Showing papers in "Annual Review of Phytopathology in 1995"

Active oxygen in plant pathogenesis.

Abstract: Plant cells produce active oxygen during interactions with potential patho­ gens. Active oxygen species, including superoxide, hydrogen peroxide, and the hydroxyl radical, could potentially affect many cellular processes in­ volved in plant/pathogen interactions. Active oxygen can be difficult to monitor in plant cells because many of the species are short-lived and are subject to cellular antioxidant mechanisms such as superoxide dismutases, peroxidases, the ascorbate/glutathione cycle, and catalase. Modifications of the luminol-dependent chemiluminescence assay have facilitated studies on both the production and scavenging of active oxygen that occurs during incompatible plantlbacteria interactions. Many potential sources for active oxygen production have been identified such as NADPH oxidases and per­ oxidases, but it is still unclear which mechanisms predominate during plant! pathogen interactions. The active oxygen produced in response to pathogens and elicitors has been hypothesized to have direct antimicrobial effects and to play a role in other defense mechanisms including lignin production, lipid peroxidation, phytoalexin production, and the hypersensitive response. tThe US Govenunent has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.

Phytoalexins, stress metabolism, and disease resistance in plants

Abstract: Phytoalexins are low-molecular-weight antimicrobial compounds that accu­ mulate in plants as a result of infection or stress. The rapidity and extent of their accumulation is determined by their release or the release of immediate precursors from conjugates andlor de novo synthesis, as well as detoxification as a result of plant or microbial enzymes. The rapidity of phytoalexin accu­ mulation is associated with resistance in plants to diseases caused by fungi and bacteria, although the genetic information for phytoalexin synthesis is found in susceptible and resistant plants. Phytoalexins are only one component of the complex mechanisms for disease resistance in plants. This review considers the contribution of phytoalexins to disease resistance and evaluates molecular and nonmolecular strategies for their biosynthesis as potential tech­ nologies for disease control.

Pathogen-derived resistance to plant viruses.

Abstract: Transformation of plants with portions of viral genomes frequently gives rise to lines of plants that are resistant to the virus from which the sequence was derived. This phenomenon has been termed "pathogen-derived resistance." The nature of the resistance obtained is variable and can be either proteinor RNA-mediated. RNA-mediated resistance often protects against very high levels of inoculum but is highly specific; protein-mediated resistance generally offers lower level but broader spectrum resistance. The mechanism of protein­ mediated resistance is probably specific to the protein concerned, while RNA­ mediated resistance appears to operate by a mechanism similar to that of cosuppression. The type of resistance obtained is governed, at least in part, by the way the trans gene is inserted into the plant chromosomes.

The secret life of foliar bacterial pathogens on leaves.

Abstract: This review focuses on the role of two distinct fitness strategies in the growth, survival, and epidemiology of foliar bacterial pathogens. A tolerance strategy requires the ability to tolerate direct exposure to environmental stresses on leaf surfaces, including UV radiation and low water availability. An avoidance strategy requires the ability to seek and/or exploit sites that are protected from these stresses, including endophytic sites. The ability to employ an avoidance strategy and grow endophytically may directly influence the potential for pathogenesis, since endophytic populations, not epiphytic populations, are likely responsible for disease induction. Furthermore, exchange between these two populations is probably crucial to the epidemiology of foliar pathogens. While foliar pathogens can grow and survive in both exposed and internal sites, indicating that they can employ both fitness strategies, the poor internal growth of most saprophytes suggests that saprophytes depend primarily on a strate...

Remote sensing and image analysis in plant pathology

Abstract: ABSTRACf Thi s paper reviews various applications of remote sensing and image anal ysis in plant path ology. It describes technical methods and their possibilities, but also emphasizes the biological prerequisites and restrictions of prac tical appli­ cations. The subject area comprises many nondestructive and noninvasive methods to detect and assess plant diseases and stress objectiv ely and cost-ef­ fectively, even though to date we cannot identify the specific cause of the damage. As a supplement to conventional methods , these new methods have great potential to facilitate and increase accuracy and precision in plant patho­ logical researc h.

Plant disease incidence: distributions, heterogeneity, and temporal analysis

Abstract: The statistical properties of disease incidence are reviewed and used to characterize spatial patterns of diseased entities (e.g. plants), satisfy assumptions of statistical analyses, and quantify change in incidence over time. Frequency of diseased plants can be represented by the binomial or, more commonly, the beta-binomial distribution. Spatial patterns of disease can be described by the aggregation parameter of the beta-binomial distribution, index of dispersion and related statistics, parameters of the binary form of the power law, as well as measures of spatial autocorrelation between sampling units. Disease incidence over time can be represented by continuous- and discrete-time nonlinear disease progress models, such as the logistic, and by autoregressive, integrated, moving average models. Ultimately, simultaneous spatio-temporal analyses can be performed to understand the dynamics of disease incidence in populations.

Involvement of bacterial polysaccharides in plant pathogenesis.

Abstract: Virulence of phytopathogenic bacteria is often correlated with their ability to produce extraceUular polysaccharides (EPSs). The composition and amount of lipopolysaccharide O-antigens and low molecular weight, cell-associated 13linked glucans may also affect virulence of Gram-negative pathogens. For a few species of Agrobacterium. Clavibacter, Erwinia. Pseudomonas, and Xan­ tlwmonas sufficient biochemical and genetical data has accumulated to permit critical evaluation of possible functions of polysaccharides during pathogene­ sis. It is clear that EPSs are necessary for several pathogens to cause normal disease symptoms such as water-soaking and wilting. Evidence is accumulating that EPSs and ceU-associated polysaccharides also promote colonization and enhance survival of some bacteria within host tissues. Further progress will require the use of thoroughly characterized polysaccharide-minus mutants. "natural" inoculation procedures, and careful monitoring of the fate of bacteria within plants.

Molecular Approaches to Manipulation of Disease Resistance Genes

Abstract: Various molecular approaches are greatly increasing our ability to characterize and manipulate disease resistance genes in plants. Molecular markers are allowing the dissection of monogenic and quantitative resistance. The utility of marker-aided selection in breeding for disease resistance is being evaluated. Markers will soon enhance the effective deployment of resistance genes to provide more stable resistance. Several resistance genes have been charac­ terized at the molecular level. This has led to the identification of several mechanistic classes and provides the opportunity for transgenic disease control strategies. As the mechanisms of detennining specificity and generating vari­ ation at resistance loci are understood in greater detail. they will be manipulated to generate new specificities. probably using microbial selection systems. Disease resistance strategies utilizing classical resistance genes will comple­ ment the use of novel trans genes.

The Relationship Between Plant Disease Severity and Yield

Abstract: The relationship between disease and yield is most often summarized as a simple empirical model that describes average crop performances is the presence of a pathogen. Such models may be robust and useful for surveys but their use is usually constrained to the specific conditions under which the modal was developed. Changes in production system usually invalidate the relationship. The alternative is to base the relationship on an epidemiological analysis of the pathogen population and a physiological concept of host growth and development. This review provides the knowledge and conceptual basis and discusses the limitations to progress in the development of such models. It is shown that a host-based assessment of disease is well suited to yield investigations and to multiple pest constraints, and that disease is logically related to yield via radiation interceptions and radiation use efficiency.

Transmission of Viruses by Plant Nematodes

Abstract: ABSTRACf Longidorid and trichodorid nematodes transmit nepo- and tobraviruses, respec­ tively. A specific association exists between vectors and their viruses which is a consequence of the nature, site and mechanism of virus particle retention within the vecto r. It is correlated with the serological properties of the virus coat protein and detennined by the RNA-2 segment of the virus genome and by an inherited character of the vector. The virus coat protein is probably involved in the recognition process between vector and virus but is not the sole determinant for transmission of tobraviruses. Genetic changes made to proteins present in the RNA-2 segment of pea early.browning tobravirus have been used to reveal the probable involvement of several proteins in vector transmission.

Conceptual and practical aspects of variability in root-knot nematodes related to host plant resistance.

Abstract: Root-knot nematodes are the leading cause of crop loss due to plant parasitic nematodes. A few species (Meloidogyne armaria, M. chitwoodi, M. hapla, M. incognita, M. javanica) have very wide host ranges that limit nonhost rotation options. Numerous host resistance genes in diverse crops are identified and could be used in cropping sequences to manage root-knot. Approaches to their combined use that maximize effectiveness and preserve durability, including pyramiding genes in a crop and in a cropping system, are considered with resistance gene specificity to nematode species and populations. (A)virulence specificity and selection in Meloidogyne for matching host resistance genes is examined, with particular reference to tomato and cowpea. Analogies are made with bacterial and fungal pathogen-host gene-for-gene interactions. A biotype scheme is offered for characterizing, categorizing, and communicating Meloi­ dogyne (a)virulence.

Biochemical and biophysical aspects of water deficits and the predisposition to disease.

Abstract: A predisposition to disease is often observed in host plants during water deficiencies. While there are no instances in which the biochemical and bio­ physical causes are known with certainty, there are changes in host plants that alter their int era ctions with other organisms an d that suggest possible mecha­ nisms of predisposition. Gradients in water potential can be altered by water deficits and prevent growth in the host without altering growth ofthe pathogen. There is a decrease in photosynthetic activity and protein synthetic activity when water deficits develop that could decrease the synthesis of metabolites and enzymes important for disease res istance. A method of supplying meta­ bolites has been used to change the flux of these molecules in intact plants and might provide tests of the causes of host predisposition to disease during water deficits.

Concepts and terminology on plant/pest relationships: toward consensus in plant pathology and crop protection.

Abstract: In plant pathology , terminological confusion still reigns despite national attempts at standardization. Terminological agreements reached within the crop protection community in The Netherlands are elaborated here and presented as an endeavor toward international consensus. Much of the on-going terminological disconcert derives from differences in outlook between academically oriented biologists (including biologically trained pathologists) and pathologists working in and for agricultural institutions where disease and harm have anthropocentric connotations. The name crop protection science more realistically covers and marks the field dealt with by most plant pathologists, and adoption of the FAO-defined term pest to encompass all biotic factors that are harmful to plants and their products is advocated. The effect of pests on plants and the interrelationships between pests and plants in dependence upon the environment, topical in resistance breeding, are especially dealt with. A diagrammatic model is used to better describe these relationships and to define the terms that denote the phenomena and mechanisms involved.

Use of Alien Genes for the Development of Disease Resistance in Wheat

Abstract: The genus Triticum contains three ploidy levels and about 30 species. Most of these species have been investigated as sources of disease-resistance genes and several have been used in successful transfers of resistance to domestic wheat (T. aestivum. genomes AABBDD). In addition, at least six genera from the tribe Triticeae have been used successfully as donors of disease-resistance genes for domestic wheat. The amount of alien chromatin involved in these transfers varies from a single gene to chromosome arms or entire chromo­ somes. No attempt was made in this review to describe all alien resistance gene transfers in wheat or to outline the various techniques involved. Alien disease resistance genes covered in detail are those that confer resistance to barley yellow dwarf virus, wheat streak mosaic virus, Cephalosporium stripe (caused by Cephalosporium gramineum) and eyespot (caused by Pseudo­ cerosporella helpotrichoides).

The impact of molecular characters on systematics of filamentous ascomycetes.

Abstract: Information derived from nucleic acid analyses either has complemented phylogenetic arguments based on phenetic characters or facilitated choice among competing hypotheses. Despite limited taxon sampling, a picture of the interrelationships of filamentous Ascomyceteas at higher taxonomic levels is developing. Intergeneric relationships within groups that include economically important fungi (e.g. Eurotiales, Hypocreales) are being clarified, and generic circumscriptions redefined. Molecular analyses have supported predictions of links between individual asexual species of groups or asexual species of the Fungi Imperfecti, and groups of Ascomycete genera and species. However, individual asexual species have not been linked unequivocally to individual Ascomycete species. Anamorph names are necessary and should be retained because teleomorphs may not be recognized in vivo nor formed in vitro. In the few cases where phenetic and molecular phylogenies seem irreconcilable, the ribosomal genes may not give the m...

The Oak Wilt Enigma: Perspectives from the Texas Epidemic

Abstract: Ceratocystis fagacearum (Bretz) Hunt, the oak wilt pathogen, is currently causing massivel osses of semievergreen live oaks (Quercus fusiformis Small and Q. virginiana Mill.) in central Texas. Given the relatively limited oak mortality caused by C. fagacearum in the deciduous forests of the North Central, Midwestern, and Mid-Atlantic United States, this Texas epidemic was not anticipated. The intensity of oak wilt in Texas is attributed to a number of factors related to host characteristics and the ability of the pathogen to adapt to limiting environmental conditions. Oak wilt management in semievergreen oaks requires considerable revision of the control techniques previously designed for deciduous oaks. The Texas oak wilt epidemic provides a new perspective from which to evaluate questions concerning oak wilt, including the origins of the pathogen as well as the potential for future losses in unaffected oak forests.

Models from Plant Pathology on the Movement and Fate of New Genotypes of Microorganisms in the Environment

Abstract: engineered host range ABSTRACf Because of their long history of studying the environmental fate of nonengi­ neered plant pathogens, plant pathologists can contribute to an evaluation of the environmental fate of genetically engineered microorganisms (GEMs). This review discusses three relevant SUbtopics: dispersal, change of ecological niche as exemplified by host range, and fitness. Dispersal of microorganisms in the daytime, turbulent atmosphere provides significant potential oflong-dis­ tance dispersal, and matters of scale are critically important in determining this potential. Studies of plant pathogen host range confirm the commonly held view that microorganisms are plastic with regard to adaptation to environment, but suggest that change in ecological niche will be inversely proportional to the magnitude of the change. Fitness of plant pathogens is not constant with regard to either environment or time, and the same is likely to be true of genetically engineered microorganisms.

Contributions of Plant Pathology to the Biological Sciences and Industry

Abstract: Research in plant pathology has made major contributions to knowledge of basic biology and genetics of plants and microorganisms as well as to devel­ opment of new products for industry. Among examples cited are first evidence of the nature of viruses as agents of disease; development of density gradient ultracentrifugat ion, a powerful tool for research in virology and cell biology; studies on the unique properties of xanthan gum, the extracellular polysaccha­ ride of a plant pathogenic bacterium; development of a method for introduction of beneficial genes into plants via a tumorigenic bacterium minus its tumor­ inducing capability; and discovery that epiphytic ice-nucleating bacteria can trigger frost damage in plants.

Pioneer Leaders in Plant Pathology: ES Luttrell

Abstract: ES Luttrell was a mycologist who spent 42 of his 44 professional years at the University of Georgia, first at the Georgia Experiment Station in Griffin and later at the main campus in Athens. He is best known for his innovative classification scheme for the perithecial ascomycetes, in which orders were based on patterns of ascomal ontogeny and mode of ascus dehiscence. This work established him as an authority on ascocarp development. His later studies on the classification of the Helminthosporium complex, in which he showed that small differences in conidium germination and structure could be correlated with distinct teleomorphs, brought him recognition from plant pathologists as well. These studies were notable for his attention to detail and the quality of his work. His contributions to mycology and plant pathology were numerous and varied and brought him various awards later in his career. To his colleagues he served as administrator, teacher, mentor and friend.