Germination rates of weedy radish populations (Raphanus spp.) altered by crop‐wild hybridisation,not human‐mediated changes to soil moisture

Cultivated plants are known to readily hybridise with their wild relatives, sometimes forming populations with weedier life‐history strategies than their progenitors. Due to altered precipitation patterns from human‐induced global climate change, crop‐wild hybrid populations may have new and unpredictable environmental tolerances relative to parental populations, which would further challenge farming and land‐management weed control strategies. To recognise the role of seed dormancy variation in weed invasion, we compared seedbank dynamics of two cross‐type populations (wild radish, Raphanus raphanistrum, and crop‐wild hybrid radish, R. raphanistrum × R. sativus) across a soil moisture gradient. In a seed‐burial experiment, we assessed relative rates of seed germination, dormancy and seed mortality over two years across cross types (crop‐wild hybrid or wild) and watering treatments (where water was withheld, equal to annual rainfall, or double annual rainfall). Weekly population censuses in 2012 and 2013 assessed the frequency and timing of seedling emergence within a growing season. Generally, germination rates were two times higher and seed dormancy was 58% lower in hybrid versus wild populations. Surprisingly, experimental soil moisture conditions did not determine seedbank dynamics over time. Yet, seed bank dynamics changed between years, potentially related to different amounts of annual rainfall. Thus, variation in seedbank dynamics may be driven by crop‐wild hybridisation rates and, potentially, annual variation in soil moisture conditions.     

Seed transmission of maize downy mildew (Peronosclerospora sorghi) in Nigeria

In an area of Nigeria where downy mildew of maize is present, histological assessment of maize seed revealed the presence of mycelium and oospores of Peronosclerospora sorghi in the kernels. Seed transmission of downy mildew of maize was demonstrated when grain purchased at local markets gave mean seedling infection rates of 12·3% (untreated seeds) and 10·0% (in metalaxyl-treated seeds) within 7 days of emergence, after storage in a desiccator for 30 days. When untreated seeds taken from nubbin ears of systemically infected plants from four states in southern Nigeria were planted at 9 days (17–22% moisture content) and 27 days (9–22% moisture content) after harvest, 20·0% infected seedlings resulted in both trials. Seeds from Borno state in northern Nigeria had 26·6% systemic seedling infection after 9 months of storage at 11% moisture content. When seeds harvested from maize plants inoculated with P. sorghi through silks were examined histologically, hyphae of P. sorghi were observed mostly in the scutellum of the embryo. Transmission of disease to seedlings was observed when the silk-inoculated seeds (9% moisture content) were planted in pots in a greenhouse; however, no disease transmission was observed when such seeds were planted in the field. The epidemiological significance of seed transmission is discussed with particular reference to survival of inoculum and development of epidemics. Also noteworthy is the overall significance of seed transmission in Nigeria, where the major source of seed is that saved by farmers from their grain crop, occasionally supplemented by seed bought from the local market.     

黄瓜花叶病毒CA株系在花生上流行的研究

 据1985~1990年在密云县试验观察,黄瓜花叶病毒(CMV)在花生上流行受病毒种传率、苗期蚜虫发生数量及苗期温度和雨量的影响。花生苗期和花针初期为易感病时期,随后抗性有所增强,但不明显。苗期和花针期发病植株,CMV种传率最高,之后带毒率有所下降。CMV种传率高低与病害流行关系,在同一年份呈极显著正相关关系,而在年度间未达显著水平。花生田间蚜虫发生盛期与病害发生高峰期是吻合的,蚜虫发生数量与病害流行关系显著。根据花生种传率和苗期蚜虫发生数量两因素6年次资料配合的病害流行预测式,其回报可靠度的置信范围在93%左右。     

Simulation of herbicide persistence in soil .II. Simazine and linuron in long-term experiments

The rates of degradation of simazine and linuron were measured in soil from plots not treated previously with these herbicides. Degradation of both compounds followed first-order kinetics and soil temperature and soil moisture content had a marked effect on the rate of loss. With linuron, half-lives increased from 36 to 106 days with a reduction in temperature from 30° to 5°C at 4% soil moisture, and from 29 to 83 days at 12% soil moisture. Similar temperature changes increased the half-life of simazine from 29 to 209 days and from 16 to 125 days at soil moisture contents of 4 and 12% respectively. A computer program which has been developed for simulation of herbicide persistence was used in conjunction with the laboratory data and the relevant meteorological records for the years 1964 to 1968 in order to test the model against previously published field persistence data for the two herbicides. The results with simazine showed a close correspondence between observed and predicted residue levels but those for linuron, particularly in uncropped plots, were satisfactory for limited periods only.     

大豆花叶病的流行因素及综合防治

1985年设置流行观察圃,以4个品种、4种播期、两种种子传毒率的种子,系统观察各因素对病害流行的作用。另设综合防治田,品种与观察圃相同;采取隔离种植、种传率低、适期播种及早期拔除病苗等措施。1986年用相同的4个品种,设5个隔离试区,各播种不同种传率(0—33％)的种子,测定播种种子传毒率对流行的影响。结果证明:传毒种子是当地最主要的侵染来源,田间杂草不传病,蚜虫传播距离多在100米以内,花期以后被侵染的植株,其种传率显著降低。播种种子传毒率与田间流行程度及收获种子传毒率相关显著。所用防病措施效果明显,可控制田间病情指数为4％、种传率为0％、种子斑驳率低于4％。     

Biological control of Botrytis cinerea on chickpea seed with Trichoderma spp. and Gliocladium roseum: indigenous versus non-indigenous isolates

Establishment of chickpea seedlings from seed inoculated with Botrytis cinerea was increased under controlled environmental conditions by treatment of seed with Gliocladium roseum or Trichoderma virens . An isolate of G. roseum (GrH) from Horsham in the chickpea-growing region of western Victoria outperformed T. virens in three soil types, including one to which T. virens was indigenous. There was no effect of soil moisture on seedling establishment from seed treated with GrH, but recovery of G. roseum from seed in continuously wet soil was lower than from wet/dry soil. A factorial assay with three isolates of G. roseum and the soils from which they were isolated showed no interaction between isolate and soil type and no difference between isolates for establishment or recovery. In a field trial at Horsham sown in winter (July), seed treatment with GrH increased establishment from 26.0% (control) to 37.5%. Biocontrol by GrH was ineffective in a winter-sown field trial at Bundoora, but all three isolates increased establishment from 47.5% (control) to up to 87.5% in a trial at the same location sown in spring (October). The commercial implications of these observations are discussed.     

Factors Influencing Transmission of Didymella rabiei (Ascochyta Blight) from Inoculated Seed of Chickpea Under Controlled Conditions

Ascochyta blight of chickpea (Cicer arietinum), caused by the fungus Didymella rabiei, has the potential to cause 100% crop loss in severe epiphytotics. Management of this disease often involves reducing sources of inoculum. The influence of sowing depth, host resistance, seed infection level and soil temperature on disease transmission was investigated in a series of glasshouse and growth room trials using seed artificially inoculated with D. rabiei. A positive correlation (R²=0.9992) was observed between rate of seed infection and the incidence of disease on seedlings. Disease transmission to seedlings was not significantly influenced by sowing depth (1, 3 and 6 cm) in separate trials on two cultivars. Susceptibility of the host showed no obvious influence on the frequency of disease transmission in two trials conducted using four cultivars ranging from highly susceptible to moderately susceptible/moderately resistant. Trials conducted in controlled conditions showed that there was no obvious relationship between soil temperature (5, 9, 14 and 19 °C) and the incidence of disease on seedlings.     

土壤湿度对小麦矮腥黑穗病菌(TCK)冬孢子萌发的影响

通过土壤湿度对小麦矮腥黑穗病菌(Tilletiacontronversa Kühn,TCK)萌发率的影响试验研究表明,其冬孢子在土壤质量含水量为1%～28%(相对含水量3.57%～100%)范围内均可萌发,其适宜萌发的土壤质量含水量范围为10%～25%(相对含水量17.85%～89.3%),最佳土壤相对含水量范围在65%～75%之间;不同分离物在相同土壤湿度培养下,多数分离物冬孢子的萌发率之间差异不显著。根据分离菌T_t1和T_t2在不同土壤湿度下培养50d和60d的冬孢子萌发率,建立了TCK冬孢子萌发率与土壤相对含水量的关系模型,此结果为TCK的风险评估提供了基础数据。     

干旱区土壤蒸发及水热耦合运移模式研究

干旱地区土壤蒸发模拟及水热耦合运移研究,对干旱地区水资源利用、陆面过程及气候模式研究具有重要意义.通过对土壤蒸发与内部水分传输机理的深入了解,推导出土壤水热传输与土壤蒸发模式.为了比较模式模拟结果,利用本模式对CoLM陆面过程模式进行改进,并采用黑河沙漠站观测资料,对模式改进前后土壤蒸发与土壤温度和湿度模拟结果进行了对比.结果表明:改进后模式能够更好地模拟土壤温度与土壤蒸发,干旱条件下应考虑水汽对土壤水分与温度传输的影响.     

干旱区保护性耕作对土壤水分的影响研究

为了研究保护性耕作对土壤水分的影响,选择翻耕地和免耕地两种土地耕作类型,分析不同耕作方式下土壤水分的时空变化规律。在保护性耕作实行的两年时间里,测定了土层不同深度土壤水分,结果表明:(1)在作物生长的整个生育期内土壤水分含量有随降雨量周期变化的趋势,无论是何种耕作方式,除生育期降雨外,播前底墒也是土壤供水的重要组成部分;(2)土壤贮水能力免耕地都是大于翻耕地的;0-80cm土壤水分的空间剖面自上而下分为两个层次:表层失水层(0-30cm)和中层贮水层(30-80cm)。(3)用SPSS统计软件进一步对土壤水分与土层深度的关系进行拟合,结果表明土壤水分与土层深度的关系具有极强的相关性,其关系方程为拟和三次抛物线相型。     

Simulating evolution of glyphosate resistance in Lolium rigidum II: past, present and future glyphosate use in Australian cropping

Glyphosate is a key component of weed control strategies in Australia and worldwide. Despite widespread and frequent use, evolved resistance to glyphosate is rare. A herbicide resistance model, parameterized for Lolium rigidum has been used to perform a number of simulations to compare predicted rates of evolution of glyphosate resistance under past, present and projected future use strategies. In a 30‐year wheat, lupin, wheat, oilseed rape crop rotation with minimum tillage (100% shallow depth soil disturbance at sowing) and annual use of glyphosate pre‐sowing, L. rigidum control was sustainable with no predicted glyphosate resistance. When the crop establishment system was changed to annual no‐tillage (15% soil disturbance at sowing), glyphosate resistance was predicted in 90% of populations, with resistance becoming apparent after between 10 and 18 years when sowing was delayed. Resistance was predicted in 20% of populations after 25–30 years with early sowing. Risks of glyphosate resistance could be reduced by rotating between no‐tillage and minimum‐tillage establishment systems, or by rotating between glyphosate and paraquat for pre‐sowing weed control. The double knockdown strategy (sequential full rate applications of glyphosate and paraquat) reduced risks of glyphosate and paraquat resistance to <2%. Introduction of glyphosate‐resistant oilseed rape significantly increased predicted risks of glyphosate resistance in no‐tillage systems even when the double knockdown was practised. These increased risks could be offset by high crop sowing rates and weed seed collection at harvest. When no selective herbicides were available in wheat crops, the introduction of glyphosate‐resistant oilseed rape necessitated a return to a minimum‐tillage crop establishment system.     

气象因子对祁连山北坡天然草地土壤水分动态变化的影响

利用2004年祁连山北坡五类天然草地生长季的土壤水分数据和同期的气象资料,以回归和相关分析的方法分析了气象因子对五类天然草地土壤水分动态变化的影响.结果表明:(1)土壤水分的动态主要受降水量及其季节分配的影响,并建立了降水量对土壤含水量的回归方程;(2)利用ASCE-PM模型计算了五类草地群落的潜在蒸散量,潜在蒸散量对...     

Influence of soil moisture on the competitive ability and seed dormancy of Sinapis arvensis in spring wheat

Two experiments were carried out, one in 1995 and one in 1997, to investigate the competitive abilities of two spring wheat cultivars with Sinapis arvensis L. The spring wheat cultivars (Baldus and Canon) of contrasting growth habit were grown with and without S. arvensis under two different moisture regimes (10% and 70% of field capacity). In 1995, S. arvensis was found to be less competitive when subjected to moisture stress, resulting in smaller wheat yield losses in dry soil than in moist soil. In both years, seed production of S. arvensis was reduced by competition and moisture stress, and the seeds produced by plants that had been grown in drier soil were small and had negligible dormancy. Hence, in dry conditions, the competitiveness of S. arvensis and its potential to produce persistent seed may be reduced. Some differences between the two wheat cultivars were evident: cv. Baldus was more competitive against S. arvensis than cv. Canon. This could be attributed to differences in canopy structure.     

影响大豆花叶病毒种子传毒和种子斑駁因素的研究

 本文研究了影响大豆花叶病毒种子传毒及种子斑驳的有关因素,以及种子传毒与种子斑驳的关系。结果表明,种子传毒率的高低受大豆品种,大豆花叶病毒(SMV)毒株,大豆感病早晚以及环境条件的影响。这些因素之间对种子传毒还存在交互作用。供试的12个大豆品种中,传毒率最高的为47%,最低的为25%;包括SMV Ⅰ,Ⅱ,Ⅲ号3个株系群的8个毒株在东农64-3513上传毒率的变化范围为1-33%,在抗霉2号上的变化范围为9-48%;大豆在开花以前感染SMV,种子传毒率可达45%,开花以后感病,种子基本不能传毒。
种子斑驳率因大豆品种,SMV毒株不同而异,并受品种与毒株交互作用的影响。种子斑驳既不代表种子带毒,也不代表种子传毒;从感病植株上采收的大豆种子,其斑驳种子与非斑驳种子的传毒率基本相同(P>0.05),因此,不能从种子斑驳率预测种子传毒率。     

Population regulation of Galinsoga ciliata and G. parviflora

The population response of the two sympatric annual weeds, Galinsoga ciliata (Raf.) Blake and G. parviflora Cav., which occur abundantly in croplands and early secondary successional communities in the north-eastern hill region of India was studied in relation to sowing pattern (clump or scattered sowing), population density and soil moisture and texture. In a given sowing pattern, seed germination of both species declined with increase in sowing density and moisture stress. However, sowing of seeds in clumps and an increased proportion of sand in soil enhanced the germination, Fewer weeds died in the population raised from scattered sowing than from clumped distribution. Plants grown at high moisture level matured earlier, and fertility of the survivors was negatively correlated with both density and moisture stress in both species although G. ciliata was less affected. Seed output and dry matter yield per pot of both species increased with population density but the increase was not linearly related to the increase in plant numbers, and density-induced reduction in seed output was observed at high densities. Galinsoga ciliata appears to tolerate greater density-stress than G. parviflora. Asubstantial decrease in production of capitula, seeds and dry matter at low moisture level indicates the role played by soil moisture in regulating the population growth of the two weeds. An increased proportion of sand in soil resulted in significantly greater seed production in G. parviflora. Galinsoga ciliata also showed a similar trend although the differences were not statistically significant.     

Simulation of atrazine persistence in Spanish soils

The persistence of atrazine was monitored in three fields at different sites in Spain during two consecutive years (1990 and 1991). Laboratory assays for determining the influence of temperature and soil moisture content on the rate of herbicide degradation were carried out on soil samples from the same fields. The degradation constants derived from these assays, together with weather records for the period of the field experiments, were used in a computer program which simulated herbicide persistence in the field. Some adjustments were made to adapt the model to Spanish conditions. The model predicted with reasonable accuracy the persistence of the herbicide in two soils, although there was a tendency to overestimate the residues at early dates. Discrepancies between predicted and measured residues were greater in the third soil, due to rapid initial losses that were not predicted by the program. In this case, the agreement was improved if the program was run taking time zero to be one month after herbicide application. Possible reasons for these discrepancies are discussed.     

The importance of the infected seed tuber and soil inoculum in transmitting Potato mop‐top virus to potato plants

Potato mop‐top virus (PMTV), the cause of spraing in potato tubers, is transmitted by Spongospora subterranea, the cause of powdery scab, and by planting infected seed tubers. This study was undertaken to determine the relative importance of these sources of infection in seed potato production in Scotland. The transmission of PMTV from tested seed tubers to daughter plants was examined over 2 years and six cultivars. The development of foliar symptoms varied with year and cultivar. Infection of daughter tubers derived from PMTV‐infected seed tubers was more prevalent on plants affected by foliar symptoms than those without symptoms. The rate of transmission of PMTV from infected seed tubers to daughter tubers ranged from 18 to 54%. Transmission was affected by cultivar and by origin of seed tubers used for a cultivar, but not by a cultivar's sensitivity to PMTV infection. The incidence of PMTV in daughter tubers of cv. Cara grown from seed potatoes from one source (common origin) by more than 25 seed producers was examined over two successive generations. The incidence of PMTV in daughter tubers was not correlated with that in the seed tubers but appeared to be strongly associated with soil inoculum. The incidence of PMTV was correlated with powdery scab in those crops in which both were present. There was some evidence from soil tests conducted in 2006 using a tomato bait plant and real‐time RT‐PCR that planting PMTV‐infected seed potatoes could increase the risk of introducing the virus into land not infested by PMTV.     

Dormancy and viability of Phalaris minor seed in a rice–wheat cropping system

Seed placement, soil temperature and soil moisture content influenced the process of after-ripening in Phalaris minor seeds. Seeds of P. minor collected from the soil just after wheat harvesting exhibited higher germination than seeds from P. minor threshed directly. There was a pronounced impact of periodic inhabitation of seed into the soil on germination after its dispersal. Germination was strongly inhibited when the seed was kept in soil at more than field capacity (FC) or in water. Maximum germination of seed incubated in soil at FC occurred at 30°C while a temperature of 40°C favoured after-ripening of seed when mixed with dry soil or kept dry without any medium. Release from conditional dormancy was quicker in the seed retrieved from the soil kept at 20°C than at 10°C. Seed release from conditional dormancy and germination increased with a rise in temperature from 30 to 40°C when the seed was retrieved from incubation in soil at FC for 70 days. The seed kept immersed in water was least responsive to a rise in temperature. Seed recovered from dry soil, or kept without any medium, responded quickly at both temperatures. Light enhanced the germination of Phalaris minor seed. The seedbank subjected to rice (Oryza sativa) field management conditions lost vigour in comparison with the seed stored in laboratory. There was significant variability in seed viability when exposed to differential water management conditions in rice.     

Seed survival and predation of Anoda cristata in soyabean crops

Anoda cristata is a troublesome annual broad-leaved weed in summer crops in the rolling Pampa in Argentina; seeds are the only source of regeneration of this species. Seed persistence or depletion is the result of survival and loss processes, including predation. The objective of this study was to determine survival at two burial depths in undisturbed soil and predation rates of A. cristata seeds in soyabean crops in different rotations and tillage systems. Survival was discontinuous and decreased to 25% after 35 months, after which no further reduction in survival was observed to the end of the experiment at 96 months. No differences in seed survival between seeds placed on the soil surface and buried 5 cm below the soil surface were found at 80 months, but at later times survival was lower for seeds placed on the soil surface. Predation rates ranged between 0.3% day⁻¹ and 6.7% day⁻¹. Of the models tested, a polynomial regression of the rate of predation with time gave the best representation of seed predation. From January to July, predation was higher in non-tillage plots in the wheat/soyabean rotation. There was no significant difference in predation rates between tillage systems in the soyabean monoculture and no difference between planting densities. Higher crop residue levels in non-tillage plots in the wheat/soyabean rotation was the dominant factor influencing seed predation, probably because such habitat favours the presence of seed predators.     

The question of seed transmissibility of Plum pox virus

For many years, Plum pox virus (PPV) was considered to be transmissible by seed, increasing the fear of long-distance spread of the disease. In the late 1970s, it was claimed on the basis of biological transmission of the virus to herbaceous indicator plants and the development of serological diagnosis based on polyclonal antibodies, that PPV was seed-transmitted, with a different infection rate according to the plant species and part of the seed which was tested. In the 1990s, PPV was characterized into four different types, and specific monoclonal antibodies were produced for them. These new and more sensitive diagnostic techniques, together with RT-PCR with different sets of specific primers, were used to approach once again the problem of PPV transmission through seeds. The virus was detected in seed coats and cotyledons, but embryonic tissue and seedlings obtained from germinated seeds never showed symptoms, and gave negative results for PPV with both ELISA and PCR assays. No PPV isolate is currently recognized to be seed transmitted, so vertical transmission of PPV from infected mother plants to their progeny does not occur. Hypothetically, the only possibility of seed transmission would arise from a mutation in the helper component of the virus, associated with high susceptibility of the infected Prunus cultivar.