Showing papers in "Annual Review of Phytopathology in 1976"

Papillae and Related Wound Plugs of Plant Cells

Abstract: Over 100 years ago deBary (29) discovered localized apparent wall thickenings on the inner surfaces of plant walls at sites of penetration by fungi. Shortly thereafter (30, 31) he described these structures as cell wall ingrowths and noted a correla­ tion between their occurrence and the failure of further fungal development. By the turn of the century these wall-like depositions were thought to be paramu­ ral, i.e. located between the plasmalemma and cell wall (106) (Figure 2). The depositions were later shown to be extremely common, and although not ubiqui­ tous (5, 47, 54, 67, 120), they apparently accompanied every host wall penetration in certain parasite-host pairings (3, 8, 37, 47, 54, 71, 96, 107). If intracellular fungal development is slight or absent at an encounter site (potential or actual penetration site) the depositions are usually hemispherical (3, 46, 54, 71, 96, 107), but if considerable development has occurred there, they may conform to some extent to the intracellular parasite structures around which they form and thus encase them (Figure I) ( 2, 17, 30, 31, 37, 46, 53, 71, 77, 80). Be­ cause these various forms of wall-like depositions commonly have similar sub­ structure, composition, origin, and mode of formation (Figure I), I refer to them all as papillae (106) [or callosity, lignituber, callus (4)]. The term papillae is often used to refer to hemispherical depositions only, with other terms being applied to morphological variants (15). However, viewing them all as papillae emphasizes the concept that they are all products of similar processes. The term sheath was rather consistently used in the old literature (97) to refer to papillae, but it has quite a different meaning in current literature ( 15). Several articles (4, 15, 20, 38, 110, 119) have briefly reviewed some aspects of papillae and should be consulted for additional information and viewpoints. Attack by pathogenic agents other than fungi may result in papilla formation or related responses. Ingestion of plant cell contents by nematodes is sometimes preceded by rapid aggregation of cell cytoplasm at the feeding site (122, 123), a

Microbial Colonization of Plant Roots

Abstract: Research in microbial growth and interactions on roots is like ly to increase in the future as the need for integration of plant pathology and soil microbiology is recognized. On the one hand, some diseases are suppressed by soil microorgan­ isms (4) and on the other hand, the soil microflora can also.suppress beneficial root symbionts; for example, the establishment of a selected mycorrhizal fungus is easier in sterile than in n onsterile soil (21, 143) . Also, poor responses to Rhizobium inoculation may result from competition from less effective indige­ nous strains and antagonists in soi l . Interest in controlling the rhizosphere comes also from the effects of certain rhizosphere microorgan isms themselves on plant growth; many organisms in the general soil microflora may be subclinical pathogens reducing root and root hair growth or acting directly on the plant (8 , 19). Inoculation of many plant species with certain soil bacte ria sometimes in­ creases yield, advances flowering, or increases internode extension (27, 93, 117). After a long period devoted largely to isolation of organisms from roots, there is new impetus into finding mechanisms of colonization of the root; also, ex­ perimental approaches to the population dynamics of microorganisms on roots are emerging. In this review we summarize the present information and suggest possible future research . Good data are sparse and conclusions have necessarily often been made on the basis of very few experiments.

Epidemiology and Control of Tomato Mosaic Virus

Abstract: The cultivated tomato ( Lycopersicon esculentum) is subject to a range of diseases resulting from infection with certain strains of tobacco mosaic ¥irus (TMV). The mosaic disease in tomato was described in the Netherlands in 1910 (158), and in the USA in 1916 (9). Since then considerable efforts have been made to under­ stand its epidemiology and to devise control measures. The fact that the disease remains widespread suggests that the efforts to date have not been entirely successful. TMV is the most infectious plant virus known; young tomato plants are in­ fected when rubbed with infected tomato sap diluted one part in 5 million parts of water (27). It is also the most persistent virus in terms of its ability to survive outside plant cells and in dead tissues (38). Its numerous strains infect a very wide range of plant species, although there is considerable specificity between strain and host. It is not surprising, therefore, that the disease is ubiquitous wherever tomatoes are grown under glass and in many areas of outdoor cultivation. Tomatoes are not a cheap crop to produce, particularly under glass, and until recently about 20% of world production probably was lost because of TMV. The literature on TMV is voluminous; so also is the literature on the disease in tomato. Many of the papers mentioned in my 1960 review (25) are omitted from this paper.

Diseases as a Factor in Plant Evolution

Abstract: Disease epidemics are among the most spectacular of biological phenomena. Not only can mortality be enormous but it is selective, sparing the occasional resistant or escapt(d individual and destroying the susceptibl; ones. Crops, domesticated animals, and man himself have suffered repeated epidemic disasters throughout recorded history. Of the many plagues that raged across Europe and Asia, the one in the winter and spring of 1347 -1348 was especially virulent and the death rate exceptional. In many communities, less than 10% of the population survived and there were not enough healthy people to bury the dead (16). In 1633, a smallpox epidemic swept through the New England Indian population with such virulence that some villages were carried off entirely (14). Even measles, a com­ paratively mild disease in Europe, took a devastating toll of Polynesians in Hawaii (61). From the havoc wrought on the forces of Sennacherib (2 Kings 19:35) and the plagues visited on Egypt to the 20th century, history has been punctuated by the dread and terror of epidemic disease. Most of us have seen towns and cities stripped of their best shade trees by Dutch elm disease, and some of us can recall the great, stark, grey hulks of dead chestnut trees that once dotted our eastern forests. We can, somehow, get Py without shade and timber trees, but when epidemics strike the food plants we li.ve by, famine and starvation may result. We all know of the great potato famines of 1846-1851 which had such devastating effects on the population of Ireland. Perhaps we are not so well informed on the great famine in Bengal of 1943 (65) caused primarily by Helminthosporium oryzae (Cochliobolus miyabeanus). Even a nation producing substantial food surpluses like the United States can suffer heavy financial loss in epidemic years. In countries that normally import some food and that have limited exchange resources, epidemics of crop disease are utter disaster. Plant diseases can cause humans to die (of starvation) as well as the black death or cholera. One must suppose that any agent that selectively destroys genotypes, populations, or whole species as efficiently as epidemic disease surely has played a role in the evolution of modern faunas and floras.

Fire and Flame for Plant Disease Control

Abstract: Fire was the first great force employed by man, and the discovery of a method to ignite vegetable matter looms as one of humanity's greatest achievements. The earliest evidence of deliberate use of fire dates from Peking man, or as much as 500,000 years ago, and use of fire is suspected to have been known to man thousands of years earlier (83). Some of primitive man's reasons for the burning-over of land were to clear forest (or, rarely, grassland) for agriculture; to improve grazing land for domestic animals or to attract game; to deprive game of cover, or to drive game from cover in hunting; to kill or drive away predatory animals, ticks, mosquitoes, and other pests; to repel the attacks of enemies or to burn them out of their refuges; to ex­ pedite travel; to protect villages, settlements, or encampments from great fires by controlled burning; and to gratify sheer love of fires as spectacles (7). Indians of North America lighted bushes and trees so that the roots would burn slowly and provide a continuous source of fire (83). Wildfire, along with soil, water, and temperature, has been one of the major forces that shaped the earth's vegetation. Most of the grasslands and many of the forests represent subclimax vegetation that from antiquity has been dependent on periodic fires for existence. The winter burning of dead grass left unconsumed from the growth of the previous season is a worldwide practice of ancient origin, particularly in humid regions where uncut grass does not cure into palatable winter forage. Grass fires set by the American Indians and by lightning main-

Ethylene in Soil Biology

Abstract: Ethylene, a simple unsaturated hydrocarbon gas, is well known in biology for its profound and often spectacular effects on plant growth. It is an endogenous plant growth regulator functional at concentrations as low as 0.04 ppm, with most growth responses saturated at about I ppm. Ethylene is produced by most plant parts, and directly or indirectly affects the physiology of most organs and virtual­ ly all developmental stages of plants. It is also involved in some host-pathogen interactions. These aspects of ethylene in plant biology have been thoroughly reviewed (\, 17, 18, 46, 87). More recently, ethylene has been identified as a common constituent of the soil atmosphere of both aerobic and anaerobic soils. This ethylene is produced by microbial activity and concentrations are high enough to be biologically active. Important, far-reaching claims have been made about its role in soil biology. This review concentrates on ethylene in this context. Most emphasis is placed on ethylene as a critical regulator of microbial activity in soil with its implications in the important soil processes of rates of turnover of organic matter, availability of essential plant nutrients, and the incidence and control of soil-borne plant dis­ eases. Cursory attention is paid to the significance of this exogenous source of ethylene on plant root growth and seed germination.

Genetic Change in Host-Parasite Populations

Abstract: Although the total amount of factual genetic information relating to population changes in host-parasite systems is small, the recent literature in phytopathology abounds with interpretation and discussion of events that have taken place or may have taken place at the population level. Accordingly, this review does not catalog the detailed results of a large number of studies. Its purpose is to review, in the light of the theory of population genetics, the mainly speculative literature that is accumulating. Because the existing observational and experimental data pertain almost exclusively to parasitic systems in which host populations have been manipulated by man, this review centers mainly on changes that have oc­ curred, or may occur, in parasite populations as a direct consequence of human intervention. The process by which pathogen populations change as a direct result of human activity was referred to by Johnson ( 17) as "man-guided" evolution. The need for accurate information that would have relevance to this interpretation was referred to in an earlier review (27). That need still exists. There is also a pressing need for adoption of a clearly defined set of terms (a problem recently discussed in another connection by Nelson, 26). It is also important to distinguish between organisms that reproduce asexually (either as haploids or diploids) and those that do not. Population genetics has traditionally dealt with those that do not repro­ duce asexually. Relatively little attention has been directed to organisms such as the rusts, mildews, and other parasites that undergo repeated cycles of asexual division which are only occasionally interrupted by nuclear fusion and meiosis, and in which genetic recombination may occur only occasionally or not at all.