Showing papers in "Annual Review of Phytopathology in 1980"

Lignification as a Mechanism of Disease Resistance

Abstract: This review deals with the formation of lignin as a mechanism of resistance to plant disease. Lignin is one of the most abundant biopo1ymers on earth (120, 140) and is resistant to degradation by most microorganisms (83,84). It is thought to be formed commonly as a response to microbial penetration and mechanical damage (51, 71, 111, 140), but the role of lignification in disease resistance has been investigated in only a limited number of cases. The possible specificity of lignin formation in disease resistance remains unresolved.

The Biology of Striga, Orobanche, and other Root-Parasitic Weeds

Abstract: This review deals with higher plants that are parasitic on the roots of other higher plants. These unique organisms in the genera Striga and Orbanche, are both weeds and parasites. As weeds they show great phenotypic plas­ ticity, wide environmental tolerance, prefer permanently disturbed habitats, and are part of a plant guild associated with colonizing or crop complex species. As parasites, they depend upon another vascular plant for food or water, which flows from host to parasites through haustoria. These struc­ tures form a morphological and physiological graft with the roots or other underground parts of the host. The remainder of the root system of these parasites is usually condensed and lacks typical root hairs and root caps. All species of Orobanche and most Striga are obligate parasites-they will not develop at all without a host. Facultative parasites such as most of the parasitic species in the Scrophulariaceae are not nutritionally dependent on a host plant but they are invariably attached to one in nature. Root parasitism is found in the following diverse dicotyledon families of seed plants: Hydnoraceae, Balanophoraceae, Krameriaceae, Lennoaceae, Santalaceae (and segregate and allied families), Scrophulariaceae, and Oro­ banchaceae (88, 176). No parasitic monocots are known and there is only one parasitic gymnosperm, Parasitaxus ustus; this organism is endemic to New Caledonia-it has an unique host-parasite interface involving a graft­ like union of tissue (L. J. Musselman, unpublished).

Soil anaerobiosis, microorganisms, and root function

Abstract: Anaerobic (oxygen-free or anoxic) conditions result from restriction by excess water of the free diffusion and convection of oxygen between the atmosphere and the soil. The ensuing changes in the chemical and biological properties of the waterlogged soil greatly affect the activities of soil microor­ ganisms and plant roots, and depress the growth of plants not especially adapted to wet-land conditions. Characteristic symptoms of damage during temporary waterlogging in herbaceous and woody species when tempera­ tures are high include a slowing of leaf and shoot extension and a yellowing of the older (lower) leaves suggesting premature senescence. Wilting, leaf epinasty, and leaf abscission may occur; in Gramineae tillering is sup­ pressed; and some diseases may become more prevalent. Phloem transport from the shoots into the original roots is soon inhibited (51, 205) and the roots cease extension and dry matter accumulation and often degenerate, particularly at the apex (23,51,93, 165,214,215). Prolonged waterlogging can lead to death of the entire plant in species that do not adapt (203). The effects of poor soil aeration on root function and plant growth have been the subject of an extensive literature (reviewed in 18, 71, 88, 203) but the causes of injury to plants, particularly the role of soil microorganisms, remain obscure, perhaps because of the multiplicity of mechanisms. The primary cause of impairment to root metabolism is still a matter for conjec­ ture. One possibility is that the generation of ATP in anaerobic respiration is inadequate for maintenance of cell metabolism in higher plant cells. With

Pathogen Variability and Host Resistance in Rice Blast Disease

Abstract: Almost 90 years have passed since Cavara described the fungus Pyricularia oryzae, which causes rice blast disease. Rice blast occurs in all major rice growing regions of the world and accounts for devastating epidemics and a considerable decrease each year in rice production. In some countries large quantities of fungicides have provided temporary relief. Many resis­ tant cuItivars have also been developed, but the resistance has been short lived. Thus, the disease continues to be a problem in rice growing areas. The literature on rice blast is voluminous, however, the mystery, confu­ sion, and controversy over older subjects, such as classification, nomencla­ ture, and host range of the fungus, have not been resolved, and the controversy surrounding the newer subjects of variability, cytology, genet­ ics, and host resistance has increased (49). In continuing the attempt to develop resistant cultivars, a reexamination of pathogen variability and host resistance deserves our special attention. Because of space limitations it is not possible to cite all the literature on the subject. This review focuses on the host-parasite relationship, which establishes a type of stable resistance. It departs somewhat from conven­ tional thinking, and therefore may be considered controversial. We are reminded by Stakman & Christensen (65) that "many plant disease fungi

Biological Control of Soilborne Fungal Propagules

Abstract: In the light of present-day constraints on plant disease control practices, especially those imposed on the use of pesticides, biological control is increasingly capturing the imagination of many plant pathologists and is gaining stature as a possible practical agricultural method for soilborne pathogen control. The enhanced research activity on the subject of biologi­ cal control is in line with increased effort and determination by plant pathologists and soil microbiologists to adapt to the conceptual scheme of integrated pest management as an acceptable ecosystem approach to disease control and to realize that biological control must become one of the basic components in pest management practices. The growing realization that biological control of plant pathogens can be successfully exploited in modem agriculture has enjoyed the attention of several major symposia during the last 15-20 years and stimulated the publication of an excellent book (13). Numerous reviews have also covered the status of biological control (32, 33, 45-47, 85, 102, 105, 135, 141, 151) and its principles and mechanisms (14, 95, 149). Many ecological aspects of propagule survival in soil have also been reviewed (21, 30, 86, 93). The enhanced research activity of the last 15-20 years has greatly in­ creased our knowledge of microbial interactions in soil and of basic princi­ ples and concepts. We must admit, however, that our accomplishments in the field of applied biological control lag behind those accomplishments in the theoretical field. We must realize that few researchers seem to have

Biological Control of Nematode Pests by Natural Enemies

Abstract: +3741 Various aspects of biological control of plant-parasitic nematodes have been reviewed a number of times (5, 9, 16, 39, 56, 75), but the numerous recent developments in this area and the impending loss of several of the most widely used nematicides for reasons of hazards to human health have prompted this examination of the status of natural enemies for control of nematode pests. It is not my intent to list or consider all organisms in this category because little information has been acquired on many of them since previous reviews. This review deals only with the most important and promising natural enemies on which there is sufficient information to make at least preliminary interpretations of their value and potential role in biological control. It also attempts to analyze recent studies, compile re­ ports of new organisms and, I hope, stimulate and guide future research in this important field.

Reactions of nonsuscepts to fungal pathogens

Abstract: A nonsuscept is traditionally defined as a plant in which the growth of a particular plant pathogen, even in the appropriate tissue, is far less than in a susceptible host; usually this restriction in fungal growth is accompanied by a lack of sporulation. Using this definition, it becomes clear that most higher plants are nonsuscepts for most fungal plant pathogens. Nonsuscept­ ibility (used here as synonymous with resistance) to pathogenesis is usually shown by all individuals of a plant species, and this species is regarded as a "nonhost" for the fungus concerned. However, nonsuscepts may also be found among varieties and cultivars of what is considered the host plant although this nonsusceptibility may be shown only toward certain races of the pathogen. In nonsusceptible hosts and nonhosts, resistance to pathogenesis usually is accompanied by some detectable reaction of the plant. Such reactions have been described in many reviews (e.g. 1, 17,48,78,79,80,87,146, 149, 150), but the most recent of these have tended to provide detailed discussion of only one type of response. The purpose of this review, therefore, is to provide an overview of the topic, with emphasis on investigations carried out in the last five to ten years; for comprehensive summaries of earlier work the reader is referred to the excellent reviews by Tomiyama (146) and Kuc (79). The reactions discussed below are primarily those of the nonsuscept which differ from those elicited by the same pathogen in its susceptible host; however, the definition of a nonsuscept implies nothing about the absolute degree of vegetative fungal growth or the type of plant response. Since the term depends on the corresponding definition of susceptibility, situations

Management of Viruses by Alteration of Vector Efficiency and by Cultural Practices

Abstract: The management of plant virus diseases may seem like simply a popular philosophy for the control of plant diseases; however, careful crop manage­ ment is as basic today for the control of most important virus diseases as the use of modern pesticides for control of other plant pathogens. Far too often, growers concentrate solely on producing a crop, and incidentally become concerned with the managing of virus diseases. But, as we wish to establish in this review, sound crop production practices exist which not only result in a better crop, but also can minimize virus diseases by altering vector efficiencies and by using crop management strategies. In many cases, these measures are the only options available for the control of plant virus diseases. The biological factors that account for virus spread are as intricate as one might imagine considering that three factors (host, vector, and virus) are closely involved. Plant viruses, more so than any other group of pathogens, depend almost totally on insect vectors for their dissemination, which al­ lows for some interesting phenomena. This almost total dependence is further complicated by the close association between host and pathogen

Nontarget Effects of Pesticides on Soilborne Pathogens and Disease

Abstract: The application of pesticides into the environment generally can be ex­ pected to result in a diverse array of effects on target and nontarget organ­ isms. Because of this, it often is not immediately apparent what we should define as a target or nontarget effect. Only when the side effects become apparent are questions raised regarding the adequacy of a pesticide for gen­ eralized field use, the environmental and economic importance of the side ef­ fect, and the desirability of testing for such effects prior to use of the material. The terminology used for interpretation of nontarget effects of pesticides may at times cause some confusion. Most pesticides have a wide range of biological activities extending beyond the specific function intended by the manufacturer. Nevertheless, groups of pesticides are named according to their intended use. Thus, "fungicides" are formulated to provide a certain spectrum of activity against specific pathogenic fungi, "nematicides" are designed to control undesirable nematodes, and "herbicides" are intended to control noncrop (weed) plants. From within the context of this com­ monly accepted terminology, we proceed in this chapter to discuss the unintended or nontarget activities of certain compounds that may alter pathogen behavior and influence the induction of severity of disease.

Effects of Fungal Viruses on Their Hosts

Abstract: Although the presence of viruses in fungi has long been suspected (102, 113), experimental evidence was not forthcoming until 1962, when virus particles were demonstrated in diseased mushroom (48, 53). The subse­ quent discovery that virus particles in Penicillium spp. (44, 68) contain double-stranded RNA (dsRNA) greatly stimulated the search for virus in other filamentous fungi. To date viruses or virus-like particles (VLPs) have been reported in over 100 species of fungi belonging to all major taxa (8, 55, 74, 76). The discovery of fungal viruses (mycoviruses) in the 1960s unleashed great expectations, as virus infection was suggested to account for some of the abnormalities and unusual biological activities associated with certain fungi. Many of the reported examples of cytoplasmic inheri­ tance in fungi have not as yet been assigned to any particular genome (105), and the possibility that virus infection rather than a mutation in a cytoplas­ mic gene may cause some of these conditions appeared attractive (57,81). The fact that virus-infected, but not virus-free, strains of the bacterium Corynebacterium diphtheriae produce diphtheria toxin (46) provided the rationale for investigating the role of virus infection in toxin production by certain plant pathogenic fungi (13, 43). Furthermore, virus infection was sought to explain the variation in antibiotic production and the instability of some strains of industrial fungi (75, 77). The possibility that viruses with potentially adverse effects on phytopathogenic fungi might be exploited as a means of biological control was pursued with enthusiasm in several labo­ ratories around the world (69, 95).

New Concepts in Breeding for Disease Resistance

Abstract: Any scientific discipline has a basis in theory, and theory may be defined as a coherent system of concepts. Breeding plants for resistance to crop parasites embraces several disciplines within crop science and conceptually, these disciplines have developed at different rates. The major concepts in genetics were developed some three quarters of a century ago, while those in epidemiology were developed only recently, largely as a result of the work of Van der Plank (17-20). The conventional approach to breeding for resistance is consequently based almost exclusively on genetic theory; cru­ cial aspects of epidemiological theory have been ignored for the obvious reason that they were unknown. These new aspects of epidemiology, and their likely influence on plant breeding, were reviewed recently [Robinson (14)], and the present paper is an attempt to expand some of the more interesting of those ideas. While knowledge of this recent review would be an advantage, this is not essential as its important features are summarized here.

Cytogenetics and Morphology in Relation to Evolution and Speciation of Plant-Parasitic Nematodes

Abstract: Adaptation to plant parasitism has occurred extensively in nematodes in the order Tylenchida and four genera of the order Dorylaimida. The ancestors of plant parasites probably possessed a protrusible stylet for feeding, before they became gradually adapted to plant parasitism. First, they may have lived in the rhizosphere and fed on fungi and other lower microorganisms; then they fed on plants as ectoparasites of roots and above-ground parts; and finally, they entered the various plant tissues to become highly adapted endoparasites (2, 35, 38). Once some weakly plant-parasitic forms became

Biological Significance of Multicomponent Viruses

Abstract: One of the significant advances in plant virology in the past decade has been the development of knowledge of multicomponent plant viruses. The sub­ ject has been reviewed periodically and in some detail (7, 21, 38, 40, 54, 60, 63). The purpose of this paper is not to present another review, but to speculate on why different plant viruses exist as multicomponent systems. The term divided genome (60) may be more proper than multicomponent when applied to these viruses, because their significant characteristic seems to be that the genetic material is distributed among two or more nucleo­ protein particles. In the case of the rod-shaped tobraviruses (tobacco rattle and pea early browning viruses) particles are rods of two different lengths. This is also the case with the smaller, round-ended rods of alfalfa mosaic virus and some other ilarviruses. Most of the multicomponent viruses, however, are isometric. The particles of some, such as tobacco streak virus, differ in diameter; others, such as the comoviruses and nepoviruses, have all particles the same size. The fact that some particles contain more nucleic acid than others results in differences in sedimentation rates upon ultracentrifugation. When these viruses, as well as those with particles differing in size or shape, are layered on the top of sucrose density gradients and centrifuged for several hours, the different particle types (components) form discrete bands some distance down the tube. For certain other multicomponent viruses, bromoviruses and cucumoviruses for example, quantitative differences in nucleic acid content of different particle types are not great enough to provide significant differences in sedimentation rates. The particle types do differ, however, in