Comparison of extraction procedures for detection of Xanthomonas axonopodis pv. phaseoli in common bean seed

Xanthomonas axonopodis pv. phaseoli (Xap) is an important seedborne pathogen of Phaseolus vulgaris. Accurate seed health testing methods are critical to protect seed quality and meet phytosanitary requirements. Currently employed selective media‐based methods include several variations in extraction procedures. In order to optimize pathogen extraction from seeds, the influence of different extraction steps on the sensitivity of Xap detection was assessed. Seeds were inoculated by vacuum infiltration with Xap to achieve inoculum levels from 10¹ to 10⁵ CFU per seed; one contaminated seed was mixed into 1000‐seed subsamples of uncontaminated P. vulgaris seeds. Thirty subsamples of 1000 seeds were tested using each different extraction procedure. These included soaking whole seeds in sterilized saline phosphate buffer, either overnight at 4°C or for 3 h at room temperature, with or without vacuum extraction, and either with or without concentrating the seed extract by centrifuging. Seed extract dilutions were cultured on semiselective agar media MT and XCP1. The percentages of positive subsamples were compared to measure the effects of each extraction step on detection sensitivity. Vacuum extraction and centrifugation of seed extracts increased sensitivity; the highest sensitivity was obtained with the 3 h vacuum extraction followed by centrifugation. These results were confirmed with naturally infested seeds; Xap was detected in 48 of 70 samples using the 3 h vacuum extraction with centrifugation, whereas only 35 of 70 field samples tested positive using overnight soaking, a significant difference. The results suggest that these steps would be valuable modifications to the current method approved by the International Seed Testing Association (ISTA).     

Factors that influence the level of weed seed predation: A review

Herbicides applied alone have failed to reduce weed populations in agricultural ecosystems. The use of this management strategy has followed other problems such as damage to ecosystems and organisms that live in it. Herbicide resistance in weeds has also occurred more quickly based on the application of herbicides. Today, most scientists are looking for new integrated weed management programs in agricultural ecosystems. Biological control is one of the most effective ways of weed management. In this way, living organisms are used to reduce and control weed species in arable lands. Weed seed predators, including predispersal and postdispersal predators as biological control agents, can be primary factors of weed seed mortality in agricultural ecosystems. This review has discussed factors influencing seed predator populations, activity, and granivory. Knowledge of the interrelations between factors and seed predators can contribute to designing future strategies that augment weed seed predator activity and enhance weed seed consumption as a component of an integrated weed management system.     

Seed retention of ten common weed species at oat harvest reveals the potential for harvest weed seed control

We assessed the seed production and shedding pattern of 10 common weed species in two oat fields in Denmark. The aim was to evaluate the possibility of harvesting retained seeds on weeds at crop harvest by a combine harvester based on estimation of weed seed retention. Before flowering, ten plants of each weed species were selected and surrounded by a seed trap comprising of a porous net. When the plants started shedding seeds, the seeds were collected from the traps and counted weekly until oat harvest. Just before oat harvest, the retained seeds on the plants were counted and the ratio of harvestable seeds and shed seeds during the growing season was determined. The seed production and shedding patterns varied between the 2 years. Across both years, Anagallis arvensis L., Capsella bursa-pastoris L. Medik, Chenopodium album L., Geranium molle L., Persicaria maculosa Gray, Polygonum aviculare L., Silene noctiflora L., Sonchus arvensis L., Veronica persica Poir. and Viola arvensis Murray retained on average 61.6%, 52.7%, 67.2%, 58.4%, 32.05%, 59.5%, 95.7%, 23.5%, 51.7% and 33.9%, respectively, of their produced seeds at crop harvest. Silene noctiflora was classified as a good target for harvest weed seed control; C. bursa-pastoris, C. album, G. molle, P. maculosa, S. arvensis and V. arvensis were classified as intermediate targets; and A. arvensis, P. aviculare and V. persica were classified as poor targets.     

Correlation between the soil seed bank and weed populations in maize fields

Annual weed populations establish every year from persistent seed banks in the soil. This 3 year study investigated the relationship between the number of weed seeds in the soil seed bank and the resultant populations of major broadleaf and grass weeds in 30 maize fields. After planting the crop, 1 m² areas were protected from the pre-emergence herbicide application. Soil samples were collected soon after spraying to a depth of 100 mm and the weed seeds therein were enumerated. The emerged weed seedlings in the field sampling areas were counted over the following 8 weeks. Up to 67 broadleaf species and five grass weeds were identified, although not all were found at every site and some were specific to a region or soil type. For the most abundant weeds in the field plots, on average 2.1–8.2% of the seeds of the broadleaf species and 6.2–11.9% of the seeds of the grass weeds in the soil seed bank emerged in any one year, depending on the species. Overall, the results showed a strong linear relationship between the seed numbers in the soil and the seedling numbers in the field for all the grasses and for most broadleaf weeds. For some species, like Trifolium repens , only a weak relationship was observed. In the case of Chenopodium album , which had the largest seed bank, there was evidence of asymptotic behavior, with seedling emergence leveling off at high seed numbers. An estimate of the soil seed bank combined with knowledge of the germination and behavior of specific weed species would thus have good potential for predicting future weed infestations in maize fields.     

Diversity in weed seed production and the soil seed bank: Contrasting responses between two agroecosystems

This study was carried out to compare the diversity in seed production and the soil seed bank in a dryland and an irrigated agroecosystem in the dry tropics. Both agroecosystems showed a comparable number of species, but only 25% and 38% similarity during the winter and rainy cropping seasons, respectively. In the irrigated agroecosystem, the amount of seed production diversity was almost double in the winter season, compared to the rainy season. The weed seedbank diversity was low but was sensitive to cropping practices and seasons in both agroecosystems. A considerably smaller soil seedbank size in the irrigated agroecosystem (cf. dryland) was related to lowered weed seed production. The dryland agroecosystem showed a greater accumulation of the seeds of broad‐leaved weeds, whereas the irrigated agroecosystem accumulated more seeds of the grasses or sedges. About three‐fourths of the seeds during the winter season were accounted for by Anagallis arvensis and Chenopodium album in the dryland agroecosystem and by C. album and Melilotus indica in the irrigated agroecosystem. However, during the rainy season, Ammannia baccifera, Echinochloa colona and Cyperus rotundus dominated in both agroecosystems. The changes in the weed seed bank and its diversity are mainly attributed to differences in water management, which tends to reduce species diversity, especially at a lower depth, but leads to the dominance of some potentially noxious weeds (e.g. Phalaris minor and M. indica). Approximately double the soil seedbank size and a greater diversity at a lower depth might indicate an adaptive mechanism in the storage of weed seeds in the dryland agroecosystem.     

A survey of weed seed contamination of rice paddy in Cambodia

Cambodia has experienced a rapid shift from transplanted to hand broadcast seeded rice, with a consequent increase in seeding rates from 25–30 to 100–200 kg ha^?1. To reduce costs, farmers keep their own seed for sowing with the risk of greater weed seed contamination of the sowing seed. A survey of weed seed contamination in harvested rice paddy was conducted in two provinces of Cambodia (Battambang and Takeo) at the end of the wet season in 2016. Farmers were interviewed about rice‐seeding practices, and a total of 110 farmers' fresh paddy samples were inspected for weed seed contamination from the two provinces. Sowing seed samples collected from 28 seed producer lots and 71 samples of farmer‐kept seed were also analysed for weed seed contamination. In both provinces, the majority of farmers kept their own seed or bought seed from a neighbour. Farm‐kept seed for sowing accounted for 88% of sown seed in Battambang and 89% in Takeo. Seeds of 41 different weed species from 13 plant families were found in the farmers' freshly harvested paddy samples. Overall, farmers managed to reduce the number of weed propagules by 60% and seed producers by 95%. There was no significant difference between farmer‐kept seed and seed producer/seed company seed for the total number of weed seeds present. When shown photos, farmers' rankings of the 10 most common weed species found in freshly harvested paddy did not closely correspond to the actual weed seed frequency in the paddy. When farmers were asked to rank the frequency of weeds in their fields without the option to choose from a list, they ranked the weeds differently. Farmers ranked Ischaemum rugosum, Echinochloa spp. and Fimbristylis miliacea as the three most frequent weed species in their fields. The most frequent weeds in harvested paddy, apart from weedy rice, were I. rugosum and Melochia corchorifolia. Farmers did not rank M. corchorifolia as a frequently occurring weed, and most farmers could not recognise M. corchorifolia from photographs. The priority for improved seed hygiene is to place the emphasis on assisting farmers to further improve their seed purification techniques and to caution them to inspect seed before purchasing from neighbours, seed producers and seed companies in the absence of the implementation of seed certification regulation.     

进境菜豆种子中普通细菌性疫病菌的分离和鉴定

对经甘肃口岸进境的30批菜豆Phaseolus vulgaris种子进行了普通细菌性疫病菌的检测,利用选择性培养基MT从波兰进境菜豆种子上分离到1株细菌597,对该分离物进行菌落形态特征观察、致病性测定、16S rDNA及16S-23S rDNA ITS序列分析和特异性PCR检测。结果表明,该分离物在MT培养基上菌落呈黄色、圆形、黏稠、表面光滑向外隆起、菌落周围有水解圈。分离物597接种菜豆幼苗后导致叶片枯萎,接种点干枯。结合菌落形态、16S-23S rDNA ITS序列、特异性PCR检测结果,将分离物597鉴定为地毯草黄单胞杆菌菜豆致病变种Xanthomonas axonopodis pv.phaseoli。     

沿江圩区长期稻麦连作田土壤杂草种子库的研究

通过对沿江圩区长期稻麦连作田土壤杂草种子库研究,结果表明,土壤杂草种子库中小麦田有8科14种、水稻田有9科17种。2种类型田块的种子库主要分布在0～10cm的土层中。小麦田该层出草量达77.74%,水稻田为72.05%。麦田土壤总出草量为11930株/m2,主要是日本看麦娘、草和稻槎菜,出草量占总量的59.96%;稻田土壤总出草量达9855株/m2,主要是稗草、节节菜、千金子和异型莎草,出草量占总量的51.03%。上述7种杂草是该区农田的主要杂草,均为当前防除的重点,其中日本看麦娘和稗草分别是小麦、水稻田的优势种。通过研究提出了减少土壤杂草种子库种源的对策。     

Occurrence and transmission of sowbane mosaic virus in seed from naturally infected plants of spinach (Spinacia oleracea)

Sowbane mosaic virus was isolated as a single infectious component from seedlings and seeds from a seed-lot of spinach (Spinacia oleracea) propagated in Hungary, and was identified by experimental host-range, thermal-inactivation point, dilution end-point, electron microscopy and serology.Chenopodium quinoa was more susceptible and/or sensitive to infection thanC. amaranticolor and was a better indicator host, butC. amaranticolor is of diagnostic value. This is the first report on natural infection of spinach by the virus and on its natural seed-transmission in spinach. Growing-on tests on whole seeds and infectivity tests on separate embryos and seed-coats showed that over 30% of the seeds' embryos were infected and c. 80% of the seed-coats contained the virus. The incidence of infection in the spinach crop from which the seed was obtained must have been high.     

Comparison of three methodologies for efficient seed extraction in studies of soil weed seedbanks

Determination of weed seeds in the soil is tedious and time consuming. To evaluate the different seed extraction methods to improve seedbank estimations, three extraction methods (sieving, cloth bag and flotation) were compared, based on their accuracy and time needed for separation processes and enumeration. Seeds of Datura stramonium,Amaranthus retroflexus,Portulaca oleracea and Plantago major were used to artificially infest soil samples of four textures, namely clay, clay loam, loam and sandy loam containing 19%, 27%, 38% and 65.5% sand respectively. Soil textures had a significant effect on counting time in all extraction methods. In flotation, four submethods involving different solutions and centrifuge rotation speed were examined, but showed no differences, so data were pooled. Counting time in flotation, cloth bag and sieving methods was 9, 16 and 30 min respectively. However, when the time needed for other processes was taken into consideration, flotation and bag methods did not differ significantly. Species‐wise seed recovery was not affected by soil texture in the bag method, suggesting an advantage for this approach as its accuracy is not soil texture‐dependent. Total seed recovery for flotation, sieving and bag methods was 61%, 67% and 75%, respectively, and was not significantly different. Considering the efficiency of methods, the cloth bag technique could be recommended, because it was as time consuming as the flotation method, but required the same minimum equipments and costs as the sieving method.     

Ecological aspects of seed dormancy-break and germination in Heteranthera limosa (Pontederiaceae), a summer annual weed of rice fields

Summary Heteranthera limosa seeds were buried in flooded and in non-flooded soil and exposed to natural seasonal temperature changes in Lexington, Kentucky, USA. Seeds exhumed after various periods of burial ranging from 2 to 36 months were tested for germination under both flooded and non-flooded conditions. Seeds were dormant at maturity in September and became non-dormant during winter. Seeds buried in non-flooded soil during winter germinated to higher percentages and over a wider range of temperatures when tested under flooded conditions (in light) during spring and summer, than did those buried in flooded soil during winter. Thus, the water regime associated with rice culture (non-flooded in winter and flooded in summer) is optimal for dormancy-break and germination of H. limosa seeds. A portion of the buried seeds exhibited an annual dormancy/non-dormancy cycle, whereas others had a conditional dormancy/non-dormancy cycle. Regardless of the type of cycle, seeds buried in non-flooded soil retained the ability to germinate in light at high temperatures under flooded conditions throughout the summer. Thus, seeds potentially can germinate at any time during the growing season, whenever rice fields are flooded. Flooding fields during winter and/or sowing rice relatively early in the growing season may help in establishing rice before seeds of H. limosa germinate.     

The influence of the soil management systems on aboveground and seed bank weed communities in olive orchards

The biodiversity of farmed landscapes is, in the context of agricultural intensification, a key aspect with regard to improving the sustainability of agroecosystems. Olive groves are undergoing rapid changes because of the spread of intensive farming systems, which may have negative environmental impacts. This paper reports a survey on the aboveground flora and seed banks in five olive groves located in Andalusia (Southern Spain). In this study, the following three management systems have been compared: no‐tillage, with the mowing of spontaneous weedy vegetation; no‐tillage, with the mowing of planted cover crops (Poaceae); and conventional tillage practices. Results showed that coverage and an abundance of vegetation are favored by spontaneous weedy vegetation with mowing management, while the richness of aboveground species was affected by landscape diversity and the presence of edges, which increases the richness and diversity of aboveground flora species in olive groves. Seed bank composition showed a low relationship with aboveground flora in the three cover crop management systems. The multivariate analysis performed pointed to those seed species that have a major influence on the aboveground flora communities of each of the three agricultural systems. The seed bank was clearly impoverished in terms of both abundance and species richness after the long‐term conventional tillage practices. We conclude that the intensive long‐term conventional tillage dramatically reduces weed communities in olive orchards and the subsequent ecosystem services provided by them.     

The effect of crop,cultivation and seed addition for birds on surface weed seed densities in arable crops during winter

Weed seeds present an agronomic threat, but are also an important food resource for wildlife in winter. Weed seed densities on the soil surface in winter were examined from 1999 to 2002 in 105 fields on three different farms in UK. The effect of the preceding crop, cultivation, position within the field and the application of seed for birds (bird seed) on surface seed abundance and species composition was tested. Six or fewer species comprised c. 80% of the weed seeds. By January of each study year, the densities of seeds important for farmland birds (key seeds) were 73% or 87% lower compared with early winter on two of the farms, but were stable on the third where seeds were incorporated through cultivation. At the edge and mid‐field, seed densities only exceeded 400 m^?2 in 17%, 10% and 12% of fields for total, key and dicotyledonous seeds respectively. The preceding crop only affected seed densities at one site; stubbles of winter barley had fewer seeds compared with winter wheat or spring barley. Seed densities varied between the edge and mid‐field, but trends were inconsistent between sites. The density of the larger seeds (Atriplex patula, Viola arvensis, Polygonum aviculare and Chenopodium album) were reduced in fields receiving bird seed. The objectives of weed control and conservation may not be mutually exclusive because seed return was most reduced where the ground remained uncultivated through the winter, yet this also provided the best foraging opportunities for surface feeding seed predators.     

Factors affecting the seed germination and seedling emergence of redflower ragleaf (Crassocephalum crepidioides)

Redflower ragleaf (Crassocephalum crepidioides) is a weed, as well as a minor vegetable, in tropical and subtropical regions of the world. The influence of environmental factors and seed conditions on the germination and emergence of redflower ragleaf have been evaluated in order to help understand its distribution and to develop effective management strategies. The seeds germinated at a constant temperature in the range of 10–30°C and reached a maximum at 15–20°C. The highest germination rate was recorded at an alternating temperature of 20/15°C (day/night). The seeds germinated over a wide pH range (2–12), with the highest germination rate at between 4 and 10. Germination under saturated and flooded conditions was also high. The germination of seeds from opened (mature) capitula was significantly higher than from partially opened or unopened capitula. The germination of seeds without a pappus was significantly higher than for seeds with a pappus. The germination rate of 1 year old seeds decreased drastically when compared to that of freshly harvested seeds. The seedling emergence rate was ～63% for those seeds placed on the soil surface, but no seedling emerged from a depth of ≥1 cm. These results indicate that redflower ragleaf seeds can germinate in various environmental conditions, but that the percentage that germinates will be different in different environments. Regeneration could be effectively prevented by at least a 1 cm soil covering or by destroying the plant before the capitula open. In contrast, freshly harvested seeds from opened capitula should be sown on the soil surface when redflower ragleaf is to be cultivated as a vegetable.     

Comparison of growth,survivorship, seed production and shedding of eight weed species in a wheat crop in Western Australia

Development of integrated weed management strategies is dependent on a thorough knowledge of the demography of individual species. The current research established eight winter or summer weed species in a winter annual wheat cropping system at Wongan Hills, Western Australia, and investigated emergence of the first cohort of each species, survivorship, plant size, seed production and seed shedding over three years (2016–2019). The winter weeds Bromus diandrus and Lolium rigidum emerged at the same time as the wheat crop, and the initial cohort of marked plants had 100% survival to seed production in each year. By comparison, other winter weed species like Hordeum leporinum, Rumex hypogaeus, Sonchus oleraceus and Polygonum aviculare frequently emerged later than the crop and had a lower percentage of plants surviving to seed production. However, individual S. oleraceus and P. aviculare plants had the greatest seed production compared to other species. All winter weeds had variable patterns of seed shedding between years, with the exception of L. rigidum. Summer weed species emerged at the same time, but plants in the initial cohort of each species did not always survive to produce seed. The early emergence and high survivorship of B. diandrus indicates high competitive ability, but shedding commenced at a similar time to L. rigidum and harvest weed seed control may be a viable control method for this species.     

Real‐time quantitative PCR assays for the rapid detection and quantification of Fusarium oxysporum f. sp. phaseoli in Phaseolus vulgaris (common bean) seeds

Fusarium oxysporum f. sp. phaseoli (Fop) is a devastating pathogen that can cause significant economic losses and can be introduced into fields through infested Phaseolus vulgaris (common bean) seeds. Efficient seed health testing methods can aid in preventing long‐distance dissemination of this pathogen by contaminated seeds. In order to improve detection of Fop in seed, a rapid, accurate and sensitive real‐time PCR assay (qPCR) protocol was developed for detection of Fop in common bean seeds. Seed lots of seven cultivars with infection incidence ranging from 0·25 to 20% were prepared by mixing known amounts of Fop‐infected seeds with Fop‐free seeds. Direct comparisons between SYBR Green and TaqMan qPCR methods were performed using primers based on the Fop virulence factor ftf1. The primers developed in this study produced a 63 bp product for highly virulent strains of Fop but did not produce an amplicon for nonpathogenic or weakly pathogenic isolates of F. oxysporum from P. vulgaris or other hosts. Under optimized conditions, both qPCR assays detected Fop infection at low levels (0·25%); however, the results suggest the TaqMan assay was more reliable at quantification than the SYBR Green assay. Linear regression models were fitted to the relationships between results of qPCR assays and infection incidence, but the models differed among cultivars. Fungal biomass per seed differed among cultivars and was related to seed size. The results indicate that the TaqMan assay developed in this study is a useful tool for the detection and quantification of Fop in bean seeds.     

Detection of Acidovorax valerianellae in corn-salad seeds, seed transmission of the pathogen and disease development in the field

Acidovorax valerianellae , the causal agent of bacterial black spot of corn salad and responsible for severe economic losses to this vegetable in France, was successfully transmitted to corn-salad plants by artificially inoculated seeds in glasshouse and field experiments. In the field experiments, climatic data recorded under plastic tunnels indicated that increasing temperature and relative humidity increased symptom development. To investigate the possible contamination of commercial seedlots of corn salad, a seed test was developed consisting of soaking batches of seeds (five batches each of 5000, 1000, 500 and 100 seeds) overnight at 4°C in distilled sterile water, followed by dilution-plating of seed extracts on TSAV (tryptic soya agar for A. valerianellae ) semiselective medium. Suspected colonies were identified by biochemical and pathogenicity tests or, within 24 h, using antibodies specific to A. valerianellae . Acidovorax valerianellae was detected in three lots. Seed infection levels ranged from 0·10 to 0·89% of contaminated seeds and a single seed carried up to 1800 A. valerianellae colony-forming units.     

Effect of different weed management systems on the weed populations, and seedbank composition and distribution in tropical coconut plantations

Weeds are a perennial problem in coconut plantations and cause significant losses in the nut yield. The occurrence of a wide range of weeds also causes difficulties in their eradication. The influence of five different weed management practises on the distribution and composition of the soil weed seed bank in coconut plantations in the low-country dry zone of Sri Lanka was evaluated. The treatments imposed included the application of glyphosate (N-[phosphonomethyl]-glycine), cover cropping with Pueraria phaseoloides , tractor harrowing, tractor slashing, and tractor plowing. All the treatments were applied twice per year, except for the cover cropping treatment. In terms of a reduction in the weed biomass, the application of glyphosate and cover cropping ( Pueraria ) were more efficient in reducing the ground weed population. These methods were very effective in reducing the weed seed density in the top soil layers. Plowing and harrowing significantly reduced the seed bank in the top soil layers and shifted significant numbers of weed seeds to deeper soil profiles. However, the total germinated weed seed count increased by 123.5, 691.4, 1133.1, and 1216.5% in the 10–15, 15–20, 20–25, and 25–30 cm soil depths, respectively, compared with the initial germinated weed seed count in the plowing treatment. Considering all the soil layers, the decline in the germinating weed seed count was very high in the treatment plots with cover cropping and the application of glyphosate; thus, these are considered to be the best practises to reduce the germinating weed seed count in the soil of coconut plantations.