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.     

植物病害分子流行学概述

 本文概要地介绍了植物病理学新研究领域——植物病害分子流行学的基本概念、进展和在几个主要方面的研究实例。分子生物技术在病原鉴定方面得到了广泛应用;定量的分子生物技术在测定病菌初侵染源方面显示出特有的快速、准确的优势;分子流行学应用分子生物技术监测病害和病原菌群体的动态,克服了传统流行学方法的弱点;作为有力的补充,分子生物技术正在用于探讨和推测病原菌远距离传播的路径,并注重研究病原菌群体的时、空动态变化,病原菌的长期进化,以及与病害发展的关系;病原群体的竞争将得到更深入的研究以揭示其变化是如何导致植物病害大流行;应用分子流行学手段,植物抗病性的鉴定将大大加速和简化;病害防治策略的制定将具有更科学的依据。宏观与微观研究手段的结合将越来越显示其在植物病害流行学研究中的巨大潜力。     

Systems approaches in epidemiology and plant disease management

This contribution to Zadoks’ liber amicorum reviews the developments in quantitative epidemiology during the last decades. It elucidates the progress in this field and shows how empirical crop protection with many phenomenological aspects transformed into a science-based (inter) discipline.The availability of experimental tools and the rapid development and introduction of computers enabled the application of systems approaches which stimulated a revolution in thinking and caused a considerable improvement of strategic and tactical decision making in crop protection. Zadoks played a crucial role in that development.     

Systems approaches in epidemiology and plant disease management

This contribution to Zadoks’ liber amicorum reviews the developments in quantitative epidemiology during the last decades. It elucidates the progress in this field and shows how empirical crop protection with many phenomenological aspects transformed into a science-based (inter) discipline. The availability of experimental tools and the rapid development and introduction of computers enabled the application of systems approaches which stimulated a revolution in thinking and caused a considerable improvement of strategic and tactical decision making in crop protection. Zadoks played a crucial role in that development.     

Nonparametric tests in plant disease epidemiology: characterizing disease associations

ABSTRACT Nonparametric tests are suited to many statistical applications, including experimental design, regression, and time series analysis, for example. Often these tests are thought of as alternatives to their parametric counterparts when certain assumptions about the underlying population are questionable. Although suited for this scenario, there are a number of nonparametric tests that fill unique niches in the analysis of data, for example, characterizing interspecific associations. Quantifying the degree of association between two or more pathogens or diseases at a defined spatial scale is essential to gain a thorough understanding of disease dynamics, generate testable hypothesis behind the mechanisms that cause association, and is often necessary in modeling applications. In this paper, nonparametric approaches to characterizing interspecific associations will be covered. Specifically, I will address the use of rank correlation coefficients and the development of a randomization procedure for testing the Jaccard index of association against a null model.     

Registration of plant protection products in EPPO countries: current status and possible approaches to harmonization

A survey concerning registration of plant protection products was carried out in 41 EPPO member countries. Twenty six countries responded and results are summarized in three groups. Various models are discussed which seek to adapt product dosage to the crowns of fruit-tree crops. The tree row volume model (TRV) is favoured by the authors since it can adequately cope with the wide variability of orchards encountered across Europe. It is suggested that experimenters who carry out registration field trials should measure the height and width of the tree crowns and the distance between the tree rows to facilitate comparison of registration trials including residue data.     

Challenges in monitoring and managing plant diseases in developing countries

Journal of Plant Diseases and Protection - Plant diseases heavily reduce crop yields in developing countries and tropical ecosystems. Lack of capacity to manage the diseases makes these countries...     

实时荧光定量PCR在植物病害流行学中的应用

实时荧光定量PCR广泛应用于植物病理学研究的各个领域。近年来已开始应用于植物病害流行学的定量研究中,本文对近年来实时荧光定量PCR技术在病菌初始菌源的定量分析、病害流行动态的监测、寄主抗病性的快速鉴定和植物病原菌的抗药性监测等方面的应用进行了综述。     

Fitting and testing spatio-temporal stochastic models with application in plant epidemiology

We propose and illustrate a likelihood-based method for fitting spatio-temporal stochastic models for the spread of a plant disease to experimental observations. The models considered are individual-based, with members of the population occupying discrete sites on a two-dimensional lattice. The disease is assumed to be characterized by presence/absence, and infection of susceptible individuals by infected individuals is represented as a stochastic process. The method described can be applied to estimate parameters in models of this kind when observations consisting of temporal sequences of disease maps are available. The use of measures of spatial aggregation as measured from simulated and real epidemics is proposed as a means of assessing the relative merits of alternative models for the spread of a disease. To illustrate the technique we fit and compare two models, which differ in the relationship between infective pressure and distance, to observations of an epidemic of citrus tristeza virus (CTV). It is demonstrated that a model in which this relationship is a power-law is superior to one which uses a negative exponential and the importance of model choice for the design of control strategies is discussed briefly.     

Molecular characterisation and diagnostics of some Longidorus species (Nematoda: Longidoridae) from Russia and other countries using rRNA genes

The needle nematodes of the genus Longidorus can cause diseases of various crops and trees, and are comprised of more than 150 valid species. Eleven valid and six unidentified species of the genus Longidorus collected in different regions of Russia, two states of USA, Germany, New Zealand and Ukraine were molecularly characterized using analysis of the partial 18S rRNA and the D2–D3 expansion segments of the 28S rRNA gene sequences. Fifty-four partial 28S rRNA and fifteen partial 18S rRNA gene sequences were obtained for the present study. Using molecular criteria, we confirmed the morphological identification and distinguished between the following species: L. aetnaeus, L. africanus, L. andalusicus, L. artemisiae, L. caespiticola, L. distinctus, L. elongatus, L. euonymus, L. intermedius, L. leptocephalus and L. lignosus. Two longidorid populations from Russia and four from California were not identified to a species level. We obtained the full length D2–D3 of 28S rRNA gene sequence from several freshly-collected L. artemisiae samples. We confirmed the identity of the D2 region of 28S rRNA gene sequence with a short D2 of 28S rRNA gene fragment sequence previously obtained from formalin-fixed nematodes embedded in the L. artemisiae paratype slides. Longidorus lignosus was molecularly characterized and L. aetnaeus was reported from Russia for the first time. PCR-D2-D3-RFLP diagnostic profiles generated by five restriction enzymes: AluI, HinfI, Bsp143I, Tru1I and RsaI are presented for sixteen Longidorus species.     

A perspective on the measurement of time in plant disease epidemiology

The growth and development of plant pathogens and their hosts generally respond strongly to the temperature of their environment. However, most studies of plant pathology record pathogen/host measurements against physical time (e.g. hours or days) rather than thermal time (e.g. degree-days or degree-hours). This confounds the comparison of epidemiological measurements across experiments and limits the value of the scientific literature.     

Molecular characterization, ecology, and epidemiology of a novel Tymovirus in Asclepias viridis from Oklahoma

Native virus-plant interactions require more understanding and their study will provide a basis from which to identify potential sources of emerging destructive viruses in crops. A novel tymovirus sequence was detected in Asclepias viridis (green milkweed), a perennial growing in a natural setting in the Tallgrass Prairie Preserve (TGPP) of Oklahoma. It was abundant within and frequent among A. viridis plants and, to varying extents, within other dicotyledonous and one grass (Panicum virgatum) species obtained from the TGPP. Extracts from A. viridis containing the sequence were infectious to a limited number of species. The virus genome was cloned and determined to be closely related to Kennedya yellow mosaic virus. The persistence of the virus within the Oklahoma A. viridis population was monitored for five successive years. Virus was present in a high percentage of plants within representative areas of the TGPP in all years and was spreading to additional plants. Virus was present in regions adjacent to the TGPP but not in plants sampled from central and south-central Oklahoma. Virus was present in the underground caudex of the plant during the winter, suggesting overwintering in this tissue. The RNA sequence encoding the virus coat protein varied considerably between individual plants (≈3%), likely due to drift rather than selection. An infectious clone was constructed and the virus was named Asclepias asymptomatic virus (AsAV) due to the absence of obvious symptoms on A. viridis.     

Host genetic resistance to root‐knot nematodes,Meloidogyne spp., in Solanaceae: from genes to the field

Root‐knot nematodes (RKNs) heavily damage most solanaceous crops worldwide. Fortunately, major resistance genes are available in a number of plant species, and their use provides a safe and economically relevant strategy for RKN control. From a structural point of view, these genes often harbour NBS–LRR motifs (i.e. a nucleotide binding site and a leucine rich repeat region near the carboxy terminus) and are organised in syntenic clusters in solanaceous genomes. Their introgression from wild to cultivated plants remains a challenge for breeders, although facilitated by marker‐assisted selection. As shown with other pathosystems, the genetic background into which the resistance genes are introgressed is of prime importance to both the expression of the resistance and its durability, as exemplified by the recent discovery of quantitative trait loci conferring quantitative resistance to RKNs in pepper. The deployment of resistance genes at a large scale may result in the emergence and spread of virulent nematode populations able to overcome them, as already reported in tomato and pepper. Therefore, careful management of the resistance genes available in solanaceous crops is crucial to avoid significant reduction in the duration of RKN genetic control in the field. From that perspective, only rational management combining breeding and cultivation practices will allow the design and implementation of innovative, sustainable crop production systems that protect the resistance genes and maintain their durability. © 2015 Society of Chemical Industry