Showing papers in "Annual Review of Phytopathology in 1971"

Current Status of the Gene-For-Gene Concept

Abstract: One of the most successful means of controlling plant diseases has been the development of varieties with major or vertical resistance genes. This type of resistance is easily manipulated in a breeding program and is efIec­ tive until strains of the pathogen to which it does not confer resistance be­ come established. Then, if another gene that conditions resistance to the new strains of the pathogen is available, this resistance gene may be incorporated into the variety by the plant breeder. In doing this, the breeder either con­ sciously or unconsciously is applying the principle of the gene-far-gene hypothesis. Plants resistant to races that are virulent on old varieties possess the new resistance gene. With the diseases of some crops, this process has becn repeated at relatively frequent intervals (4D, 42, 82). However, in some instances a single gene has conferred adequate resistance for many years 80,82). In plant diseases caused by living organisms, the same phenomena: in­ fection type in rusts, percent of infected plants in smuts of cereals, fleck or lesion in apple scab, are criteria of both the reaction of the host and the pathogenicity of the parasite. They indicate the relative resistance or sus­ ceptibility of the host and the relative avirulence or virulence of the para­ site. The gene-for-gene hypothesis was proposed (20,25) as the simplest ex­ planation of the results of studies on the inheritance of pathogenicity in the .flax rust fungus, M elampsora lini. On varieties of flax, Linum usitatissimum that have one gene for resistance to the avirulent parent race, F 2 cultures of the fungus segregate into monofactorial ratios. On varieties having 2, 3, or 4 genes for resistance to the avirulent parent race, the F2 cultures segregate into bi-, trio, or tetra factorial ratios (20-22) respectively. This suggests that for each gene that conditions reaction in the host there is a correspond­ ing gene in the parasite that conditions pathogenicity. Each gene in either member of a host-parasite system may be identified only by its counterpart in the other member of the system.

Effects of Microorganisms on Lignin

Abstract: Disintegration of plant tissues is a major aspect of pathogenesis. In a large percentage of plant diseases, tissue breakdown is the primary mode of injury and is directly responsible for loss. Consequently, many plant patholo­ gists have studied the enzymatic processes by which plant cell wall constitu­ ents are degraded. Most of these researches have emphasized the microbio­ logical degradation of herbaceous agricultural crops in which pectic materials and cellulose are the major structural components. But plant pathologists must also be concerned with the degradation of the woody tissues of perennial plants. These plants are the predominant vegetation of the earth, and the organisms that degrade their tissues play a major role not only in the loss of valuable crops, but also in the carbon cycle of the earth. In most herbaceous plants pectic materials are the cementing substances between cells. In woody tissues, by contrast, this function is performed by the complex aromatic polymer lignin. Lignin is thus a major component of peren­ nial plants, and as such is probably the second most abundant of all the con­ tinuously cycled organic materials on earth (cellulose is presumably the most abundant). As a major byproduct of pulp manufacture, lignin is also one of the largest industrial waste materials. The biological decomposition of lignin therefore is clearly an important problem, but even so it has not received nearly as much attention as has the degradation of pectic materials and cellulose. Its degradation consequently is only very incompletely understood. The lack of penetrating study reflects partly a lack of persistent interest in a refractory problem. However, it is also clear that until relatively recently the unavailability of lignin prepara­ tions of demonstrated integrity, together with difficulties in working with lignin in chemical and microbiological studies, and especially a lack of under­ standing of the chemical structure and behavior of lignin, have been im­ portant problems impeding progress. These and other problems have earned the respect of scientists who are intimately involved in lignin research, such as 1. A. Pearl, who noted recently (82) that there has been the " ... lack of appreciation of so many investigators of the many pitfalls inherent in lig­ nin research and the consequent publication of numerous papers that confuse rather than clarify the state of our knowledge." Unfortunately, this has been just as true for studies of the microbial degradation of lignin as for other

Tolerance to Plant Disease

Abstract: Resistance to plant disease, considered in the broad sense, may be brought about by various mechanisms and be present in differing degrees. The term tolerance has been used for several meanings within such a con­ text. Dictionary definitions include : the act or capacity of enduring. Other usages of the word relate to attitude and permissible variation. In plant dis­ ease literature, tolerance has usually designated either endurance or an in­ termediate level of observable resistance somewhere between immunity and full susceptibility. Maintaining both uses concurrently, is confusing. I will consider tolerance in the sense of Caldwell et al (13) in which plants en­ dure severe disease without severe losses in yield or quality. In such a sense, tolerance fits into various schematic classifications of disease resistance such as «(�) escape, (b) exclusion, (c) host-parasite interactions following infection, which lead to differing levels of disease, and (d) tolerance or endurance of a given level of disease. I have used the term resistance here in the broad sense, encompassing various phenomena, among them, toler­ ance as just described. In subsequent discussion the term resistance is used in its narrower sense to describe different levels of host-parasite interac­ tions, and is thus distinguished from tolerance an ' d may be contrasted to it. Future authors may find new specific terms for these different uses of the term resistance. Classical observations include those of Cobb (19) in 1894 and Orton (49) in 1909 on disease endurance. Cobb (19) wrote that

Interactions Between Nematodes and Fungi in Disease Complexes

Abstract: Nature does not work with pure cultures. I suspect that many plant diseases are influenced by associated organisms to a much more profound degree than we have yet realized, not only as to inhibition but also as to acceleration of the process. It may be that a number of diseases require an association of organisms for their occurrence and cannot be produced by infection of one organism alone. Pure cul­ tures we must, however, continue to use as a basis for the known mixtures and as controls on the activity of the mixtures. Research with mixtures of microorganisms will not furnish an excuse for any less care than with pure cultures in excluding organisms foreign to the mixtures. Work with mixtures, however, will not make the already complex problems of plant pathology as a whole any easier or less complex but it may throw much light on certain relationships which will never be discovered by the use of pure cultures of single organisms (23).

Role of Weeds in the Incidence of Virus Diseases

Abstract: From the standpoint o f control o f virus diseases, there is perhaps no phase of virology more important than epidemiology The role of weeds in the occurrence and spread o f plant virus diseases is an integral part o f the ecological aspect o f virus transmission L H Bailey (3) has commented that nature knows no plants as weed s Indeed, at the dawn of agriculture some 8,000 years ago, man would have recognized all plants as natural components o f the flora Man altered the earth's floral cover in his first efforts in agriculture, and weeds were born King (61) has pointed out that perhaps the shortest definition of a weed is that attributed to Professor W J Beal, who defined a weed as "a plant out of place" King has summarized some thirty definitions into ten commonly accepted characteristics of a weed: growth in an undesired location; com­ petitive and aggressive habits; wild and rank growth; resistant to control; large p opulations, with abundant, rank and extensive growth; useless, un­ wanted, and undesirable; harmful to man, animals, and crops; spontaneous growth, appearance without being sown or cultivated; of high reproductive capacity, and unsightly with disfigurement of the landscape Thus, weeds are plants, with harmful or objectionable characteristics, that grow where they are unwanted Whether a plant is considered a weed depends not only on its characteristics but also on its relative position with reference to other plants and to man The same plant may be a weed in one environment and an economically important plant in another Thus, in at least one incident, sugar beets were considered to be noxious weeds during certain times of the year in legislation proposed to enforce a beet-free period Numerous reports (5, 14, 32, 118) have indicated a close correlation be­ tween incidence o f beet yellows virus and overwintering beets In Europe, the sources of beet yellows virus include beet seed crops and infected man­ golds in clamps; in the United States, they include escaped beets growing in waste places and beets in overwintering fields The beet mosaic virus (32, 118) has been shown to be even more closely associated with overwintering

Mycoplasmas, Rickettsiae, and Chlamydiae: Possible Relation to Yellows Diseases and Other Disorders of Plants and Insects

Abstract: In 1967, based on their discovery of mycoplasma- like bodies in si eve ele­ ments of yellows-infected plants and the therapeutic effect of tetracycline antib iot ics, Doi, Ish iie, and coworkers (4 2, 95) pro pose d that the causal agents of yel lows disease might be mycoplasma- or chlam ydia-like organ­ isms. Since that time, considerable rese arch has confirmed the morph ologi­ cal si milari ties between the presumed yellows agents and the Mollicutes (my ­ copla sma) 2 and has greatly stren gthened the hypothesis of mycoplasmal etiology (14, 18, 137, 225). Yet, the intracellular multiplication of the myco­ plasma -li ke organisms in yello ws-infected hosts contrasts with the usual ex­ trac ellular multiplication of vertebrate mycoplasmas. Moreover , in spite of numerous attem pts, yellows agents have not convincingly been demonstrated to grow in cell-free media. Thes e aspe cts suggest that it may be useful, in st udies on yel lows diseases, to consider certain features of obligate intra­ cellular parasi tism. Thus, although we emphasize the recen t literature on yellows agents and discuss the mycoplasmas in some detail , in this review we also briefly re view othe r pro car yotes, suc h as chlamydiae and rickettsiae, not known to be patho genic in plan ts and therefore generally unfam iliar to plan t pathologists. We know of no fundamen tal reason, however, why such organisms could not reside in plan t tiss ues and even ind uce disease. Indeed, our attention is drawn to this possibilit y by the occurrence of ri cket tsi a- like organisms in phy tophagous insects (13 0), by the possible resemblance of certain Ricket tsi ales to "mycoplasma-li ke" organisms in some yello ws-in­ fe cted hosts, and by re cent suggestions of rickettsia-like organisms in dodder (69) and in sali va of homo pterous insects ( 128). 1 The following abbreviations will be used: A Y (aster yellows); CS (corn stunt) ; WX (Western X-disease). • The vernacular terms mycoplasma and mycoplasmas properly signify only members of the genus Mycoplasma; however, in this review th ey will be used in a broa der (al though improper ) sense to si gnify any members of the Mollicutes until a suitable ver nacular name for this class of organisms comes into use.

Fine Structure of the Host-Parasite Interfaces in Mycoparasitism

Abstract: This review is not intended as a computerized compendium of literature citations but as a personalized discussion utilizing selected references in ad­ dition to unpublished data to synthesize previous work, illustrate the present state of knowledge, and propose fruitful areas for further investigations. The emphasis throughout is on broad concepts as they relate to the host­ parasite interface in plant disease, rather than on minute details. Every host-parasite-environment combination is unique in itself, and every investi­ gator has his own technical "touch". It is, therefore, not unexpected that micrographs of the same disease syndrome may differ from one another in minor (or even major) ways, but a sufficient body of data has now accumu­ lated so that common factors are emerging. It seems most fruitful to place emphasis on these common factors and selective coverage of the literature so that our allotted space may be used to convey the vital concepts that are evolving as a result of just a decade of research in this area of phytopatho­ iogical research. Particular emphasis has been placed on the literature that has appeared since the reviews of Hawker (52) and Bracker (11). A living fungal cell (or part of a living cell) may enter a living host cell and establish (at least temporarily) a functional relationship there via haustorial formation as in the rusts and mildews, or via thallus formation as in cabbage clubroot. We believe that on the basis of ultrastructural research to date, three, or possibly four, major haustorial types can be distinguished from one another. The type of morphological association between the haus­ torium and the host cell is onc of the diagnostic criteria on which the separa­ tion into types is made. For purposes of this discussion, Type I is represented by the uredial stage of Puccinia graminis tritici; Type II by the conidial stage of Erysiphe graminis,' Type III by Phytophthora infestans; and the still uncertain type IV by various Peronospora spp.

Viruses that Infect Fungi

Abstract: In recent years, viruses have increasingly been found in association with fungi, an association that has taken one of two forms. In the first, the fun­ gus is the vector of the virus. So far, virus transmission has been demon­ strated only in Oomycete fungi ( Chytridiales and Plasmodiophorales ) . The virus-vector relationship appears to be highly specific and there is at present no evidence that any of thc viruses concerned can multiply in its fungal vector. Thus, some very elegant electron microscopy recently indicated a purely superficial adsorption of tobacco necrosis virus on zoospores of the vector (68). This aspect of viruses and fungi has already been reviewed (28, 35), and we shaH therefore consider only the second form of virus-fun­ gus association, in which the virus infects the fungus. Several transmissible disorders of fungi were described during the 1950s, and some authors speculated that viruses might be involved (8, 49, 56), but proof of viruses in fungi was first obtained with a die-back disease of cultivated mushroom (32). In 1967, a virus was found in an Ascomycete (15 ) , and in following years several different viruses were reported from species of Penicillium and Aspergillus. These latter have been mostly the cultures used by the pharmaceutical industry. Recently, a virus-like particle has been reported from a Phycomycete (66). Thus, viruses have now been described in aH the major fungal groups and, as with flowering plants, the number of "new" viruses to be reported wiH probably be proportional to the research effort involved. We shaH discuss first the viruses of mushrooms and other Basidiomy­ cetes, and then viruses of Fungi Imperfecti and Ascomycetes.

Microbial Penetration and Immunization of Uncongenial Host Plants

Abstract: To initiate a parasitic disease the propagules of the infectious agent must contact, in an apt environment, the appropriate anatomical region of an appropriate host. Until recently phytopathologists have been mainly in­ terested in the study of the events aroused by the fulfilment of such condi­ tions, overlooking what happens when the propagules of a pathogen fall on plants not belonging to its natural circle of hosts, or when a normally non­ pathogenic microorganism comes in contact with a plant. Only lately has greater attention been paid to this second type of combination. The root system of a plant is permanently in contact with a multitude of microorganisms. But the quantity of nonpathogenic contaminants of both "resident" or "casual" types (sensu Leben, 73), on the aerial surface of a plant is generally higher than the quantity of pathogens. At least in theory it must be assumed that the avirulent microorganisms can induce physiologi­ cal changes in the plant and thus variations in its reactivity towards subse­ quent infection by specific pathogens. Besides the direct antagonistic activ­ ity (140) and the physical and chemical alterations that are produced espe­ cially in the soil environment, this is another way through which micmflora may affect the establishment of a parasitic process. The relations between plants and avirulent germs, and the influence of the latter on the likelihood of subsequent infections by normal pathogens, can be studied better in the laboratory, where they can be mimed and mag­ nified under controlled conditions, than in nature where they cannot. A great deal of progress has been made in the similar field of plant virology where investigations carried out with combinations of different viruses and virus strains have led to evidence and to codifying the complex phenomena known as "acquired immunity following recovery," "cross protection," "lo­ calized and systemic interference" (108). This paper is primarily concerned with the "immunizing" effect that the "infections" with avirulent fungi and bacteria are capable of inducing in plants. We shall therefore examine (a) whether nonpathogens can pene­ trate and induce morphophysiological responses ill the plant, (b) whether

Protein Changes in Diseased Plants

Abstract: The purpose of this review is to present the current survey of protein changes in diseased plants, in connection with the metabolic alterations. Be­ sides the survey, some possible mechanisms of protein changes are also de­ scribed, together with some problems to be solved in the field. This review also deals with the relation of protein changes in disease with those in senescence and ripening of plants, and their role in the defense reaction, wound healing, and deterioration. There are several kinds of plant pathogens such as fungi, bacteria, and virus. Most pathogens cited here are fungi and bacteria. Viral diseases are not specially cited, because viral mUltiplication in plants, which is the main subject of protein changes in viral disease, proceeds in a different way from that of fungi and bacteria. Nematode infection is also excluded from this review. Some reviews on metabolism as related to protein changes in viral and nematode diseases, have been presented (23,25,72, 103, 107, 129), The review is organized as follows:

Fluorides as air pollutants affecting plants

Abstract: /Pathological effects of fluoride have been known for nearly 1000 years when the ancient disease of gaddur, or gaddjax was first described in early Icelandic literature. Later known in English as dental fluorosis of cattle, the disease developed on domestic animals feeding on grass contaminated by ash deposited following volcanic eruptions (67). Fluoride has been recognized as injurious to plants only since 1883 when Schroeder & Reuss first reported damage to vegetation in Germany (75). Reports of damage in areas near superphosphate and glass factories followed shortly (42,66,89). Not until the 1940s though, with the tremendous expansion of the alumi­ num and phosphate industries, was fluoride recognized as a threat to agri, culture and forestry in local areas of the United States. Research during this period was stimulated by serious crop losses and injury to forests near major new phosphate, aluminum, and steel plants largely in Florida, Wash­ ington, and Utab. Fluoride is a natural component of the earth's ernst, with concentrations in soils commonly exceeding 0.05% and occasionally 1 %. Plants normally accumulate small amounts of this fluoride; on rare occasions, toxic concen­ trations may be absorbed from acid soils, but polluted atmospheres provide the main source of the .fluoride accumulated by plants. Fluoride is released whenever the clays, rocks, coal, or ores containing it are heated, or when fluoride is used as a flux and allowed to escape into the atmosphere. Fluorides are well recognized as air pollutants, and their harmful effects on plants have received considerable attention in the past two decades. Their role in plant disease , and the concept of air pollutants as disease inci­ tants dates back nearly a hundred years (70) and was fully described by Darley & Middleton in 1966 (19). Their statement that "the increasing im­ portance of air pollution injury to vegetation as a factor in agricultural pro­ duction emphasizes the need for plant pathologists to become more aware of pollution-incite d diseases" is still timely and is certainly true for fluorides.