental stage, and exposure time of insects to residues. Among the insect's developmental stages, B. brassicae nymphs were more susceptible to biopesticides than adult females. In addition to acute toxicity, the biopesticides reduced the number of offspring and the amount of honeydew excreted by B. brassicae adult females. Furthermore, the biopesticides induced a repellent effect on B. brassicae adult females. Despite their high toxicity, in semi-field conditions, the azadirachtin- and oxymatrine-based biopesticides showed low biological persistence [mortality >80% up to 3 and 1 day after spraying (DAS), respectively] in comparison to the synthetic insecticide flupyradifurone (mortality >80% up to 7 DAS), which was used as a positive control. However, in the field, the biopesticides demonstrated high efficacy in controlling the B. brassicae population, with no significant impact on the population levels of different species of lacewings, ladybugs, and syrphids, which are considered the main insect predators of the cabbage aphid in agroecosystems. Therefore, azadirachtin- and oxymatrine-based biopesticides constitute an important management measure for B. brassicae in agroecosystems, including those with an ecological basis where management tools are limited, and effective control tactics must be incorporated.

Combined cytogenetic and molecular methods for taxonomic verification and description of Brassica populations deriving from different origins

Agriculture faces great challenges to overcome global warming and improve system sustainability, requiring access to novel genetic diversity. So far, wild populations and local landraces remain poorly explored. This is notably the case for the two diploid species, Brassica oleracea L. (CC, 2n=2x=18) and B. rapa L. (AA, 2n=2x=20). In order to explore the genetic diversity in both species, we have collected populations in their centre of origin, the Mediterranean basin, on a large contrasting climatic and soil gradient from northern Europe to southern sub-Saharan regions. In these areas, we also collected 14 populations belonging to five B. oleracea closely related species. Our objective was to ensure the absence of species misidentification at the seedling stage among the populations collected and to describe thereafter their origins. We combined flow cytometry, sequencing of a species-specific chloroplast genomic region, as well as cytogenetic analyses in case of unexpected results for taxonomic verification. Out of the 112 B. oleracea and 154 B. rapa populations collected, 103 and 146, respectively, presented a good germination rate and eighteen populations were misidentified. The most frequent mistake was the confusion of these diploid species with B. napus. Additionally for B. rapa, two autotetraploid populations were observed. Habitats of the collected and confirmed wild populations and landraces are described in this study. The unique plant material described here will serve to investigate the genomic regions involved in adaptation to climate and microbiota within the framework of the H2020 Prima project ‘BrasExplor’.

Pathotype Characterization of Plasmodiophora brassicae by European Clubroot Differential and Williams Sets in China.

Clubroot disease caused by the soil-borne Plasmodiophora brassicae is devastating to Brassicaceae crops and spreading rapidly in China in recent years, resulting in great yield losses annually. Virulence of P. brassicae populations specializes and is in dynamic change in the fields. Information on the pathotypes and their distributions is crucial to control the clubroot disease. Presently, the pathotypes of P. brassicae prevalent in China, however, are not well determined. In this study, we used 16 Brassica hosts, including the European Clubroot Differential (ECD) and Williams sets, to designate the pathotypes of 33 P. brassicae populations from 13 provinces. The 33 P. brassicae populations could be divided into 26 pathotypes by the ECD set or seven pathotypes by the Williams set, revealing ECD16/15/31 and ECD16/31/31 or P4 and P2 as the predominant pathotypes. We found that the Brassica rapa differentials ECD01 to ECD04 showed stable and high levels of resistance to most pathotypes of P. brassicae in China, thereby providing valuable resources for clubroot-resistance breeding of Brassicaceae crops. The ECD set exhibited much higher discernibility and further divided the isolates that belonged to the P4 pathotype into 10 ECD pathotypes. Isolates of ECD16/23/31 and ECD16/15/31 were strongly virulent on Huashuang 5R, the first and widely used clubroot-resistant cultivar of oilseed rape in China. As we learn, 26 pathotypes are the most diverse populations of P. brassicae characterized until now in China. Our study provides new insights into virulence specialization of P. brassicae and their geographical distributions, contributing to exploitation of clubroot-resistant resources and the field layout of the present resistant Brassica crops in China.

Effect of Mulching on Soil Temperatures and Its Impact on Plasmodiophora brassicae and Clubroot.

Clubroot caused by Plasmodiophora brassicae is a serious soilborne disease on cruciferous crops worldwide. Agricultural practice is a preferable clubroot management strategy because of its low investment requirement and environmental safety. Among the agricultural practices, solarization has been widely applied in the integrated management of other soilborne diseases. However, only few reports exist on the effect of solarization on clubroot management. In this study, we measured the effect of plastic mulching on soil temperature at different depths and on clubroot incidence and severity under greenhouse and field conditions. The pathogen density in the soil after solarization was measured by quantitative PCR analysis. Results indicated that the mulching treatment increased soil temperature especially in the soil layer ranges of 0 to 20 cm. Solarization with mulching also effectively reduced the incidence and severity of clubroot in the greenhouse assay and the field trial by decreasing the P. brassicae population in the soil. This study suggested that solarization with mulching can impair clubroot development and thus contribute to the sustainable management of clubroot.

Population modeling of oilseed brassica aphids and their biocontrol agents with weather parameters in the north-west Himalayan region of India.

We studied the impact of weather parameters on the population build-up of Brevicoryne brassicae (L.) (Cabbage aphid), Lipaphis erysimi (Kalt.) (Mustard aphid), Myzus persicae (Sulzer) (Green peach aphid) and their biocontrol agents (coccinellids, syrphids, and a parasitoid, Diaeretiella rapae M'Intosh) on oilseed brassicas in Himachal Pradesh, India, during winters from 2016-2017 to 2018-2019. The temperature and sunshine resulted in the build-up of B. brassicae and their biocontrol agents' population, while rainfall and relative humidity caused a negative influence at surveyed locations. The L. erysimi and M. persicae populations showed an inverse correlation with the density-independent factors at most locations. Correlation coefficients indicated a negative correlation of the coccinellids population with the build-up of L. erysimi and M. persicae, while the predator population was positively related to the B. brassicae population at maximum locations. Parasitization by D. rapae showed a negative relationship with the aphid population. Stepwise regression analysis showed that minimum temperature and rainfall had a significant effect on the variability in the population of aphids. The predictive model could interpret more than 90% variation by minimum temperature in the coccinellid population at the surveyed locations. Further, regression analysis with temperature could explain up to 94% variability in parasitization by D. rapae. This study will contribute to predicting the changes that may occur in a population of aphids concerning the weather.

In situ occurrence and protection of crop wild relatives in Italian sites of natura 2000 network: Insights from a data-driven approach.

Aim of this work is to evaluate the in situ status of different crop wild relative species in Italy by analysing the geographic distribution of their populations and to suggests possible strategies to improve their future conservation. The work has been focused on different species of the Allium, Beta, Brassica, Secale and Triticum genera that are of priority at European and global levels for the economic importance of the related crops, the level of threat, as well as the potential for use. Using information available in the Italian National Geoportal, geographical distribution and the overall percentage of populations occurring in Natura 2000 sites was initially analysed. In addition, due to the economic importance of the genus and species distribution in Italy, Brassica glabrescens, B. insularis, B. macrocarpa, B. montana, B. procumbens, B. rupestris, B. villosa were the object of additional analyses based on more detailed occurrence data, retrieved from multiple databases, and including land cover/land use and in situ and ex situ density analyses. Geographical distribution data were retrieved for 1,996 in situ populations belonging to 60 crop wild relative species: Allium (43), Brassica (11), Triticum (4), Beta (1) and Secale (1). Percentages of population occurring in Natura 2000 sites are quite different when the different species are considered; this also applies to Brassica species in most need of protection. Results of land cover/land use analysis showed that Brassica populations outside Natura 2000 areas mainly occur in anthropized sites while those within Natura 2000 mainly in sites characterised by natural and seminatural conditions. Areas where genetic reserves could be instituted and that could be the target of future Brassica resources collection missions are also suggested. Based on a large dataset of punctual geographical distribution data of population occurrences across the territory, this research shows that, in Italy, crop wild relatives in situ are in a quite precarious condition especially when species in most need of protection are considered. Our data also highlight the role of Natura 2000 Network in favouring in situ protection of these precious resources in Europe.

Azole fungicide sensitivity and molecular mechanisms of reduced sensitivity in Irish Pyrenopeziza brassicae populations.

Light leaf spot, caused by Pyrenopeziza brassicae, is amongst the most damaging diseases of winter oilseed rape (Brassica napus), and currently the sterol 14α-demethylase (CYP51) inhibitors (azoles) represent the main class of fungicides used to control light leaf spot development. However, a shift in sensitivity to azole fungicides in P. brassicae populations has been observed in different European countries, including Ireland. To assess the sensitivity status of Irish P. brassicae populations to azole fungicides, three collections of P. brassicae from 2018-2020 were tested in vitro against tebuconazole and prothioconazole-desthio, and the PbCYP51 gene targeted by this class of fungicides was genotyped in different isolates. A change in sensitivity to azole fungicides was observed and differences in sensitivity to tebuconazole between Irish populations were present. There were two substitutions within PbCYP51 (G460S and S508T) and inserts of different sizes in its promoter region. The presence of the G460S/S508T double mutant was reported for the first time, and the diversity in insert size was greater than previously known. Compared to wild type isolates, those carrying G460S or S508T were less sensitive to both fungicides and, where inserts were also identified, they further reduced sensitivity to azole fungicides. The results of this study suggest that azole fungicides are still very effective in controlling light leaf spot in Ireland. However, using azole fungicides in mixtures of fungicides with different modes of action is recommended. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Intraspecific Variability and Distribution Difference within the Ribosomal Introns of the Discrete Plasmodiophora brassicae Group in Japan: A Case Study for Complex Dynamics of Intron Evolution

Analysis of the ribosomal introns of Plasmodiophora brassicae populations infecting the cruciferous weed Cardamine occulta revealed the complex dynamics of size, intraspecific variability, and distribution. The results showed that P. brassicae populations from the weed have lost multiple introns in the small and large subunits of the ribosomal RNA genes. Moreover, the retained introns, despite a largely mutual share of conserved parts with the cosmopolitan strains, contained numerous novel structures. These structural differences comprise a high level of polymorphisms, such as transversion point mutations occurring at sites involving the intronic splicing sites or insertions/deletions at the binding sites. Two geographical P. brassicae populations from C. occulta carried a lengthy intron-encoded ORF and putative mobile elements established in the large subunit. A few P. brassicae populations from the Brassica crops also harbored polymorphic introns that shared common mutated motifs with the weed-affecting group. The diversity of ribosomal introns observed from those investigated populations demonstrated the genetic distinction of the P. brassicae populations from C. occulta. The genetic variations might play a key role in the adaptability of the weed-infecting populations and are more likely related to the process of pathogenesis for the cosmopolitan P. brassicae infecting the Brassica crops.

Genotypic and Phenotypic Assessments of Cruciferous Clubroot on the Paddy-Field Weed Cardamine Occulta, Revealing a Distinct Plasmodiophora Brassicae Population in Japan

Genotypic and Phenotypic Assessments of Cruciferous Clubroot on the Paddy-Field Weed Cardamine Occulta, Revealing a Distinct Plasmodiophora Brassicae Population in Japan

Spatiotemporal Quantification of Plasmodiophora brassicae Inoculum in Relation to Clubroot Development Under Inoculated and Naturally Infested Field Conditions.

Clubroot caused by Plasmodiophora brassicae is a destructive disease of cruciferous plants worldwide. A quantitative PCR (qPCR) system specific to P. brassicae was developed. Analysis of the qPCR sensitivity indicated that the lower limit of detection was 1 × 101 resting spores/ml, 1 × 102 spores/g of soil, and 1 × 103 spores/g of roots and seeds. The regression curves generated from the qPCR data of different samples had a parallel relationship. The difference between the theoretical and actual concentrations was lowest at 1 × 105 spores/g of sample, compared with other concentrations. The P. brassicae biomass in soil and plant root tissues after inoculated with different spore concentrations was correlated. A correlation analysis confirmed that the clubroot incidence and disease index at 6 weeks after inoculation increased as the spore concentration increased. Under field conditions, the natural inoculum density of the P. brassicae population decreased at the early stage and then increased, with P. brassicae mainly being detected at a soil depth of 0 to 50 cm. The horizontal distribution of P. brassicae varied in the field with occurrences of hot spots. This study established a qPCR-based method for quantitative detection of clubroot. The developed assay is useful for monitoring the spatiotemporal dynamics of P. brassicae in the field. It may also be applicable for clubroot forecasting as a part of proactive disease management.

Eradicating the large white butterfly from New Zealand eliminates a threat to endemic Brassicaceae.

In May 2010 the large white butterfly, Pieris brassicae L. (Lepidoptera: Pieridae), was discovered to have established in New Zealand. It is a Palearctic species that-due to its wide host plant range within the Brassicaceae-was regarded as a risk to New Zealand's native brassicas. New Zealand has 83 native species of Brassicaceae including 81 that are endemic, and many are threatened by both habitat loss and herbivory by other organisms. Initially a program was implemented to slow its spread, then an eradication attempt commenced in November 2012. The P. brassicae population was distributed over an area of approximately 100 km2 primarily in urban residential gardens. The eradication attempt involved promoting public engagement and reports of sightings, including offering a bounty for a two week period, systematically searching gardens for P. brassicae and its host plants, removing host plants, ground-based spraying of insecticide to kill eggs and larvae, searching for pupae, capturing adults with nets, and augmenting natural enemy populations. The attempt was supported by research that helped to progressively refine the eradication strategy and evaluate its performance. The last New Zealand detection of P. brassicae occurred on 16 December 2014, the eradication program ceased on 4 June 2016 and P. brassicae was officially declared eradicated from New Zealand on 22 November 2016, 6.5 years after it was first detected and 4 years after the eradication attempt commenced. This is the first species of butterfly ever to have been eradicated worldwide.

Responsibility of Certain Biotic Factors for Management Populations of Brevicoryne brassicae L in Cabbage fields

This work was carried out at the experimental farm of the Faculty of Agriculture, Assiut University during the 2017-2018 and 2018-2019 cabbage growing seasons. Evaluation of the seasonal abundance of the cabbage aphid Brevicoryne brassicae L in cabbage fields and responsibility of certain biotic factorsfor managing populations of this insect pest was the cornerstone of this investigation. Cabbage aphid, B. brassicae was found to be active on cabbage plantations and recorded in low numbers (99.71 individuals / 7 Cm2 / leaf / infested plant) at the beginnings of the season (November) with gradual increase until harvesting. Three amongst the six evaluated cabbage cultivars and/or hybrids were appeared as susceptible (S) and harbored so high numbers of the pest. However, the remaining cultivars showed some sort of resistance to this insect pest. The local cultivar (Ganzory) harbored the least aphid numbers and appeared as moderately resistant (MR) cultivar. This finding could be attributed to the antixenosis and/or antibiosis phenomena presented by this local cultivar. Proteins and amino acid contents showed a highly significant positive correlation (r) with B. brassicae populations, however, chlorophyll showed highly significant negative (r). The obtained data reflect the role of the cabbage aphid parasitoid Diaeretiella rapae (McIntosh) to manage B. brassicae populations. Mummified aphids (parasitism rate) before harvesting were found to be equal 6.16-fold of that recorded before ripening stages. High percentages of D. rapae were succeeded to emerge from the mummified B. brassicae with diverse extrusion rates. Multiple increases were recorded on the percentages of infested plants when cabbage undergo toward ripening at the 9th stage till harvesting. No or less existence of unmarketable plants (through the periods of the low infestation)before cup formation and/or plant ripening period was recorded. So, it can be concluded that cabbage cultivar, plant age, and stage in addition to plant metabolites combined with the naturally occurring biological control agent (Aphid parasitoid) can be affected and manage the population trend of cabbage aphid B. brassicae infesting cabbage in the field.

Modeling Overdispersion, Autocorrelation, and Zero-Inflated Count Data Via Generalized Additive Models and Bayesian Statistics in an Aphid Population Study.

Count variables are often positively skewed and may include many zero observations, requiring specific statistical approaches. Interpreting abiotic factor changes in insect populations of crop pests, under this condition, can be difficult. The analysis becomes even more complicated because of possible temporal or spatial correlation, irregularly spaced data, heterogeneity over time, and zero inflation. Generalized additive models (GAM) are important tools to evaluate abiotic factors. Moreover, Markov chain Monte Carlo (MCMC) techniques can be used to fit a model that contains a temporal correlation structure, based on Bayesian statistics (BGAM). We compared methods of modeling the effects of temperature, precipitation, and time for the Brevicoryne brassicae (L.) population in Uberlândia, Brasil. We applied the proposed BGAM to the data, comparing this to the GAM model with and without autocorrelation for time, using the statistical programming language R. Analysis of deviance identified significant effects of the smoothers for precipitation and time on the frequentist models. With BGAM, the problem in variance estimations for precipitation and temperature from the previous models was solved. Furthermore, trace and density plots for population-level effects for all parameters converged well. The estimated smoothing curves showed a linear effect with an increase of precipitation, where lower precipitation indicated no presence of the aphid. The average temperature did not affect the aphid incidence. Autocorrelation was solved with ARMA structures, and the excess of zero was solved with zero-inflation models. The example of B. brassicae incidence showed how well abiotic (and biotic) factors can be modeled and analyzed using BGAM.

Most Plasmodiophora brassicae Populations in Single Canola Root Galls from Alberta Fields are Mixtures of Multiple Strains.

Clubroot, caused by Plasmodiophora brassicae, is an important disease on canola in Alberta, Canada. The pathogen is grouped into pathotypes according to their virulence reaction on differential hosts. Multiple pathotypes or strains are known exist in one field, one plant, or even one gall. This study was conducted with the objective of testing the prevalence of the coexistence of multiple strains in a single gall. In all, 79 canola clubroot galls were collected from 22 fields across Northern Alberta in 2018. Genomic DNA extracted from these single galls was analyzed using RNase H-dependent PCR (rhPCR). The rhPCR primers were designed to amplify a partial sequence of a dimorphic gene, with one primer pair specific to one sequence and the other primer pair specific to the alternative sequence. The amplification of both sequences from DNA obtained from a single gall would indicate that it contains two different P. brassicae strains. The rhPCR analyses indicated that the P. brassicae populations in 50 of the 79 galls consisted of more than one strain. This result emphasizes the need for cautious interpretation of results when a single-gall population is subject to pathotyping or being used as inoculum in plant pathology research. It also confirms that the maintenance of pathotype diversity within single root galls is a common occurrence which has implications for the durability, and stewardship, of single-gene host resistance.

Determination of the Genetic Variation within the Egg Parasitoid, Trichogramma brassicae, Bezdenko (Hymenoptera: Trichogrammatidae) Populations in Düzce Province, Turkey

Members of Trichogramma genus are the most widely used as natural enemies in biological control programs. Choice of natural enemies is known to have a critical important for the succession of biological control. Their morphologic identification has some bottlenecks as laborious, time-consuming and mostly based on very small structures of the bodies. Molecular techniques have been widely used for detection, discrimination, phylogenetic analysis, identification and characterization in biological control agents belong to Trichogramma spp. In this study, the efficacy of the direct sequencing analysis of internal transcribed spacer locus of rDNA was investigated for analyzing the genetic variation within nine Trichogramma brassicae population from corn growing areas in Düzce province. The comparison of the sequences was performed with other ITS2 locus of Trichogramma species available in GenBank database. The result of analyses indicated that no genetic variation was determined in the Düzce population.

Segregation for fertility and meiotic stability in novel Brassica allohexaploids.

Allohexaploid Brassica populations reveal ongoing segregation for fertility, while genotype influences fertility and meiotic stability. Creation of a new Brassica allohexaploid species is of interest for the development of a crop type with increased heterosis and adaptability. At present, no naturally occurring, meiotically stable Brassica allohexaploid exists, with little data available on chromosome behaviour and meiotic control in allohexaploid germplasm. In this study, 100 plants from the cross B. carinata × B. rapa (A2 allohexaploid population) and 69 plants from the cross (B. napus × B. carinata) × B. juncea (H2 allohexaploid population) were assessed for fertility and meiotic behaviour. Estimated pollen viability, self-pollinated seed set, number of seeds on the main shoot, number of pods on the main shoot, seeds per ten pods and plant height were measured for both the A2 and H2 populations and for a set of reference control cultivars. The H2 population had high segregation for pollen viability and meiotic stability, while the A2 population was characterised by low pollen fertility and a high level of chromosome loss. Both populations were taller, but had lower average fertility trait values than the control cultivar samples. The study also characterises fertility and meiotic chromosome behaviour in genotypes and progeny sets in heterozygous allotetraploid Brassica derived lines, and indicates that genotypes of the parents and H1 hybrids are affecting chromosome pairing and fertility phenotypes in the H2 population. The identification and characterisation of factors influencing stability in novel allohexaploid Brassica populations will assist in the development of this as a new crop species for food and agricultural benefit.

Variation in pathotypes and virulence of Plasmodiophora brassicae populations in Germany

Between 2012 and 2015, 49 new clubroot‐infested fields were identified in 12 German federal states. Clubroot disease incidence varied within these fields from 22% to 92%. Field information revealed that in 85% of fields, oilseed rape was grown in rotation once every 2 or 3 years. Frequency of OSR in the rotation was significantly correlated with the incidence and prevalence of clubroot disease. The disease was detected in fields with soil pH ranging from 5.1 to 8.3, and a significant negative correlation was found between soil pH and the disease incidence of infested fields. Furthermore, more cases of disease and severe incidences were observed in sandy loam and loamy sand as compared with other soil types. Pathotype classification of the 49 Plasmodiophora brassicae populations was conducted on two differential sets, the European Clubroot Differential set and the set of Somé (1996). Additionally, the degree of virulence of the collected isolates was analysed on the clubroot‐resistant oilseed rape cv. Mendel. The results showed variation in pathotype distribution in different regions in Germany. The majority of isolates according to Somé were pathotypes 1 and 3, respectively, with pathotypes 2 and 5 in the minority. Detailed classification according to Buczacki showed the dominance of 16/31/31, 16/14/30 and 16/14/31 populations among 20 distinct virulence patterns of collected isolates. From all populations tested for virulence on cv. Mendel, 15 isolates were found to be moderately or highly virulent. These virulent populations were not restricted to a small geographical area in the country.

Occurrence and morphology of small spheroid galls on clubroot-resistant Chinese cabbage

Clubroot, caused by Plasmodiophora brassicae, is a worldwide disease that can lead to significant yield losses in cruciferous crops. It causes root hypertrophy, hampering the uptake of nutrients and water. In the evaluation of resistance, small spheroid galls (SSGs), which are distinct from typical spindle galls, and are generally regarded as a host resistance reaction, were observed. In this study, the occurrence of SSGs was investigated when evaluating the host resistance of seven commercial clubroot-resistant (CR) Chinese cabbage (Brassica rapa) cultivars inoculated with 12 P. brassicae populations collected from different infested fields in Chuanyu region of China. Results showed that while all seven CR Chinese cabbage cultivars exhibited high resistance levels to the major populations tested, SSGs occurred in host-population combinations contained either Chinese cabbage cultivar ‘T1–145’ or population ‘FL’. This indicated that some CR cultivars or P. brassicae populations might be more likely to produce SSGs than others. Thus, recording the occurrence of SSGs separately in the resistance evaluation might be useful in mitigating the potential for selection of SSG producing hosts. External morphology observations revealed that both fresh root weight and gall diameter of spindle galls were significantly higher than SSGs, and histological observations indicated that delayed pathogen development in cortical infection might contribute to the restricted development of SSGs. Additionally, the number of SSGs formed per root was low (2.2 in average). This observation further confirmed that some host-population combinations cannot form large numbers of SSGs on single roots.

Emergence of new virulence phenotypes of Plasmodiophora brassicae on canola (Brassica napus) in Alberta, Canada

Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of canola (Brassica napus) in Alberta, Canada. Genetic resistance is the most effective clubroot management tool, and resistant cultivars are grown extensively in affected regions. In 2013, relatively severe symptoms of clubroot were observed in some fields of resistant canola. In greenhouse tests, four populations of P. brassicae from two of these fields caused significantly increased levels of clubroot on the cultivars from which they had been first recovered; these included three populations (L-G1, L-G2 and L-G3) recovered from the cultivar ‘L135C’, and one population (D-G3) recovered from ‘D3152’. Further testing showed that L-G1, L-G2 and L-G3 were highly virulent on a suite of six resistant canola cultivars (‘45H29’, ‘D3152’, ‘74–47CR’, ‘1960’, ‘L135C’ and ‘6056CR’) representing a cross-section of products available in Canada, while a seventh cultivar (‘9558c’) was moderately resistant to moderately susceptible. Bioassays of field soil with a dozen clubroot-resistant host genotypes confirmed that in most cases, resistance was no longer effective. Host responses to the population D-G3 were more variable, with most cultivars developing intermediate levels of disease. All four P. brassicae populations were classified as pathotypes 5, P3 and 16/6/8 on the differentials of Williams, Some et al., and the European Clubroot Differential set. The pathotype classifications, however, do not reflect the increased virulence of these populations on clubroot-resistant canola. The identification of new virulence phenotypes of P. brassicae capable of overcoming genetic resistance underscores the need for increased stewardship of resistance sources.

Landscape-level persistence and distribution of alien feral crops linked to seed transport

To assess the biotic and abiotic drivers of feral crop persistence, the occurrence and size of alien Brassica populations across an agricultural landscape in Canterbury, New Zealand, were surveyed over three years. Measures related to propagule input and site conditions were recorded and their role in explaining population occurrence and persistence assessed through GLMs and proportional-hazard models.Many Brassica populations were transient, with about 60% of populations disappearing within two years. New populations were founded at a rate that compensated for those that disappeared, and were more likely to occur along transportation routes and near seed companies, suggesting they established from seed spillage. Larger populations and those growing where habitat conditions were similar to those in which Brassica are cultivated had higher probabilities of survival.Without anthropogenic seed input to found new populations, Brassica spp. are unlikely to persist in this landscape beyond ten years. To avoid overestimating the extent of naturalised populations over time it is important to account for local population extinctions. The abundance of feral crops that occur as casuals in the landscape, along with other aliens that are maintained by external seed inputs, could be controlled by managing propagule sources. In themselves, casual populations are unlikely to facilitate gene flow or act as sources of further population spread.