A model of plant virus disease epidemics in asynchronously-planted cropping systems

A mathematical model was developed of the dynamics of a plant virus disease within a spatially-referenced lattice of fields of a host crop. The model can be applied to crops in continuous, contiguous cultivation such as tropical irrigated rice. Disease progress in each field of the host crop was assumed to be logistic and determined by incidence within the field itself as well as incidence in neighbouring fields, depending on the gradient of disease spread. The frequency distribution of planting dates (represented by the proportion of the total number of fields planted in successive months) was assumed to follow a normal distribution and the variance of planting date was used as a measure of cropping asynchrony. Analysis of the model revealed that disease incidence within the lattice (i.e. mean incidence over all fields) depended upon the infection efficiency, the slope of the dispersal gradient, and the variance in planting date. Disease endemicity depended mainly on planting date variance and disease persisted in the lattice if this variance exceeded a certain threshold. Above the threshold for persistence, the response of mean disease incidence to planting date variance was non-linear and the region of greatest sensitivity was closest to the threshold. Thus, disease systems that show moderate rather than high cropping asynchrony are more likely to be influenced by changes in the variance of planting date. Implications for the area-wide management of rice tungro virus disease are discussed.     

Comparative analysis of flexible two-parameter models of plant disease epidemics

ABSTRACT Eleven previously published models of plant disease epidemics, given as differential equations with a rate and a shape parameter, are compared using general model characteristics as well as their usefulness in fitting observed data. Six out of the eleven models can be solved analytically resulting in epidemic growth functions, while the others can be solved only numerically. When all 11 differential equations were fitted to two data sets, all models showed a similar goodness of fit, although the shape parameter in some models could not be estimated very precisely. With respect to useful characteristics (exponential population growth at the beginning, ability to generate monomolecular disease progression, and flexibility of the inflection point), the models of Fleming, Kosman-Levy, Birch, Richards and Waggoner, and Rich are recommended. Formulas were established to calculate the point of inflection as well as the weighted absolute and relative rate, respectively, depending on the shape and rate parameter. These formulas allow transformation of the parameter values of one model into those of another model in many cases. If the two models are required to have the same temporal position of the disease progress curve, then the initial disease level at the start of the epidemic or the time when the inflection point is reached have to be transformed.     

Asymptotic behaviour and threshold criteria in model plant disease epidemics

Two qualitative results concerning threshold criteria and asymptotic behaviour in plant disease epidemics are derived from Vanderplank's differential-difference equation. Analysis shows the dependence of these results on initial disease and clarifies some confusion in the literature. Results from the deterministic theory of medical epidemics, based on linked differential equations describing the dynamics of different categories of diseased individuals, are compared with the results derived from the differential-difference equation. Generally, the results correspond although the effects of initial disease need clarification. The need for a strict operational definition of the progeny-parent ratio limits its present use in plant disease epidemiology. In particular the numerical value of the ratio is not a sufficient basis for distinguishing between endemic and epidemic disease. There is a need to link theory in plant disease epidemiology with similar theory in other areas of population biology. The use of linked differential equations, rather than the differential-difference equation, provides a more flexible analytical tool.     

Use of SADIE statistics to study spatial dynamics of plant disease epidemics

Using a previously developed stochastic simulation model for plant disease epidemics, the relationship of the SADIE aggregation statistic I _a with initial epidemic conditions, spore dispersal distance, sampling quadrat size and other spatial statistics was investigated. Most variation in I _a was attributable to the initial spatial pattern of infected plants and sampling quadrat size. The importance of initial spatial pattern on SADIE clustering indices (for patches and gaps) was also demonstrated using a number of selected data sets. Correlation of I _a with clustering indices was close to 1·0. Epidemics arising from the regular and random initial patterns resulted in the smallest and greatest I _a values, respectively, at sampling times after disease spread had occurred. Furthermore, the variability in I _a between simulation runs also varied greatly with initial patterns, being lowest and greatest for the clumped and random initial patterns, respectively. I _a increased initially and then decreased with increasing incidence, especially for the clumped and random initial patterns. Overall, the effect of median spore dispersal distance on I _a was very small, especially for the random initial pattern. The correlation between I _a and intraclass correlation was generally small and varied greatly between initial patterns. However, there was a high positive correlation between I _a and a parameter describing the rate of decline of autocorrelation over spatial lags, indicating that I _a, clustering indices and autocorrelations measure some common properties of patterns.     

植物病害时空流行动态模拟模型的构建

 一个描述在二维空间中单一种植或混合种植的植物群体内病害时、空流行动态的计算机随机模拟模型构建完成。模型由寄主、病原2个组分和病斑产孢、孢子传播、孢子着落、孢子侵染、病斑潜育、寄主生长、病害控制等一系列代表病害流行生物学过程的子模型构成。模型采用了面向对象的程序设计方法,用C++语言编写,能以病害流行曲线图、空间分布图、数据列表等方式显示模拟结果。测试结果表明:模型能反映植物病害流行过程的本质规律,既可作为植物病害流行学教学工具,帮助学生理解病害流行的时、空动态规律和不同因子对病害流行的影响,也可以作为研究工具,对流行学的某些理论问题进行模拟研究     

Population biology and epidemiology of plant virus epidemics: from tripartite to tritrophic interactions

Natural enemies have long been used in biological control programs to mitigate the damage caused by herbivory. Many herbivorous insect species also act as plant virus vectors, enabling virus transmission from plant to plant and hence disease development in a plant population. Whilst an intuitive assumption would be to expect a decrease in vector numbers to lead to subsequent reductions in virus transmission, recent evidence suggests that introduction of natural enemies (parasitoids and predators) may in some cases increase plant virus transmission while at the same time reducing vector numbers. In this paper we review the evidence for plant-virus-vector-natural enemy interactions, the signalling mechanisms involved and their implications for virus transmission, and show how a modelling approach can assist in identifying the key parameters and relationships involved in determining the disease outcome. A mathematical model linking the population dynamics of a vector-parasitoid system with virus transmission was used to investigate the effects of virus inoculation and acquisition rates, parasitoid attack rate and vector aggregation on disease dynamics across a wide range of parameter value combinations. Virus spread was found to increase with enhanced inoculation, acquisition and parasitoid attack rate but decrease with high levels of vector aggregation.     

植物诱导抗病机制的研究进展

植物诱导抗病性是植物抵御病害侵染的重要机制,并且作为一种经济有效的抗病策略,在农林业可持续病害防控中具有广阔应用前景,受到人们的日益关注.从组织结构、生理生化乃至分子生物学的一系列变化,阐述了植物诱导产生抗病性的机制.同时,指出了植物诱导抗病性研究中有待进一步研究加以解决的问题和发展前景.     

Effect of crop growth and canopy filtration on the dynamics of plant disease epidemics spread by aerially dispersed spores

Most mathematical models of plant disease epidemics ignore the growth and phenology of the host crop. Unfortunately, reports of disease development are often not accompanied by a simultaneous and commensurate evaluation of crop development. However, the time scale for increases in the leaf area of field crops is comparable to the time scale of epidemics. This simultaneous development of host and pathogen has many ramifications on the resulting plant disease epidemic. First, there is a simple dilution effect resulting from the introduction of new healthy leaf area with time. Often, measurements of disease levels are made pro rata (per unit of host leaf area or total root length or mass). Thus, host growth will reduce the apparent infection rate. A second, related effect, has to do with the so-called "correction factor," which accounts for inoculum falling on already infected tissue. This factor accounts for multiple infection and is given by the fraction of the host tissue that is susceptible to disease. As an epidemic develops, less and less tissue is open to infection and the initial exponential growth slows. Crop growth delays the impact of this limiting effect and, therefore, tends to increase the rate of disease progress. A third and often neglected effect arises when an increase in the density of susceptible host tissue results in a corresponding increase in the basic reproduction ratio, R(0), defined as the ratio of the total number of daughter lesions produced to the number of original mother lesions. This occurs when the transport efficiency of inoculum from infected to susceptible host is strongly dependent on the spatial density of plant tissue. Thus, crop growth may have a major impact on the development of plant disease epidemics occurring during the vegetative phase of crop growth. The effects that these crop growth-related factors have on plant disease epidemics spread by airborne spores are evaluated using mathematical models and their importance is discussed. In particular, plant disease epidemics initiated by the introduction of inoculum during this stage of development are shown to be relatively insensitive to the time at which inoculum is introduced.     

植物抗病毒的可能机制--基因沉默

根据目前RNA介导的植物病毒抗性(RMVR)、转录后基因沉默(PTGS)的研究成果,综述了基因沉默可能是植物抵抗病毒的一种机制,深入研究基因沉默不仅在抗病机制上有重要的理论意义,而且对彻底解决植物病毒问题有较大的潜在实用价值.     

Studies on the mechanism of transmission of pollen-associated tobacco streak ilarvirus virus by Thrips tabaci

Mixed instars of Thrips tabaci were allowed to feed on test seedlings of cucumber, the thrips were removed and wounds caused by thrips feeding dusted with tobacco streak virus (TSV) carried in tomato pollen. Transmission of TSV occurred in three out of the four experiments conducted. When Chenopodium amaranticolor test seedlings were dusted with infective tomato pollen, and thrips introduced for varying periods before being killed with insecticide, TSV transmission occurred after exposure to thrips for 1 h, 5 h, 1 day or 2 days, but not 5 min. Pollen-washing experiments indicated that TSV was carried both internally and externally in pollen of tomato and several weed hosts. T. tabaci was able to transmit TSV from pollen of Nicotiana clevelandii and Nicandra physalodes , and TSV from tomato pollen was transmitted to several weed species. It is concluded that T. tabaci transmits TSV associated with tomato pollen by a mechanical mechanism whereby virus carried externally, or released from inside the pollen, infects feeding wounds.     

Transmission of tobacco streak virus by Thrips tabach a new method of plant virus transmission

When adults or nymphs of Thrips tabaci were mixed with virus-carrying pollen from Lycopersicon esculentum infected with tobacco streak virus and then placed on Chenopodium amaranticolor test seedlings, the virus was regularly transmitted. The virus was also regularly transmitted when virus-carrying pollen was placed on the leaves of C amaranticolor test seedlings and the thrips then introduced. No transmission occurred when test seedlings were exposed to virus-carrying pollen in the absence of the thrips or to the thrips without pollen. Further, no transmission occurred when the thrips were fed on virus-infected leaves and then transferred to test seedlings in the absence of virus-carrying pollen. The evidence indicates that the transmission of tobacco streak virus by Thrips tabaci depends on the presence of pollen-borne virus, which presumably infects via wounds made by the thrips. This method of virus transmission has not previously been reported.     

Effects of initial epidemic conditions, sporulation rate, and spore dispersal gradient on the spatio-temporal dynamics of plant disease epidemics

ABSTRACT A stochastic model that simulates the spread of disease over space and time was developed to study the effects of initial epidemic conditions (number of initial inocula and their spatial pattern), sporulation rate, and spore dispersal gradient on the spatio-temporal dynamics of plant disease epidemics. The spatial spread of disease was simulated using a half-Cauchy distribution with median dispersal distance mu (units of distance). The rate of temporal increase in disease incidence (beta(I), per day) was influenced jointly by mu and by the sporulation rate lambda (spores per lesion per day). The relationship between beta(I) and mu was nonlinear: the increase in beta(I) with increasing mu was greatest when mu was small (i.e., when the dispersal gradient was steep). The rate of temporal increase in disease severity of diseased plants (beta(S)) was affected mainly by lambda: beta(S) increased directly with increasing lambda. Intraclass correlation (kappa(t)), the correlation of disease status of plants within quadrats, increased initially with disease incidence, reached a peak, and then declined as disease incidence approached 1.0. This relationship was well described by a power-law model that is consistent with the binary form of the variance power law. The amplitude of the model relating kappa(t) to disease incidence was affected mainly by mu: kappa(t) decreased with increasing mu. The shape of the curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. Generally, the relationship of spatial autocorrelation (rho(t,k)), the correlation of disease status of plants at various distances apart, to disease incidence and distance was well described by a four-parameter power-law model. rho(t,k) increased with disease incidence to a maximum and then declined at higher values of disease incidence, in agreement with a power-law relationship. The amplitude of rho(t,k) was determined mainly by initial conditions and by mu: rho(t,k) decreased with increasing mu and was lower for regular patterns of initial inocula. The shape of the rho(t,k) curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. At any level of disease incidence, autocorrelation declined exponentially with spatial lag; the degree of this decline was determined mainly by mu: it was steeper with decreasing mu.     

Modelling and control of non-persistent plant virus transmission for annual production cycles

When an insect carrying a non-persistent virus punctures a plant it loses part, or may be all, of its viral load. Using a differential equation model, we show that this is a critical factor affecting disease incidence levels when crops are under annual production cycles. Computer simulations suggest that relatively low vector pressure at the beginning of planting cycles decrease the disease progression. The model provides also approximations to disease incidences in subsequent plantings. Conditions for incidences to decrease or increase in time are supplied, which may be useful to assess the impact of some control strategies.     

Attenuated plant viruses: preventing virus diseases and understanding the molecular mechanism

Attenuated viruses have been isolated and studied not only as a practical means of controlling virus diseases but also to gain a molecular understanding of viral virulence and cross protection. They have been isolated from crop fields and generated through high/low temperature treatment or by mutagens such as nitrous acid and ultraviolet irradiation. Some viruses have been beneficially used in fields and evaluated for one or more decades. Molecular genetic studies on attenuated viruses have revealed that amino acid substitutions are located in replicase and the movement protein in tobamovirus, protein 2b for cucumovirus, and P1 and HC-Pro for potyvirus. In most cases, with a few exceptions, symptom attenuation is positively correlated with a reduced level of RNA silencing suppression. Molecular mechanisms underlying virus attenuation and cross protection and the rationale for practical use of attenuated viruses for effective virus disease control are discussed.     

重大和突发植物疫情协作防控工作进展

为有效控制红火蚁等重大植物检疫性有害生物的扩散与为害,保护我国农业生产、生态环境和农产品贸易安全.支持粮食增产和农民增收,2006年以来.农业部成立了红火蚁等14个重大植物检疫性有害生物联合监测与防控协作组(以下简称"协作组").     

病毒侵染后植物叶绿体光合作用变化的分子机制

 当植物遭受病毒侵染后通常会引起植物重要生理功能的改变,包括光合作用与呼吸作用的变化、核酸与蛋白质的变化、酚类代谢的变化、水分生理的变化以及体内激素的紊乱和寄主膜结构的破坏等,其中对光合作用的影响是最关键的因素。     

Spatiotemporal effects of cultivar mixtures on wheat stripe rust epidemics

The use of cultivar mixtures is increasingly practical in wheat stripe rust management. Field experiments with wheat cultivar mixtures were conducted to determine their effects on temporal and spatial patterns of stripe rust epidemics in three regions. In the Beijing and Gangu fields, where the epidemics were caused by artificial inoculation, disease incidence and the area under the disease progress curve (AUDPC) of the cultivar mixtures were significantly lower (P < 0.05) than those of the susceptible pure stands. We defined the relative effectiveness of cultivar mixture on disease development related to that in pure stands (REM). The results demonstrated that in many treatments of mixtures of susceptible cultivar with resistant cultivars at various ratios in different locations, their effects on disease reduction were positive (REM < 1). The reduction of epidemic rate in cultivar mixtures expressed in either early season or late season depended on the initial pattern of disease and cultivar mixture treatments. Semivariograms were used to determine the spatiotemporal patterns of disease in the Gangu field. The spatial analysis showed clear spatial patterns of the disease in all four directions of the fields on susceptible pure stands but not on cultivar mixtures. The results implied that the mechanisms of cultivar mixture on disease management might include the interruption of disease spatial expansion and a physical barrier to pathogen inoculum by resistant cultivars.     

Host mixture efficacy in disease control: effects of lesion growth analyzed through computer-simulated epidemics

Lesion growth varies among foliar parasites and in order to study the effect of lesion growth on the efficacy of host mixtures to control epidemics, we altered the epidemic simulator Epimul by integrating a lesion growth function into the model. A theoretical study was performed by simulating epidemics caused by parasites with different lesion growth rates, spore dispersal gradients and multiplication rates. We found that increases in lesion growth rates resulted in large decreases in the effectiveness of mixtures for disease control and interacted strongly with parasite multiplication rate and spore dispersal gradient. The decline in mixture efficacy for epidemics with high lesion growth rates was reduced when parasite multiplication rate was higher and spore dispersal gradient steeper. Our results suggested that the lower number of infections on susceptible plants in the mixture as a result of inoculum loss on resistant hosts was partially compensated by lesion growth.     

植保有害生物风险分析理论体系的探讨

植保有害生物风险分析需要正确地收集大量数据。并急需建立实用的或可操作的有害生物风险分析体系，针对这种情况，本文将风险相关因素归纳为天气，地理和生物3大类，提出了有害生物的天气-地理-生物复合体系。该体系以生态网为出发点，对有害生物风险条件，事件及风险种类按层次和等级进行风险因子划分，然后进行风险因子模拟，风险分析和计算，实现有害生物的多因子调控管理。从而在风险分析，风险预测和风险决策的基础上，完成从风险确定，风险评价，风险管理到风险交流的风险全过程管理。