Mycelial growth rate and toxin production in the seed pathogen Pyrenophora semeniperda: resource trade‐offs and temporally varying selection

Pyrenophora semeniperda, an important pathogen in Bromus tectorum seed banks in semi‐arid western North America, exhibits >4‐fold variation in mycelial growth rate. Host seeds exhibit seasonal changes in dormancy that affect the risk of pathogen‐caused mortality. The hypothesis tested is that contrasting seed dormancy phenotypes select for contrasting strategies for increasing pathogen fitness, and that increased fitness on nondormant seeds involves a resource trade‐off between toxin production and growth. The strategy for successfully attacking rapidly germinating nondormant seeds at high inoculum loads in autumn involves increased post‐infection aggressiveness to prevent seed escape through germination. An earlier study demonstrated that slow‐growing strains caused higher mortality than faster‐growing strains on nondormant host seeds at high inoculum loads. In this study, production of the toxin cytochalasin B was significantly higher in slower‐growing strains, and was induced only in seeds or in seed‐constituent‐containing media. Its production was reduced in vivo by Bromus tectorum seeds, suggesting direct involvement in pathogenesis on seeds. Fast‐growing strains caused significantly higher mortality than slow‐growing strains at low inoculum loads on dormant seeds, which apparently have resistance that is overcome at high loads or through rapid mycelial proliferation. In a co‐inoculation study, the fast‐growing isolate produced 3 × more stromata than the slow‐growing isolate on dormant seeds, whereas the slow‐growing isolate was twice as successful on nondormant seeds. These results provide evidence that mycelial growth rate variation and associated variation in cytochalasin B production represent a trade‐off maintained through temporally varying selection resulting from seasonal variation in host seed dormancy status.     

Seed dormancy and soil seedbank of the invasive weed Chenopodium hybridum in north‐western China

Seed dormancy and persistence in the soil seedbank play a key role in timing of germination and seedling emergence of weeds; thus, knowledge of these traits is required for effective weed management. We investigated seed dormancy and seed persistence on/in soil of Chenopodium hybridum, an annual invasive weed in north‐western China. Fresh seeds are physiologically dormant. Sulphuric acid scarification, mechanical scarification and cold stratification significantly increased germination percentages, whereas dry storage and treatments with plant growth regulators or nitrate had no effect. Dormancy was alleviated by piercing the seed coat but not the pericarp. Pre‐treatment of seeds collected in 2012 and 2013 with sulphuric acid for 30 min increased germination from 0% to 66% and 62% respectively. Effect of cold stratification on seed germination varied with soil moisture content (MC) and duration of treatment; seeds stratified in soil with 12% MC for 2 months germinated to 39%. Burial duration, burial depth and their interaction had significant effects on seed dormancy and seed viability. Dormancy in fresh seeds was released from October to February, and seeds re‐entered dormancy in April. Seed viability decreased with time for seeds on the soil surface and for those buried at a depth of 5 cm, and 39% and 10%, respectively, were viable after 22 months. Thus, C. hybridum can form at least a short‐lived persistent soil seedbank.     

Germination characteristics and seedbank of the alien species Leucaena leucocephala (Fabaceae) in Brazilian forest: ecological implications

Plant species invasiveness is frequently associated with rapid proliferation and production of seeds that can persist in the soil for long periods of time. Leucaena leucocephala (Fabaceae) is an alien and invasive species, for example in Brazilian forest and savannah ecosystems. This study quantified the invasive potential of this species by analysing its seed rain (using seed collectors), seed longevity in the soil (stored in buried bags) and the germination capacity of the soil seedbank (by collecting soil samples in the study area). Our results showed that seed rain occurred throughout the year, although more intensely from July to September, with about 5500 seeds m^?2 year^?1 being released. The numbers of seeds in the buried bags diminished over time and intact seeds showed low germinability (approximately 15%), although their viability remained >80% of the recovered seeds after two years of in situ storage. The germinability of seeds collected directly from the soil was approximately 40%, indicating that more than half of the seeds of soil seedbank were dormant (physical dormancy). Leucaena leucocephala produces large numbers of seeds and is able to form a persistent short‐lived seedbank (viability 1–5 years). These factors may contribute significantly to its invasive potential, which makes it difficult to control this species once it becomes established. As control costs become higher over time, immediate public efforts are needed to counter this threat.     

Variation in the annual dormancy cycle in buried seeds of the weedy winter annual Viola arvensis

Seeds of Viola arvensis collected in different years and in different months within those years were buried in soil under natural seasonal temperature cycles, and changes in their germination requirements monitored. Seeds were dormant at maturity in May or June, but nondormant by autumn. During winter, some seeds entered dormancy, while others entered conditional dormancy, i.e. retained the ability to germinate at 15/6 and 20/10^oC but not at other thermoperiods. Dormant and conditionally dormant seeds became nondormant the following summer. Seeds collected in 1981 exhibited an annual dormancy:nondormancy cycle, while those collected in 1982 exhibited an annual conditional dormancy:nondormancy cycle. The type of dormancy cycle found in these seed lots during their first year of burial persisted in subsequent years. Thirty–five and 36% of seeds collected in May 1983 and 1986, respectively, were conditionally dormant the following May, while only 5 and 9% of those collected in the same field in June 1983 and 1986, respectively, were conditionally dormant. Dormant seeds collected in 1981,1982 and 1984 and buried at 5^oC during summer germinated to 0, 33 and 0% respectively, at 15/6^oC in autumn. After the 1982 seeds became nondormant during summer, only 25% entered conditional dormancy when buried at 5^oC, but after the 1981 and 1984 seeds became nondormant, 100% entered conditional dormancy at 5^oC. Thus, the persistent seed bank of V. arvensis at a population site may consist of seeds with an annual dormancy:mondormancy cycle and others with an annual conditional dormancy:nondormancy cycle. This is the first report of the two types of annual seed dormancy cycles in the same species.     

Minimal soil disturbance combined with spring cropping can halt soil seedbank accumulation of Alopecurus myosuroides

The basic mechanism of soil inversion tillage for control of annual weeds is based on the vertical translocation of weed seeds from the soil surface to deeper soil layers. Buried weed seeds either remain dormant in the soil seedbank and are exposed to biological and chemical decay mechanisms, or they germinate but the seedlings cannot reach the soil surface (fatal germination). However, depending on the seed biology of the respective target species, frequent inversion tillage can lead to a build-up of the soil seedbank. For soil seedbank depletion based on available knowledge of the biology of Alopecurus myosuroides seeds, soil inversion tillage is suggested to be reduced to every third or fourth year with reduced or even no-tillage (direct seeding) in between (rotational inversion tillage systems). Including spring crops in the crop rotation could further help dampening the population growth and hence the seed return into the seedbank. This study investigated the effect of rotational inversion tillage in combination with reduced tillage or direct seeding on the soil seedbank and population development of A. myosuroides. In a long-term field trial, set up in 2012, these tillage strategies were compared with continuous inversion tillage in a 3-year crop rotation with two consecutive years of winter wheat (Triticum aestivum) followed by spring barley (Hordeum vulgare). The results showed a significant decline in the soil seedbank following the spring crop, irrespective of the tillage system. The continuous inversion tillage system and inversion tillage before spring cropping with reduced tillage (shallow tillage with a disc harrow) before winter wheat both led to accumulation of seeds in the soil seedbank. In contrast, inversion tillage before spring cropping with direct seeding of winter wheat depleted the soil seedbank significantly after only one crop rotation. Although only covering one intensively studied field site, these findings highlight the need for diversified cropping systems and indicate potential avenues for reducing soil tillage while controlling economically important weeds.     

A comparison of techniques for estimation of arable soil seedbanks and their relationship to weed flora

Summary. Post harvest soil samples taken during the autumn of 1985 and 1986 were split and estimates made of the weed seedbank using two methods: (1) a physical separation of seed from the soil mineral fraction by a sieving/flotation procedure and (2) by placing soil in shallow containers in a greenhouse where seeds could germinate and be periodically counted over a period of eight months. Seedbank estimates derived from each procedure were analysed to determine the suitability of each technique for detecting treatment differences from an experiment evaluating tillage/herbicide effects on weed populations. Both techniques were suitable for determination of seedbank changes due to different tillage treatments and herbicide inputs. The two techniques also proved effective for detection of individual species in the seedbank and the two techniques provided comparable estimates of the relative density of individual weed species in the seedbank. Weed seedbank estimates obtained by the physical extraction procedure from the autumn 1985 soil samples were correlated with weed seedling counts made in the spring of 1986. In most cases, weed seedlings represented less than 10% of the number of seeds estimated by physical extraction the preceding autumn. Individual species seed estimates and subsequent weed counts were poorly correlated which indicated that the seed count estimates alone were poor predictors of weed flora.     

Vegetation and seedbank response to Eragrostis lehmanniana removal in semi‐desert communities

Eragrostis lehmanniana (Lehmann lovegrass) is an invasive perennial bunchgrass in semi‐desert grasslands in the USA. We removed E. lehmanniana using glyphosate at three locations in south‐eastern Arizona in 2003 and 2004 to evaluate the community response. We also sampled the seedbank five times during the course of the experiment to characterise viable seeds at the sites. Two of the sites with histories of livestock grazing responded similarly to the continued removal of E. lehmanniana, with significant increases in native cover and species richness, mainly attributable to herbaceous annual species. Over the course of the study, progressively fewer E. lehmanniana seeds and higher numbers of native annual herb seeds were germinated from seedbank samples of treated plots. At the third site, characterised by a history of intense agricultural use and drier conditions than the other sites, there was a large decrease in E. lehmanniana cover in all plots, followed by an increase in aggressive non‐native species as well as native grasses. The seedbank at this site suggested potential for restoration, with native perennial grass species present in increasing quantities. The results of this study have implications for site restoration, should a means of permanently removing E. lehmanniana be found.     

Demography of Anoda cristata in wide- and narrow-row soyabean

Summary The annual weed Anoda cristata is recognized as a problem of increasing importance in soyabean crops. As seeds are the only source for the renewal of A. cristata populations, knowledge of its seedbank characteristics and seed production is useful in improving weed management decisions. There is a lack of information about the effect of the planting pattern of soyabean on weed population dynamics. The effects of 70-cm and 35-cm soyabean row spacings on A. cristata seedbank, seedling recruitment and mortality, plant biomass, seed production and seed shed were determined. Soyabean density was higher in 35-cm row spacings as an increase in planting rate in narrow-row soyabean is recommended for producers in Argentina. The different planting patterns modified A. cristata demography through changes in plant biomass and, subsequently, in seed production. Seedling emergence, plant mortality and the pattern and duration of seed shed were similar in both planting patterns and thus independent of crop spatial arrangement and density. Increments in A. cristata seedbank can be reduced by planting soyabean at narrower row spacings and by bringing forward soyabean harvest. The first practice reduces A. cristata biomass and consequently seed production, and the second practice decreases the amount of seeds contributing to the soil seedbank.     

Burial depth and cultivation influence emergence and persistence of Phalaris paradoxa seed in an Australian sub-tropical environment

Emergence and persistence characteristics of Phalaris paradoxa seeds in no- and minimum-till situations and at different burial depths were studied in a sub-tropical environment. Three experiments were carried out using naturally shed seeds. In the first experiment, seedlings emerged from May through to September each year, although the majority of seedlings emerged in July. In the second experiment with greater seed density, cultivation in March of each year stimulated seedling emergence, altered the periodicity of emergence and accelerated the decline of seeds in the seedbank compared with plots that received no cultivation. The majority of seedlings in the cultivated plots emerged in May whereas the majority of seedlings in the undisturbed plots emerged in July. Emergence accounted for only 4–19% of the seedbank in both experiments over 2 years. Seed persistence was short in both field experiments, with less than 1% remaining 2 years after seed shed. In the third experiment, burial depth and soil disturbance significantly influenced seedling emergence and persistence of seed. Seedlings emerged most from seed mixed in the top 10 cm when subjected to annual soil disturbance, and from seed buried at 2.5 and 5.0 cm depths in undisturbed soil. Emergence was least from seed on the soil surface, and buried at 10 and 15 cm depths in undisturbed soil. Seeds persisted longest when shed onto the soil surface and persisted least when the soil was tilled. These results suggest that strategic cultivation may be a useful management tool, as it will alter the periodicity of emergence allowing use of more effective control options and will deplete the soil seedbank more rapidly.     

Annual cycles of germinability and differences between primary and secondary dormancy in buried seeds of Echinochloa crus-galli

The seasonal changes in percentage of dormant seeds of Echinochloa crus-galli in the field were recorded for 4 years. The lots of seeds were wrapped in nylon fabric, buried 20 cm under the grass sward and exhumed at monthly intervals. The proportion of seeds germinating under light conditions at a constant temperature of 25 °C fluctuated between 0% and 96%, with maxima in May–July and minima in September–November. Small between-year differences in the course of summer dormancy induction and its winter termination were probably caused by variation of weather conditions.
Attributes of dormancy innate to seeds after maturation (primary dormancy) and dormancy induced in buried seeds during the summer (secondary dormancy) were compared by investigating the rate of dormancy termination during storage of (a) dry seeds at 25 °C, (b) imbibed seeds at 5°C and (c) in seeds buried under field conditions during October–June. Percentage of germination increased faster in secondary than primary dormant seeds at both constant 25 °C and 5 °C. The seeds with primary and secondary dormancy also differed in the response to `germination pre-treatment', a 10-day exposure of imbibed seeds at 25 °C that causes germination of the non-dormant fraction of seed materials. After this treatment the time to resuming germination in primary dormant seeds was substantially increased, whereas the secondary dormant seeds were much less affected. Annual variation in the proportion of germinable seeds explains the low efficiency of autumn soil cultivation for decreasing reserves of E. crus-galli seeds in the soil.     

Relative contribution of genotype, seed size and environment to secondary seed dormancy potential in Canadian spring oilseed rape (Brassica napus)

Secondary seed dormancy has been linked to seedbank persistence of volunteer oilseed rape (Brassica napus) in western Canada. It has been suggested that there is a genetic component to secondary seed dormancy expression in oilseed rape, but little is known of its importance in relation to non‐genetic factors. In a series of experiments we investigated the relative importance of genotype, seed size, time of windrowing and pre‐ and post‐harvest environment on the expression of secondary seed dormancy. We found that genotype contributed between 44 and 82% to the total variation in secondary seed dormancy. A broad range in secondary seed dormancy expression was observed among 16 genotypes examined. Nevertheless, three‐quarters of the genotypes investigated exhibited relatively high potential for the expression of secondary seed dormancy (back‐transformed mean 71% dormant seeds). Seed size contributed 21% to the total variation, while the influence of seed maturity (harvest regime) on secondary seed dormancy expression was negligible. Despite diverging environmental conditions during the four growing seasons spanning these experiments, the influence of pre‐harvest environment on seed dormancy expression was relatively small and ranged from 0.1% to 4.5%. Secondary seed dormancy potential decreased over time during seed storage. This decrease was greatest when seeds were stored at ambient temperatures and least when seeds were stored at ?70°C.     

Assessment of management options for Salsola australis in south-west Australia by transition matrix modelling

A matrix model of the life cycle of Salsola australis was constructed, based on population ecology data collected from the district of Lake Grace, Western Australia. The model was used to assess potential control strategies for this summer annual weed within the Western Australian broad acre grain cropping system. The population growth rate (λ) of S. australis in the absence of weed control strategies was 1.49 and was virtually unaffected by the dormant seedbank. However, λ increased to 8.21 if it was assumed that a constant number of seed immigrated into the area in question from neighbouring populations of S. australis , through farm-scale seed dispersal. As a result, effective weed management depended on reducing seed dispersal. The model determined that burning all senesced, mobile plants in late autumn, combined with herbicide control of the largest cohorts of S. australis in summer and autumn, reduced population growth rate to 0.79. This control strategy resulted in a 66.1% chance of the population becoming extinct over 25 years. Management strategies are proposed based on the results of the models and further research is required to validate their effectiveness and practicality in the field.     

The Potential Geographic Range of Pyrenophora semeniperda

ABSTRACT There is no evidence that Pyrenophora semeniperda, the causal agent of leaf spotting in many annual and perennial grasses, currently occurs in Europe or Asia. However, there is potential phytosanitary concern that the importation of infected commodities could result in the introduction of this fungus into Eurasia, putting crops at risk and possibly resulting in economic losses. To assist in assessing the risk of geographic range extension of P. semeniperda, an analysis was undertaken to estimate the potential global distribution of this species, based on climatic suitability. Geographic distribution data for P. semeniperda in part of its current range were used to fit parameter values in a CLIMEX pest risk assessment model, and the remaining distribution data were used to validate the model. The CLIMEX model correctly predicts that virtually all locations where P. semeniperda has been found are climatically suitable. Only five locations worldwide where the fungus was recorded present are predicted as being unsuitable. These "outliers" may have been transient populations occurring during a favorable season and then dying out. Exploratory adjustments of the model to accommodate these records created unsatisfactory distortions in the projected climatic suitability surfaces, extending the suitable climatic zone beyond well-established traditional range boundaries. We are therefore confident that the model is credibly predicting the potential distribution of P. semeniperda worldwide. The CLIMEX model suggests that P. semeniperda could potentially extend its range throughout Europe and temperate regions of Asia, Africa, and South America. Our heavy reliance upon geographic data to build this CLIMEX model departs from most previous published examples in plant pathology, which have depended primarily upon experimentally derived physiological data to estimate model parameters. The use of geographic data to infer climate parameters is popular in CLIMEX models of weeds and arthropod pests and can provide decision-makers with early risk assessments of potential pathogen invasions, particularly where the pathogens have long, or difficult-to-study, lifecycles.     

Role of temperature in regulating timing of germination in soil seed reserves of Thlaspi arvense L.

Summary. Most freshly-matured seeds of Thlaspi arvense L. (Brassicaceae) were dormant at maturity in May. Seeds sown on soil germinated in autumn and spring, but mostly in autumn. Buried seeds exhumed at monthly intervals and tested in light and darkness over a range of thermoperiods exhibited annual dormancy/non-dormancy cycles. However, the dormant period was short, usually only in April, but sometimes May, and in some years 1–6% of the seeds remained conditionally dormant. After-ripening occurred during summer, and seeds were non-dormant during autumn. Seeds entered conditional dormancy in winter and dormancy in late winter or early spring. When buried dormant seeds were kept at 25/15, 30/15 or 35/20°C for 12 weeks, they gained the ability to germinate to 95–100% at 15/6, 20/10, 25/15, 30/15 and 35/20°C. After burial for 12 weeks at 15/6 and 20/10°C, seeds germinated to 80–100% at 15/6, 20/10 and 25/15°C. but to only 11–64% at 30/15 and 35/20°C. After 4 weeks at 5°C, initially-dormant seeds germinated to 100% at all thermoperiods except 35/20°C, where only 15% of them germinated. However, after 18 weeks at 5°C, only 0–1% of the seeds germinated at all thermoperiods. Most non-dormant seeds exposed to 1, 5 and 15/6°C for 16 weeks were induced into dormancy; 1–15% entered conditional dormancy and thus germinated only at 15/6, 20/10 and 25/15°C. This study indicates that seeds of winter annual plants of T. arvense are non-dormant in autumn and enter dormancy in winter, while those from summer annuals are dormant in autumn and become non-dormant during winter.     

Fates of Setaria faberi and Abutilon theophrasti seeds in three crop rotation systems

Weed seeds in and on the soil are the primary cause of weed infestations in arable fields. Previous studies have documented reductions in weed seedbanks due to cropping system diversification through extended rotation sequences, but the impacts of different rotation systems on additions to and losses from weed seedbanks remain poorly understood. We conducted an experiment in Iowa, USA, to determine the fates of Setaria faberi and Abutilon theophrasti seeds in 2‐, 3‐ and 4‐year crop rotation systems when seed additions to the soil seedbank were restricted to a single pulse at the initiation of the study. Over the course of the experiment, seedlings were removed as they emerged and prevented from producing new seeds. After 41 months, seed population densities dropped >85% for S. faberi and >65% for A. theophrasti, but differences between rotation systems in the magnitude of seedbank reductions were not detected. Most of the reductions in seedbank densities took place from autumn through early spring in the first 5 months following seed deposition, before seedling emergence occurred, suggesting that seed predation and/or seed decay was important. For S. faberi, total cumulative seedling emergence and total seed mortality did not differ between rotation systems. In contrast, for A. theophrasti, seedling emergence was 71% lower and seed mortality was 83% greater in the 3‐ and 4‐year rotation systems than in the 2‐year system. Results of this study indicate that for certain weed species, such as A. theophrasti, crop rotation systems can strongly affect life‐history processes associated with soil seedbanks.     

The dormancy of wild oat seed (Avena fatua L.) from plants grown under various temperature and soil moisture conditions

Wild oat plants of types fA, fB and fC were grown at a constant 15 or 20°C during the period of seed maturation. Seed of the three types differed little in dormancy when grown at 15°C, but at 20°C a larger proportion of seeds of type fA were dormant compared with fB or fC. Overall, dormancy of seed produced at 15 and 20°C was 97 and 63% respectively. Plants of another collection of type fB were grown from seed at 15 or 20°C with or without water stress applied only from the time of panicle emergence. Water stress and high temperature reduced viable seed production. Seed dormancy was tested immediately after collection by planting the seed in soil, and by Petri dish tests. Further Petri dish tests were made after 6 months storage. Seedling emergence in the first autumn from seeds of plants matured without water stress at 15°C was 10% compared with 30% for seeds grown at 20°C. Seeds grown with water stress at 15°C gave 47%, and at 20°C 78% emergence. The majority of emergence from seeds formed at 15°C without water stress occurred in the second spring after burial. Petri dish tests support these findings and suggest that seeds produced in hot dry summers are less dormant than those produced in cool moist ones.     

Early autumn soil disturbance decreases persistence of volunteer summer‐annual oilseed rape (Brassica napus)

Volunteer summer‐annual oilseed rape (sOSR; Brassica napus) is an ongoing concern in Canadian crop production. Large harvest seed losses and secondary dormancy in this species generate a persistent volunteer seedbank. Yield loss in subsequent crops, potential sOSR oil profile contamination and herbicide‐resistance trait introgression create a need for effective sOSR seedbank management. This field study evaluated the effects of timing and type of implement of post‐harvest soil disturbance and seeding a winter cereal on volunteer sOSR population persistence and demographic life‐stage transition rates at five locations in Manitoba, Canada. Following sOSR harvest and supplemental seed rain, seedbank densities ranged from 6770 to 15360 and 50 to 2610 seeds m^?2 among sites in autumn and spring respectively. In contrast to European research on winter‐annual oilseed rape, early autumn soil disturbance, shortly after sOSR harvest, was the best strategy to decrease volunteer sOSR persistence (3% population persistence from autumn to spring, compared with 6% in zero tillage). Substantial autumn seedling recruitment (38% of the autumn seedbank) and subsequent winterkill contributed to lower population persistence. Soil disturbance in spring stimulated spring seedling recruitment compared with other disturbance timings (11% and 3% of the spring seedbank, respectively). The implement used for soil disturbance and seeding winter wheat (Triticum aestivum) had minimal effect on population persistence. This research showed that timing of post‐harvest soil disturbance should be utilised as an effective tactic to decrease population persistence of volunteer sOSR via stimulation of autumn seedling recruitment and concomitant winterkill.     

Life cycle and potential gene flow of volunteer oilseed rape in different tillage systems

The study examined the effect of tillage (intensive vs. zero tillage) on potential gene flow during the life cycle of oilseed rape volunteers between July 2002 and August 2003. After growing oilseed rape, 4–29% of the seeds lost during harvest entered the soil seedbank when stubble tillage was performed immediately after the seed input. The seedbank was small (0–3%) when stubble tillage was delayed. Zero tillage resulted in seedbanks from 1 to 17% of the initial seed input. The seeds were distributed mainly in the upper soil layers after zero tillage or primary tillage with a rigid tine cultivator, whereas ploughing shifted most of the seeds into deeper layers. The highest number of volunteers (1 plant m⁻²) emerged and flowered in the following crop of winter wheat either when a large soil seedbank existed and/or the seedbank was located mainly in the upper soil layer. Outcrossing with other rape crops was unlikely as volunteers flowered 1 month later than rape crops sown at the normal timing. These volunteers produced a maximum of 8 viable seeds m⁻². Ploughing preserved seeds in deep soil layers transferring the risk of gene flow to the future, whereas non-inversion tillage can cause gene flow from high numbers of flowering volunteers within the first year following oilseed rape cultivation.     

Purple witchweed (Striga hermonthica) germination and seedbank depletion under different crops,fallow, and bare soil

Seedbank density is an important aspect that determines the amount of damage that the parasitic weed, purple witchweed (Striga hermonthica; hereafter, called “Striga”), causes on its crop hosts. The seedbank depletion of Striga was measured in Mali and Niger during the 2004 rainy season under the host crops, pearl millet and sorghum, the non‐host crops, cowpea and sesame, the intercrops of pearl millet or sorghum with cowpea or sesame, and fallow with or without weeding. Two methods were used and compared; namely, a seed bag method and a soil‐sampling method. The fate of the seeds was assessed by a seed press test. Seed germination, as determined by the presence of empty seed coats, contributed most to the seedbank depletion of Striga under a variety of crop covers and fallow. The highest seedbank depletion was found under the monocultures of the host crops. The intercrops of the host and non‐host crops caused less seedbank depletion, followed by the monocultures of the non‐host crops, fallow, and bare soil. The seed bag method and the soil‐sampling method yielded similar percentages of seedbank depletion, while the former allowed for distinguishing between the germinated and diseased seeds. The results suggest that, although all the tested crop species can cause the seed germination and seedbank depletion of Striga, management by using host cereal crops causes the highest amount of germination and has the highest potential to deplete the soil seed bank, provided that seed production is prevented.     

Temperature requirements for germination of buried seeds of Aethusa cynapium L.

Freshly-collected mature mericarps of Aethusu cynapium were dormant, but some germinated at alternating (16 h low/8 high) temperatures when the seed coverings were removed. Burial during winter increased percentage germination and the temperature range over which it took place. In late spring the range narrowed, first at low and then at higher temperatures, widening again in autumn. Moist storage at both low (4°C) and high (30°C) temperatures overcame dormancy, but exposure to 30°C inhibited subsequent germination at low temperatures. Germination of intact mericarps was consistently lower than that of de-coated seeds. The cyclic change in dormancy status of the seeds appears to interact with the restricting effects of the seed coverings and perhaps other factors in determining the consistent pattern of spring emergence in A. cynapium.