The control of sclerotinia stem rot on oilseed rape (Brassica napus): current practices and future opportunities

Sclerotinia stem rot (SSR) caused by the phytopathogenic fungus Sclerotinia sclerotiorum is a major disease of oilseed rape (Brassica napus). During infection, large, white/grey lesions form on the stems of the host plant, perturbing seed development and decreasing yield. Due to its ability to produce long‐term storage structures called sclerotia, S. sclerotiorum inoculum can persist for long periods in the soil. Current SSR control relies heavily on cultural practices and fungicide treatments. Cultural control practices aim to reduce the number of sclerotia in the soil or create conditions that are unfavourable for disease development. These methods of control are under increased pressure in some regions, as rotations tighten and inoculum levels increase. Despite their ability to efficiently kill S. sclerotiorum, preventative fungicides remain an expensive gamble for SSR control, as their effectiveness is highly dependent on the ability to predict the establishment of microscopic infections in the crop. Failure to correctly time fungicide applications can result in a substantial cost to the grower. This review describes the scientific literature pertaining to current SSR control practices. Furthermore, it details recent advances in alternative SSR control methods including the generation of resistant varieties through genetic modification and traditional breeding, and biocontrol. The review concludes with a future directive for SSR control on oilseed rape.     

Efficacy of Vapam fumigant against clubroot (Plasmodiophora brassicae) of canola

Clubroot, caused by Plasmodiophora brassicae, has become a serious threat to canola (Brassica napus) production in western Canada. Experiments were conducted under greenhouse and field conditions to assess the effect of Vapam fumigant (dithiocarbamate; sodium N‐methyldithiocarbamate) on primary and secondary infection by P. brassicae, clubroot severity, and growth parameters in canola. Preliminary trials showed a 12–16‐fold reduction in primary and secondary infection and clubroot severity at all of the Vapam application rates (0·4–1·6 mL L^?1 soil) assessed. Vapam was also found to be effective in reducing clubroot severity and improving seed yield of canola under field conditions. Application of Vapam at soil moisture levels in the range of 10–30% (v:v) had a large effect on both disease severity and infection rates and plant growth parameters. The results suggest that Vapam can effectively reduce clubroot severity and may be useful for the treatment of transplant propagation beds in brassica vegetable production, and for the containment of small, localized clubroot infestations in commercial canola crops.     

Impact of cultivar on survival of volunteer oilseed rape populations in fields is more important than field management

A joint assessment of two separate approaches investigated the occurrence of volunteer oilseed rape (Brassica napus L.; OSR ) as affected by cultivar, field history and environment. Approach I comprised surveys for volunteers on >100 farmers’ fields in Germany in the years 2009 and 2010. Volunteers were assigned to the cultivars grown in previous years by inter‐simple sequence repeat‐PCR and cluster analysis. High‐dormancy cultivars resulted in 0–7 and low‐dormancy cultivars in 0–1.3 volunteers m^?2. Highest numbers originated from the most recent harvests 2007 and 2006. Approach II was a meta‐analysis based on 116 data sets from field trials and farmers’ fields in Germany to evaluate and to rank the impact of management factors on the soil seedbank and volunteers in following crops. Varietal disposition to seed dormancy turned out to be the significantly most relevant factor. The contribution of varietal dormancy to variation in the soil seedbank and of volunteers in the 1st and 2nd following crop was 1.2–2.3 times as great as the contribution of post‐harvest tillage. Up to 45% of the variation in the observations originated from factors that can be controlled by human actions on a given location in a given year. The overall analysis confirmed the results from independent short‐term trials and showed that both agronomists and breeders need to contribute to reducing OSR volunteers.     

Overexpression of barley oxalate oxidase gene induces partial leaf resistance to Sclerotinia sclerotiorum in transgenic oilseed rape

Sclerotinia stem rot (SSR) is a severe disease of oilseed rape, which severely impacts the crop productivity worldwide. Sclerotinia sclerotiorum causes SSR, resulting in the secretion of oxalic acid (OA), which can be further degraded to carbon dioxide (CO₂) and hydrogen peroxide (H₂O₂) by oxalate oxidase (OXO). In the present investigation, the barley oxalate oxidase (BOXO, Y14203) gene was introduced into oilseed rape by Agrobacterium‐mediated transformation to investigate the mechanism by which OXO promotes resistance to S. sclerotiorum. Compared to the control 72 h post‐inoculation, there were c. 15–61% fewer lesions on leaves of the transgenic oilseed rape, which thus exhibited a detectable level of partial resistance in leaf tissue to S. sclerotiorum. Transgenic oilseed rape also showed decreased oxalate and increased hydrogen peroxide levels compared to the control, and the expression of defence response genes involved in the hydrogen peroxide signalling pathway was also induced. Therefore, the improved resistance of oilseed rape could be attributed to the enhanced OA metabolism, production of hydrogen peroxide and the hydrogen peroxide‐mediated defence levels during infection.     

Effectiveness of Rlm7 resistance against Leptosphaeria maculans (phoma stem canker) in UK winter oilseed rape cultivars

The Rlm7 gene in Brassica napus is an important source of resistance for control of phoma stem canker on oilseed rape caused by the fungus Leptosphaeria maculans. This study shows the first report of L. maculans isolates virulent against Rlm7 in the UK. Leptosphaeria maculans isolates virulent against Rlm7 represented 3% of the pathogen population when cultivars with the Rlm7 gene represented 5% of the UK oilseed rape area in 2012/13. However, the Rlm7 gene has been widely used since then, representing >15% of the UK oilseed rape area in 2015/16. Winter oilseed rape field experiments included cultivars with the Rlm7 gene, with the Rlm4 gene or without Rlm genes and took place at five sites in the UK over four cropping seasons. An increase in phoma leaf spotting severity on Rlm7 cultivars in successive seasons was observed. Major resistance genes played a role in preventing severe phoma leaf spotting at the beginning of the cropping season and, in addition, quantitative resistance (QR) in the cultivars examined made an important contribution to control of phoma stem canker development at the end of the cropping season. Deployment of the Rlm7 resistance gene against L. maculans in cultivars with QR in combination with sustainable disease management practices will prolong the use of this gene for effective control of phoma stem canker epidemics.     

Weed flora shifts and specialisation in winter oilseed rape in France

Temporally repeated data sets can provide useful information about the management practices governing changes in the arable weed flora. This study aimed (i) to investigate changes in the most common weed species in winter oilseed rape crops in France between the 1970s and the 2000s and (ii) to pinpoint the main plant biological traits and associated management practices underlying the development of a specific weed flora in this crop. We compared two large‐scale surveys covering France in the 1970s and the 2000s, the later survey including several floristic samplings, on two dates, and both herbicide‐free control and treated plots. This last survey aimed to identify the species best able to maintain high densities over a growing season of oilseed rape. Since the 1970s, the frequency of two‐thirds (69%) of the 26 most common species has changed, spectacularly in some cases, with several species once considered rare becoming very common (e.g. Geranium dissectum) and, conversely, some formerly common species becoming rarer (e.g. Stellaria media). Our results indicated a general strong increase in specialist weeds of oilseed rape. Weed species success was favoured by tolerance to oilseed rape herbicides and germination synchronous with the crop. The proportion of specialist oilseed rape weed species tended to increase with herbicide treatment intensity and to decrease with increases in the proportion of spring‐sown crops in the rotation. Changes to the rotation may therefore constitute an additional or alternative means of controlling some weeds well adapted to oilseed rape crops.     

Effect of flooding on the survival of Leptosphaeria spp. in canola stubble

This study used a versatile temperature‐control device to assess the effect of temperature (12–40°C) and duration (2–12 weeks) of flooding on the survival of Leptosphaeria spp. in canola (Brassica napus) stubble. Canola basal stems with blackleg symptoms were submerged in water in small glass jars containing 20 cm³ soil on a thermogradient plate capable of simultaneously maintaining up to 96 independent temperature regimes. Flooded stems were sampled at 2‐week intervals, surface‐sterilized, and incubated on V8‐juice agar for 10 days to recover the pathogen. Flooding for 2 weeks substantially reduced pathogen recovery relative to non‐flooded controls and the pathogen was not recovered after 6 weeks of flooding, irrespective of temperature. The pathogen was eliminated slightly more rapidly at flooding temperatures >20°C than at 12–16°C. There was no difference between Leptosphaeria maculans and L. biglobosa in their ability to survive flooding. Stem tissues degraded most rapidly during the first 2 weeks of flooding, corresponding to a quick decline in pathogen survival during the same period. These results indicate that a paddy rice crop following winter rapeseed may minimize the impact of blackleg by eradicating the inoculum of Leptosphaeria spp. in stubble.     

Reducing the build‐up of Plasmodiophora brassicae inoculum by early management of oilseed rape volunteers

Clubroot of oilseed rape (OSR), caused by Plasmodiophora brassicae, is a disease of increasing economic importance worldwide. Previous studies indicated that OSR volunteers, Brassica crops and weeds play a critical role in the predisposition of the disease. To determine the effect of timing of foliar application of the herbicide glyphosate or mechanical destruction of OSR volunteers in reduction of clubroot severity and resting spore production, a series of studies was conducted under controlled conditions with a susceptible OSR cultivar and an isolate of P. brassicae. Plants were inoculated by injecting a spore suspension beside the root hairs at growth stage 11–12 (BBCH scale) and were terminated at 7 (early) or 21 (late) days post‐inoculation (dpi). Under controlled conditions, the first symptoms on roots were observed as early as 7 dpi. The early application of glyphosate as well as early mechanical destruction resulted in significant (P ≤ 0.05) reduction in the development of clubroot symptoms, root fresh weight and the number of resting spores?g root. Furthermore, the effect of volunteer management on clubroot severity in the succeeding OSR was studied by inoculating plants with the resting spores obtained from treated clubbed roots. Inoculated OSR exhibited root clubs similar to the initial symptoms after 35 dpi. Plants that were inoculated with spore suspension from early treated roots resulted in significant reductions in clubroot incidence and severity. Conversely, plants inoculated with the spore suspension from the late treated roots displayed levels of clubroot similar to the plants inoculated with the spore solutions of positive controls.     

Studies into primary and secondary infection processes by Plasmodiophora brassicae on canola

The early stages of infection of canola roots by the clubroot pathogen Plasmodiophora brassicae were investigated. Inoculation with 1 × 10⁵ resting spores mL^?1 resulted in primary (root hair) infection at 12 h after inoculation (hai). Secondary (cortical) infection began to be observed at 72 hai. When inoculated onto plants at a concentration of 1 × 10⁴ mL^?1, secondary zoospores produced primary infections similar to those obtained with resting spores at a concentration of 1 × 10⁵ mL^?1. Secondary zoospores caused secondary infections earlier than resting spores. When the plants were inoculated with 1 × 10⁷ resting spores mL^?1, 2 days after being challenged with 1 × 10⁴ or 1 × 10⁵ resting spores mL^?1, secondary infections were observed on the very next day, which was earlier than the secondary infections resulting from inoculation with 1 × 10⁷ resting spores mL^?1 alone and more severe than those produced by inoculation with 1 × 10⁴ or 1 × 10⁵ resting spores mL^?1 alone. Compared with the single inoculations, secondary infections on plants that had received both inoculations remained at higher levels throughout a 7‐day time course. These data indicate that primary zoospores can directly cause secondary infection when the host is under primary infection, helping to understand the relationship and relative importance of the two infection stages of P. brassicae.     

Overcoming seed dormancy in oilseed rape (Brassica napus L.) with exogenous compounds

Dormant seeds of oilseed rape (OSR) can persist in the soil and cause OSR volunteers in subsequent crops. Several approaches were tested in the laboratory and in the field to determine whether dormancy induction and seed persistence can be reduced by using dormancy‐breaking exogenous compounds. In a laboratory experiment, OSR seeds were coated with KNO₃, micronutrients, or gibberellic acid (GA) prior to a secondary dormancy test. In a field experiment, seeds were coated in a manner analogous to the laboratory experiment, and then buried 10 cm deep in the soil for 2.5 months. In a practical demonstration, OSR plants were sprayed with either urea ammonium nitrate (UAN) or a commercial product containing GA prior to seed maturity. Seed coating (laboratory and field experiments) reduced secondary dormancy and seed persistence in the field by up to 99%. The efficiency of the treatments for mitigating secondary dormancy (laboratory and field experiments) in decreasing order was GA > micronutrients > KNO₃ > control. With pre‐maturity spraying (practical demonstration), UAN reduced primary dormancy by up to 77% and the development of secondary dormancy by up to 38%; GA had no effect. Dormancy and seed persistence of OSR seeds may be reduced by a pre‐maturity UAN treatment of OSR mother plants, or by applying appropriate exogenous compounds to OSR seeds.     

Plant age and ambient temperature: significant drivers for powdery mildew (Erysiphe cruciferarum) epidemics on oilseed rape (Brassica napus)

Powdery mildew (Erysiphe cruciferarum) is an important disease in oilseed rape crops worldwide, but of sporadic importance in most southern Australian crops. Six Brassica napus cultivars were exposed to E. cruciferarum simultaneously in four plant age cohorts. First symptoms of powdery mildew appeared 9 days after inoculation (dai) on the oldest plants [42 days after seeding (das)], but 44 dai in the youngest plants that were exposed to inoculum from sowing, although final disease severity did not differ with the plant age at exposure. The maximum level of pod peduncle infestation was unaffected by plant age (P = 0.37) or cultivar (P = 0.28). The effect of temperature was also investigated. The development of disease on plants was slower and final severity reduced at a day/night temperature 14/10 °C compared with 22/17 °C. In vitro, maximum growth of germ tubes from conidia of E. cruciferarum was at 15–20 °C and survival of conidia reduced by temperatures >30 °C. The results explain the sporadic nature of powdery mildew outbreaks in winter‐grown oilseed rape in Australia, where slow rates of infection occur when seasonal colder prevailing winter conditions coincide with the presence of younger plants, together curtailing rapid disease development until temperatures increase in late winter/early spring. These results explain why epidemics are most severe in the two warmer cropping regions, viz. the northern agricultural region of Western Australia and New South Wales. This study suggests that with increases in winter temperatures under future climate scenarios, earlier and more severe powdery mildew outbreaks in Australia will be favoured.     

Pathogenic variation of Mycosphaerella species infecting banana and plantain in Nigeria

Mycosphaerella species that cause the ‘Sigatoka disease complex’ account for significant yield losses in banana and plantain worldwide. Disease surveys were conducted in the humid forest (HF) and derived savanna (DS) agroecological zones from 2004 to 2006 to determine the distribution of the disease and variation among Mycosphaerella species in Nigeria. Disease prevalence and severity were higher in the HF than in the DS zone, but significant (P < 0·001) differences between agroecological zones were only observed for disease severity. A total of 85 isolates of M. fijiensis and 11 isolates of M. eumusae were collected during the survey and used to characterize the pathogenic structure of Mycosphaerella spp. using a putative host differential cultivar set consisting of Calcutta‐4 (resistant), Valery (intermediate) and Agbagba (highly susceptible). Area under disease progress curve (AUDPC) was higher on all cultivars when inoculated with M. eumusae than with M. fijiensis, but significant (P < 0·05) differences between the two species were only observed on Valery. Based on the rank‐sum method, 8·3% of the isolates were classified as highly aggressive and 46·9% were classified as aggressive. About 11·5% of all the isolates were classified as least aggressive, and all of these were M. fijiensis. The majority of M. eumusae isolates (seven out of 11; 64%) were classified as aggressive. A total of nine pathotype clusters were identified using cluster analysis of AUDPC. At least one M. fijiensis isolate was present in all the nine pathotype clusters, while isolates of M. eumusae were present in six of the nine clusters. Isolates in pathotype clusters III and V were the most aggressive, while those in cluster VIII were the least aggressive. Shannon’s index (H) revealed a more diverse Mycosphaerella collection in the DS zone (H = 1·81) than in the HF (H = 1·50) zone, with M. fijiensis being more diverse than M. eumusae. These results describe the current pathotype structure of Mycosphaerella in Nigeria and provide a useful resource that will facilitate screening of newly developed Musa genotypes for resistance against two important leaf spot diseases of banana and plantain.     

Potential impacts of climate change on the threat of potato cyst nematode species in Great Britain

Potato cyst nematode (PCN) species have different temperature optima for various life cycle stages, therefore a risk assessment of the threat of PCN species under future climates is essential to guide adaptation strategies. Data defining the spatial coverage of potato crops in Great Britain were combined with probabilistic climate change data and a newly developed PCN life cycle model to project the future risk to potato crops from PCN. The model was based on the results of controlled environment experiments to investigate the effect of temperature on survival to female maturity using three PCN populations: Globodera pallida (Lindley) and G. rostochiensis from the James Hutton Institute PCN collection, and a field population of G. pallida (S‐Fife). It was found that projected increases in soil temperature could result in increased survival to female maturity for all three PCN populations, with greater increases expected for Scotland, followed by Wales then England. The largest projected increases in Scotland were for G. pallida, whereas G. rostochiensis showed the largest increases in Wales and England. The potential impact of several agronomic adaptation strategies on projected PCN risk were also investigated. The results from the model suggest that soil infestation levels would have to be reduced by up to 40% in order to negate projected increases in PCN risk, and that advancing the start date of the growing season or modifying planting patterns could be successful strategies to reduce future PCN risk.     

Fusarium species associated with stalk rot and head blight of grain sorghum in Queensland and New South Wales,Australia

Historical records report Fusarium moniliforme sensu lato as the pathogen responsible for Fusarium diseases of sorghum; however, recent phylogenetic analysis has separated this complex into more than 25 species. During this study, surveys were undertaken in three major sorghum‐producing regions in eastern Australia to assess the diversity and frequency of Fusarium species associated with stalk rot‐ and head blight‐infected plants. A total of 523 isolates were collected from northern New South Wales, southern Queensland and central Queensland. Nine Fusarium species were isolated from diseased plants. Pathogenicity tests confirmed F. andiyazi and F. thapsinum were the dominant stalk rot pathogens, whilst F. thapsinum and species within the F. incarnatum–F. equiseti species complex were most frequently associated with head blight.     

Potential biological control of clubroot on canola and crucifer vegetable crops

Clubroot caused by Plasmodiophora brassicae is an emerging threat to canola (Brassica napus) production in western Canada, and a serious disease on crucifer vegetable crops in eastern Canada. In this study, seven biological control agents and two fungicides were evaluated as soil drenches or seed treatments for control of clubroot. Under growth cabinet conditions, a soil‐drench application of formulated biocontrol agents Bacillus subtilis and Gliocladium catenulatum reduced clubroot severity by more than 80% relative to pathogen‐inoculated controls on a highly susceptible canola cultivar. This efficacy was similar to that of the fungicides fluazinam and cyazofamid. Under high disease pressure in greenhouse conditions, the biocontrol agents were less effective than the fungicides. Additionally, all of the treatments delivered as a seed coating were less effective than the soil drench. In field trials conducted in 2009, different treatments consisting of a commercial formulation of B. subtilis, G. catenulatum, fluazinam or cyazofamid were applied as an in‐furrow drench at 500 L ha^?1 water volume to one susceptible and one resistant cultivar at two sites seeded to canola in Alberta and one site of Chinese cabbage in Ontario. There was no substantial impact on the susceptible canola cultivar, but all of the treatments reduced clubroot on the susceptible cultivar of Chinese cabbage, lowering disease severity by 54–84%. There was a period of 4 weeks without rain after the canola was seeded, which likely contributed to the low treatment efficacy on canola. Under growth cabinet conditions, fluazinam and B. subtilis products became substantially less effective after 2 weeks in a dry soil, but cyazofamid retained its efficacy for at least 4 weeks.     

Isolation and characterization of Streptomyces species from potato common scab lesions in Norway

The present survey was conducted to isolate and characterize Streptomyces species from common scab lesions of potato in Norway. Bacteria were isolated from scab lesions on tubers sampled in two consecutive years at different locations in Norway spanning ～1400 km from south to north. In total, 957 independent isolations from individual tubers were performed, with 223 putative pathogenic isolates obtained from 29 different potato cultivars and 130 different fields. Streptomyces europaeiscabiei was the most abundant species isolated from common scab lesions (69%), while 31% of the isolates obtained were S. turgidiscabies. Streptomyces scabies was not found. Pathogenicity of selected Streptomyces isolates was tested on potato. The ability of the bacterial isolates to infect potato was consistent with the presence of the txtAB operon. The results revealed no pattern in geographical distribution of S. europaeiscabiei and S. turgidiscabies; both could be found in the same field and even the same lesion. Four different pathogenicity island (PAI) genotypes were detected amongst the txtAB positive isolates: nec1+/tomA+, nec1–/tomA+, nec1+/tomA? and nec1?/tomA?. The current findings demonstrate that there is genetic variability within species and that the species are not spread solely by clonal expansion. This is thought to be the most comprehensive survey of Streptomyces species that cause common scab of potato in a European country.     

Morphological and molecular diversity of Colletotrichum spp. causing pepper spot and anthracnose of lychee (Litchi chinensis) in Australia

Since the 1980s a new disease has been affecting Australian lychee. Pepper spot appears as small, black superficial lesions on fruit, leaves, petioles and pedicels and is caused by Colletotrichum gloeosporioides, the same fungus that causes postharvest anthracnose of lychee fruit. The aim of this study was to determine if a new genotype of C. gloeosporioides is responsible for the pepper spot symptom. Morphological assessments, arbitrarily‐primed PCR (ap‐PCR) and DNA sequencing studies did not differentiate isolates of C. gloeosporioides from anthracnose and pepper spot lesions. The ap‐PCR identified 21 different genotypes of C. gloeosporioides, three of which were predominant. A specific genotype identified using ap‐PCR was associated with the production of the teleomorph in culture. Analysis of sequence data of ITS and β‐tubulin regions of representative isolates did not group the lychee isolates into a monophyletic clade; however, given the majority of the isolates were from one of three genotypes found using ap‐PCR, the possibility of a lychee specific group of C. gloeosporioides is discussed.     

Identification and differentiation of Phyllosticta species causing freckle disease of banana using high resolution melting (HRM) analysis

Freckle disease of banana is caused by three closely related species of Phyllosticta, namely P. musarum, P. maculata and P. cavendishii. In this study, a high resolution melting (HRM) analysis assay was developed and its potential to identify these three fungal species is reported. The assay, which targets the ITS of the nuclear rDNA of the fungal species, generates three distinct melt profiles for the three Phyllosticta species. It is also able to distinguish a combination of up to three co‐infecting species by generating a deviant melt curve. Thirty‐five fungal cultures and infected herbarium leaf specimens, previously characterized using nucleotide sequencing as belonging to one of the three Phyllosticta species, were used for validation of the HRM analysis assay. The normalized curves generated differentiated all samples, with samples from each species correctly identified. The assay was further evaluated against 18 uncharacterized infected leaf specimens from various geographic locations and the results were verified by subsequent nucleotide sequencing. This HRM analysis assay allows rapid identification and differentiation of the three Phyllosticta species using a single primer pair in a one‐step closed‐tube system without labelled fluorescence probes. This novel assay format has potential for simultaneously identifying and differentiating other closely related species of plant pathogens, as well as the classification of infected historic specimens.     

进境澳大利亚油菜籽中茎基溃疡病菌的检测

 41 fungal isolates with similar morphological characteristics to Leptosphaeria maculans were obtained by the deep-freezing filter paper method from 2100 seeds of Brassica napus imported from Australia.The isolate 8129-5 showed a slower growth on PDA at 20℃with growth rate of 2.8 mm/day.The colonies on PDA at 20℃ had an irregular or regular margin with white or grayish white compact aerial mycelium.No diffusible pigment was produced on PDA at 31℃ or in liquid Czapek-Dox media at 20℃.PCR detection showed that the isolate 8129-5 could be amplified by L.maculans-specific primers LmacF/LmacR and got expected product of 331 bp.The sequence analysis revealed that the ITS sequence of isolate 8129-5 had 99.8% identity with L.maculans.Pathogenicity of the isolate 8129-5 was confirmed on cotyledons of rape seed by artificial inoculation compared with typical symptom of L.maculans.Based on the morphological characteristics, PCR detection and the result of pathogenicity test, the isolate 8129-5 was identified as L.maculans.     

Genetic diversity of giant reed ( A rundo donax) in Australia

The perennial grass, Arundo donax, has shown potential as a promising biomass crop. However, it has become invasive in a number of areas and declared a noxious weed in some jurisdictions, making proposals to grow A. donax for commercial use in Australia controversial. Evidence of asexual reproduction and the presence of a single genetic clone in Australia was investigated, as such characteristics would indicate a limited risk of escape and invasion. Using amplified fragment length polymorphism markers, the genetic diversity of 218 A. donax samples from across Australia was examined. The samples were found to separate into two distinct genetic groups, or clades. There was only a small amount of genetic diversity within a clade (0.9 and 1.5%). However, there was a larger difference between the clades of 19.8%, suggesting the presence of two distinct A. donax genotypes in Australia. The low level of genetic variation in Australian A. donax that was found in this study indicates that spread is essentially by vegetative means and suggests that if grown in areas where it is separated from natural water dispersal events, A. donax poses a low risk of becoming invasive.