Mowing strategies for controlling Cirsium arvense in a permanent pasture in New Zealand compared using a matrix model

Defoliation has frequently been proposed as a means of controlling Cirsium arvense (L.) Scop. (Californian thistle, Canada thistle, creeping thistle, perennial thistle), an economically damaging pastoral weed in temperate regions of the world, but its optimization has remained obscure. We developed a matrix model for the population dynamics of C. arvense in sheep‐grazed pasture in New Zealand that accounts for the effects of aerial shoot defoliation on a population's photosynthetic opportunity and consequential overwintered root biomass, enabling mowing regimes varying in the seasonal timing and frequency of defoliation to be compared. The model showed that the long‐term population dynamics of the weed is influenced by both the timing and frequency of mowing; a single‐yearly mowing, regardless of time of year, resulted in stasis or population growth, while in contrast, 14 of 21 possible twice‐yearly monthly mowing regimes, mainly those with mowing in late spring, summer, and early autumn, resulted in population decline. Population decline was greatest (with population density halving each year) with twice‐yearly mowing either in late spring and late summer, early summer and late summer, or early summer and early autumn. Our results indicate that mowing can be effective in reducing populations of C. arvense in pasture in the long term if conducted twice each year when the initial mowing is conducted in mid spring followed by a subsequent mowing from mid summer to early autumn. These mowing regimes reduce the photosynthetic opportunity of the C. arvense population and hence its ability to form the overwintering creeping roots upon which population growth depends.     

Does transmission of the rust pathogen,Puccinia punctiformis,require stem mining vectors?

Recent research in Europe has suggested that stem mining insects may be important for vectoring the pathogen Puccinia punctiformis, and largely responsible for its systemic infection in the weed, Cirsium arvense. However, here we present comparative survey data showing that the level of systemic disease is the same in Europe and New Zealand, with and without stem miners, respectively, casting doubt on the idea that these insects are necessary for transmission of the fungus.     

Classical biological control of Cirsium arvense: Lessons from the past

Cirsium arvense (L.) Scop. is a perennial herb indigenous to Eurasia that is now present throughout temperate regions of the world where it is considered one of the worst weeds of pastoral and agricultural systems. Classical biological control has been attempted in both North America (NA) and New Zealand (NZ). However, nearly 50 years after the first agent releases there are no indications of successful control. We review the status of the five agents deliberately released for control of C. arvense in NA and NZ, plus the species unintentionally introduced, and the occurrence of insects native to NA on C. arvense. We retrospectively evaluate C. arvense as a target weed, critique the agents selected for release, and contrast the different situations in NA and NZ. In retrospect, we see justification for the agents released in NA, but it is evident that these agents would not meet the more stringent host specificity requirements necessary to be released today. The failure of the program in NA is attributed to compromised safety, and lack of impact. Non-target impacts by one of the released agents, Rhinocyllus conicus, have raised safety concerns for native thistle plants. The other released agents either failed to establish, or if established, had no impact on the weed. In contrast, the situation in NZ is quite different because there are no related native thistles (Cardueae), and thus little chance of non-target impacts. Thus far, failure in NZ is attributed to lack of effectiveness due to non-establishment, or no impact, of released agents. In the past, the same agents that were released in NA were subsequently released in NZ, without considering whether or not these were the best choices. Thus, the past failure in NZ might be due to the previous lack of a NZ-specific approach to biocontrol of thistles in general and C. arvense in particular. A new approach taking into consideration the absence of native Cardueae has resulted in the release of agents more likely to be effective, and has potentially set NZ on track towards successful biological control of C. arvense, and other thistles.     

Effects of pasture competition and specialist herbivory on the performance of Cirsium arvense

Combining specialist herbivory with interspecific plant competition can be an effective means of controlling pasture weeds. Cirsium arvense (Canada thistle, Californian thistle, creeping thistle) is one of the worst weeds of pastoral production systems in New Zealand (NZ). The oligophagous leaf-feeding beetle, Cassida rubiginosa, was recently released in NZ for control of C. arvense. To assess the impact of this biocontrol agent we conducted an outdoor potted-plant experiment with low and high densities of Cassida larvae combined with different levels of interspecific competition from typical NZ pasture species. Secondly, we carried out a field-release experiment to quantify the impact of high densities of Cassida under more natural conditions. Interspecific competition reduced all measured plant parameters of C. arvense except mean shoot height and base diameter. Herbivory by Cassida only reduced root biomass, and showed a weak additive response when combined with competition. All other measured parameters of C. arvense showed a substitutive response, with competition being the only factor having a significant impact on the weed. There were no significant synergistic interactions with competition and herbivory on C. arvense. Interestingly, the number of root buds per plant was significantly greater in the presence of herbivory by Cassida, suggesting that C. arvense may compensate for defoliation. Similar to the potted-plant experiment, Cassida had no significant effect on shoot growth and development in the field-release experiment. The results of this study indicate that competition from typical NZ pasture species is a more important factor than herbivory by Cassida, and unless Cassida reaches outbreak densities, it will likely have an insignificant impact on this weed.     

Influence of environmental conditions on spore dispersal and infection by Septoria nodorum

In field experiments using healthy trap plants it was found that pycniospores of S. nodorum were dispersed from diseased wheat plants whenever rain fell and occasionally in the absence of rain. Of two spring wheats tested, cv. Kolibri seemed a better ‘source’ plant and cv. Maris Butler a better ‘receptor’ when rainfall was light, but this difference was not apparent when rainfall was heavy. On 44 occasions, plants of Kolibri and M. Butler were also exposed to natural conditions immediately following artificial inoculation with S. nodorum. Infection of plants occurred on 10 occasions and was associated with the following minimum conditions: r.h, at inoculation >63%; and in the following 24 h, minimum temperature <6°C, at least 4 h with r.h. >90% and not more than 4 h with r.h. >60%.     

The symptoms and causal agents of early-dying disease (Verticillium wilt) of potatoes

The only characteristic symptom produced by Verticillium albo-atrum and V. dahliae in infected potato plants is unilateral chlorosis and necrosis: this was not shown until the approach of host maturity, and was distinguishable from symptoms of natural senescence only in its slightly earlier expression. Of six species of Verticillium tested against potato (King Edward), V. albo-atrum, V. dahliae, V. nigrescens and V. nubilum were pathogenic (all produced ‘wilt’ symptoms and relative virulence was in that decreasing order) but V. tricorpus and V. lateritium did not induce disease. Isolates of V. albo-atrum and V. dahliae, obtained from a number of other host plants, were also pathogenic to potato. Possible reasons are given for the fewness of records of ‘Early dying’ disease (Verticillium wilt) of potatoes in the field.     

Isolation of polymorphic microsatellites in the stemless thistle (Cirsium acaule) and their utility in other Cirsium species

The genus Cirsium includes species with both widespread and restricted geographical distributions, several of which are serious weeds. Nine polymorphic microsatellite loci were isolated from the stemless thistle Cirsium acaule. Eight were polymorphic in C. acaule, six in C. arvense and seven in C. heterophyllum. One locus monomorphic in C. acaule showed polymorphism in C. heterophyllum. The mean number of alleles per locus was 4.1 in C. acaule, 6.2 in C. arvense and 2.9 in C. heterophyllum. These nine loci were also amplified in C. eriophorum and C. vulgare, suggesting that these markers may be of use throughout the genus.     

Effect of Inoculum Density on Verticillium Wilt Incidence in Commercial Olive Orchards

Verticillium wilt, caused by Verticillium dahliae Kleb., is presently the most destructive disease of olive, particularly in Andalucía (southern Spain). ‘Picual’ and ‘Arbequina’ are the dominant cultivars being planted in Spain. Both cultivars are highly susceptible to the defoliating pathotype of V. dahliae when artificially inoculated by root‐dipping or stem injection. Conversely, ‘Arbequina’ is considered more resistant than ‘Picual’ based on field observations and farmer's experience. In this study, the differential reaction between of cultivars was confirmed by surveys of naturally infested orchards with different inoculum densities of the pathogen. The average percentage of affected olive trees of ‘Picual’ was 60.2%, while only 13.1% of trees of ‘Arbequina’ showed disease symptoms. Overall, the pathogen caused extensive wilting of branches and defoliation on the trees of ‘Picual’, whereas ‘Arbequina’‐infected trees showed chlorotic symptoms and slight defoliation. The relationship between inoculum density and disease incidence fit a logarithmic function for both cultivars. The percentage of affected trees of ‘Arbequina’ per year increased linearly (y = 0.3559x, R² = 0.5652, and P = 0.0195) with the inoculum density in the soil, whereas this relationship was not observed for the ‘Picual’. Planting density had no effect on disease incidence for any of the two cultivars.     

Asymptomatic systemic disease of Canada thistle (Cirsium arvense) caused by Puccinia punctiformis and changes in shoot density following inoculation

Canada thistle (Cirsium arvense) is one of the worst weeds in temperate areas of the world. A rust fungus, Puccinia punctiformis, was first proposed as a biological control agent for C. arvense in 1893. The rust causes systemic disease which ultimately kills C. arvense plants. In 2013 it was demonstrated in four countries, that inoculation of C. arvense rosettes in the fall with ground telia-bearing leaves can initiate epidemics of systemic rust disease with an average of 28% of inoculated rosettes producing a systemically diseased shoot the following spring. Other rosettes that emerged near inoculation points in spring were stunted and appeared diseased. To determine whether other rosettes were diseased, a chemiluminescence western slot blot test, applying polyclonal antibodies raised against P. punctiformis antigens, was developed to detect the fungus in roots. Rosettes were inoculated with telia-bearing leaves in the fall in Maryland, USA and Veroia, Greece. Roots of asymptomatic rosettes that emerged adjacent to inoculation points the following spring were tested for the presence of the fungus with the slot blot test. Rosettes that had diseased shoots were recorded. Based on the slot blot tests, 50–60% of the asymptomatic rosettes adjacent to inoculation points were positive for presence of the rust and likely to be systemically diseased. To demonstrate that systemic disease leads to C. arvense decline, C. arvense shoot densities were measured annually at 10 sites, in three countries, that had been inoculated with telia-bearing leaves in the fall between 2008 and 2012. Changes in C. arvense shoot densities over time were calculated. Average reductions in C. arvense density across the 10 sites were 43.1 ± 10.0% at 18 months after inoculation, 63.8 ± 8.0% at 30 months after inoculation, and 80.9 ± 16.5% at 42 months after inoculation, and 72.9 ± 27.2% at 54 months after inoculation; the 54 month reduction was, however, based on only two sites.     

Some dynamics of spread and infection by aeciospores of Puccinia punctiformis,a biological control pathogen of Cirsium arvense

Systemic disease of Cirsium arvense caused by Puccinia punctiformis depends on teliospores, from telia that are formed from uredinia, on C. arvense leaves. Uredinia result from infection of the leaves by aeciospores which are one main source of dispersal of the fungus. However, factors governing aeciospore spread, germination, infection, and conversion to uredinia and telia have not been extensively investigated. In this study, effective spread of aeciospores from a source area in a field was fitted to an exponential decline model with a predicted maximum distance of spread of 30 m from the source area to observed uredinia on one leaf of one C. arvense shoot. However, the greatest number of shoots bearing leaves with uredinia/telia was observed within 12 m of the source area, and there were no such shoots observed beyond 17 m from the source area. Aeciospore germination under laboratory conditions was low, with a maximum of about 10%. Temperatures between 18 °C and 25 °C were most favorable for germination with maximum germination at 22 °C. Temperature and dew point data collected from the Frederick, MD airport indicated that optimum temperatures for aeciospore germination occurred in late spring from about May 18 to June 20. Dew conditions during this period were favorable for aeciospore germination. A total of 122 lower leaves, 2 per shoot, on 61 C. arvense shoots were individually inoculated in a dew tent in a greenhouse by painting suspensions of aeciospores onto the leaves. Of these inoculated leaves, 47 produced uredinia within an average of 21.2 ± 6.9 days after inoculation. Uredinia were also produced, in the absence of dew, on 17 non-inoculated leaves of 12 shoots. These leaves were up to 4 leaves above leaves on the same shoots that had been individually and separately inoculated. Results of PCR tests for the presence of the fungus in non-inoculated leaves that were not bearing uredinia, showed that 44 leaves above inoculated leaves on 27 shoots were positive for the presence of the fungus. These leaves were up to 5 leaves above inoculated leaves on the same shoot. Uredinia production and positive PCR results on leaves above inoculated leaves on the same shoot indicated that aeciospore infection was weakly systemic. In other tests in which all leaves of plants were spray-inoculated with aeciospores, uredinia were produced by 10 days after inoculation and converted to telia and sole production of teliospores in about 63 days after inoculation. Successful systemic aeciospore infections in late spring would be expected to result in uredinia production in excess of a 1:1 ratio of aeciospore infections to uredinia and ultimately telia production in late summer. In this manner, systemic aeciospore infections would promote increased density of telia that lead to systemic infections of roots in the fall.     

Annual burning of semi‐natural grasslands for conservation: winners and losers among plant species

Species‐rich semi‐natural grasslands in Europe are a main target for conservation efforts, and alternative methods to the traditional management of mowing or grazing would be welcome due to the difficulties in maintaining traditional management practices. One such method proposed is burning of grassland vegetation during late winter or spring. To evaluate the effects of annual spring burning vs annual mowing on semi‐natural grassland vegetation, we compared the frequency of species in eleven field experiments in southern Sweden after ca 14 years. Out of the 88 species analyzed, five were more frequent in burnt plots compared with mowed plots (Vicia cracca, Cirsium arvense, Urtica dioica, Galium verum, Convallaria majalis). In contrast, 37 species were significantly less frequent in burnt plots compared with mowed ones, those with the largest differences being Ranunculus acris, Briza media, Veronica chamaedrys, Festuca ovina, Plantago lanceolata and Anthoxanthum odoratum. Tall‐grown species and those with preferences for N‐rich soils increased in frequency under an annual spring‐burn regime, compared with annual mowing, as did species producing larger amounts of nectar. Hence, although vegetation composition becomes more trivial with annual spring burns, there might be long‐term benefits for nectar‐feeding insects.     

Verticillium dahliae (Kleb.) Infecting Pumpkin Seed

This study investigated the natural occurrence of Verticillium dahliae (Kleb.) infection in pumpkin (Cucurbita pepo L.) seed. The mean incidence of infection was found to be 21.0%. Isolates recovered from seeds were pathogenic to pumpkin (cultivar ‘Jamaican squash’). Surface sterilization by immersion in 0.6% sodium hypochlorite for 20 min eradicated V. dahliae from infected pumpkin seeds without affecting germinability. Plating of seed components revealed that the fungus was present in the seed coat but not in the embryo or cotyledons. In a growing‐on test, 25% of 6‐week‐old plants grown from untreated seeds were infected. Germination and production of normal seedlings were unaffected by V. dahliae infection of seeds. Verticillium dahliae in pumpkin seed was found to be external and transmissible to plants. The findings of this study are important in devising disease control strategies.     

Genetic and phenotypic differences between thistle populations in response to habitat and weed management practices

Rapid evolutionary change is increasingly being recognized as commonplace, but the evolutionary consequences for species and ecosystems under human‐induced selection regimes have not been explored in detail, although many species occur in such environments. In a common garden experiment and with amplified fragment length polymorphism markers, we examined whether genetic differentiation has taken place between spatially intermixed populations of creeping thistles Cirsium arvense (Asteraceae) collected from a natural habitat (maritime shores), a semi‐natural habitat (road verges) and arable fields under two management regimes: conventional and organic farming. Populations of C. arvense have altered genetically and locally adapted their growth patterns with changed land use. Although plants from different habitats showed similar total biomass production, shoot and root production was higher for maritime populations, suggesting selection for increased competitive ability. Competitive ability then declined in the order semi‐natural, conventional farms and organic farms. Thistles in arable fields may be more selected for tolerance against disturbances from herbicides and mechanical weed control. In addition, early shoot sprouting and genetic analysis showed differentiation between plants originating from conventional farms and farms that were converted to organic 9–30 years ago, suggesting some adaptation to altered crop cultivation practices. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 797–807.     

I. Pathogenic variation in fungi and bacteria and mycorrhizal compatibility: Pathogenic variation in Verticillium albo-atrum isolates from hop (Humulus lupulus)

Non-lethal (M strain) ‘fluctuating’ wilt was first identified in hops in 1924. Lethal (V strain) ‘progressive’ wilt appeared in the 1930s and spread rapidly through Kent, destroying all commercial varieties. By 1965 the disease was largely controlled by use of new resistant cultivars, accompanied by restricted nitrogen fertilisers, non-cultivation (with weed control by overall herbicides), and high standards of field hygiene. Between 1968 and 1971 severe wilt developed in previously highly resistant clones, used as ‘controls’ in routine field resistance tests. Verticillium isolates from the two farms supplying the natural inoculum for these tests during 1968–71 were more virulent than previously encountered V strains, some causing severe wilt in all resistant cultivars. Subsequent surveys supported by pathogenicity tests against resistant cultivars indicated the existence of two ‘super virulent’ strains on eight farms in Kent. The appearance of mild (M) strains in the 1920s, virulent (VI) strains in the 1930s, and ‘super-virulent’ (V2 and V3) strains in the 1960s follow 400 years of apparent freedom from wilt. This suggests that the initial breakage of resistance barriers was critical and subsequent variations in an apparently complex, polygenically controlled resistance system are rapidly being matched by the pathogen.     

Effects of Pratylenchus penetrans on development of strawberry wilt caused by Verticillium dahliae

The nematode Pratylenchus penetrans or the closely related P. fallax occurred in three out of four strawberry plantations infested with Verticillium dahliae surveyed in Kent. When plants of cv. Cambridge Vigour were inoculated with P. penetrans and V. dahliae together in field ‘micro-plots’ the nematode increased the rate of wilt development except when the concentration of V. dahliae microsclerotia in the soil was very low. In a pot experiment, under conditions not conducive to symptom expression, the incidence of infection was increased by nematodes in Cambridge Favourite, Cambridge Vigour and Redgauntlet; the incidence was least in Red-gauntlet, the most resistant cultivar. The nematode appeared to cause local changes in the root cortex which aided hyphal penetration of the adjacent tissues. Growth of strawberry plants in the absence of V. dahliae was not significantly affected even by large populations of the nematode, but growth was reduced by V. dahliae at all rates of inoculation in the field. Infection by Verticillium did not affect the total numbers of nematode per plant at the end of the experiments, although the number per g fresh weight was greater on the small amount of white root on severely diseased plants than on the larger amount on healthy or mildly diseased plants.     

<Emphasis Type="Italic">Plectosphaerella cucumerina</Emphasis> as a bioherbicide for <Emphasis Type="Italic">Cirsium arvense</Emphasis>: proof of concept

Plectosphaerella cucumerina (Lindf.) W. Gams was evaluated as a bioherbicide for Cirsium arvense L. (Scop.) using a Canadian and a New Zealand isolate. Both isolates defoliated C. arvense when applied at 10¹³ conidia ha^?1 in water volumes ranging from 250 to 6400 l ha^?1 with a rapid decline in effect with declining conidial dose. Repeat application and the addition of the adjuvant Pulse^® penetrant to the conidial suspension increased the disease severity in C. arvense. Maximum disease occurred at 20 °C with a 48 h post-application dew period. The experiments demonstrate that P. cucumerina can defoliate C. arvense under the environmental conditions of temperate pastures where the weed is problematic. The results also show that modifications to formulation and strategic application may reduce the 48 h dew period requirement and risk to non-target species respectively, supporting the conclusion that the fungus has potential as a bioherbicide for C. arvense.     

Direct and Indirect Effects of a Shoot-Base Boring Weevil and Plant Competition on the Performance of Creeping Thistle, Cirsium arvense

Creeping thistle or Canada thistle, Cirsium arvense (L.) Scop., is considered one of the world's worst weeds and the third most important weed in Europe. Biological control of this indigenous weed in Europe by use of native agents may provide a low-cost alternative to use of chemical or mechanical control measures and contribute to a more sustainable weed management. We investigated the potential of a shoot-base boring weevil, Apion onopordi Kirby (Coleoptera: Apionidae), for biological weed control, in the presence or absence of plant competition by three grass species. Infestation of thistle shoots by A. onopordi at natural infestation levels reduced above- and belowground plant performance after 2 years. Plant competition at natural levels had an overall greater effect than that of herbivory, significantly reducing both above- and belowground thistle performance in both years, thereby slowing the propagation of the weed. Weevil infestation and grass competition had a synergistic effect on C. arvense growth; the combined effects of the two factors was greater than the sum of both single-factor effects. The experiment revealed that A. onopordi promotes systemic infections of the rust fungus Puccinia punctiformis (Str.) Röhl in the year following weevil infestation. Systemically infected thistle shoots died before the end of the growing season. Although the direct effect of A. onopordi may not be sufficient to control creeping thistle, the synergistic interaction with plant competition and the indirect effect via promotion of systemic rust infections makes A. onopordi a promising agent for the biological control of this weed.     

Survival of the causal agents of 'early-dying disease' (Verticillium wilt) of potatoes

Although the isolation of Verticillium albo-atrum and V. dahliae from soil and dried moribund stems following infection of a potato crop proved extremely difficult, both fungi were equally capable of overwintering in these substrates and of inducing disease in a subsequently planted susceptible crop. In the absence of a susceptible crop some weed species became colonized. The two species, however, appeared to differ in their capacity for survival both beneath a monocotyledonous crop and within the potato tubers. Colonization of the roots of wheat, barley, oats, rye and maize was observed with V. dahliae but not with V. albo-atrum. The latter appeared to be capable of prolonged survival in the tubers, whereas V. dahliae did not remain viable in storage over winter. Consequently only tubers infected with V. albo-atrum produced infected plants. The presence of the fungi within the tubers affected neither dormancy nor the initial development of the sprouts. Some correlation was noted between tuber size, the percentage of tubers infected, the distribution of V. albo-atrum within the tubers and the development of disease in plants subsequently grown from these tubers.     

Transient peak in moth diversity as a response to organic farming

Few initiatives to preserve and enhance biodiversity on farmland have been as thoroughly evaluated and debated as the agri-environment schemes (AES). Yet, little is known how confounding factors co-varying with the specific AES measures may affect species responses. Here, we quantify the influence of one such factor, the time since transition to organic farming, on moth diversity patterns. We found that species richness and abundance of moths were higher on new organic farms (years since transition ≤6) compared to old organic (≥15 years) and conventional farms, indicating a transient diversity peak. This correlates with the abundance patterns of the weed Cirsium arvense, which also reached its highest densities on new organic farms. Weeds such as C. arvense constitute a notorious problem in organic farming. However, they also provide various resources for farmland biodiversity, and our results strongly suggest that the transient weed peak may be important in influencing the parallel peak among the moths. This stresses the problem in balancing out production and conservation values. More generally, our results show that rather than having static effects on the environment, AES can have an important temporal component and result in a dynamic interplay between different trophic levels.     

High‐Throughput Assessment and Genetic Investigation of Vegetative Compatibility in Verticillium dahliae

Classification of isolates into vegetative compatibility groups (VCGs) using nitrate‐non‐utilizing (nit) mutants has been widely used for the characterization of Verticillium dahliae populations. However, certain methodological limitations prevent its application on a large scale. Furthermore, systematic investigations into the genetics underlying complementation tests between nit mutants of fungal isolates (i.e. heterokaryon formation) are lacking for Verticillium species. In this work, a diverse collection of 27 V. dahliae isolates – including representatives of all VCGs, both mating types, and heterokaryon self‐incompatible isolates – was employed for the development and optimization of (i) a protocol for the rapid generation of nit mutants of V. dahliae isolates using UV‐irradiation and (ii) a reproducible high‐throughput procedure for complementation tests between nit mutants in liquid cultures using 96‐well microplates. The genetic analysis of selected heterokaryons demonstrated that the frequently encountered ‘weak’ cross‐reactions between VCGs and their subgroups can be actually heterokaryotic, implying the absence of strict genetic barriers between VCGs. In conclusion, we provide in this work an optimized method for the high‐throughput VCG assignment of V. dahliae populations and a genetic analysis of heterokaryons that may have serious implications for the interpretation of VCG classification data. These advancements in the available methodology and the genetic background of vegetative compatibility grouping may contribute to a better understanding of the population biology of V. dahliae and possibly other mitosporic fungi.