Plant Diseases and Vectors: Ecology and Epidemiology

367 Pages·1981·6.993 MB·English

by  Karl Maramorosch (Eds.)

Plant Diseases and Vectors:
Ecology and Epidemiology
EDITED BY
KARL MARAMOROSCH
Waksman Institute of Microbiology
Rutgers University
New Brunswick, New Jersey
KERRY F. HARRIS
Department of Entomology
Texas A&M University
College Station, Texas
1981
ACADEMIC PRESS
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COPYRIGHT © 1981, BY ACADEMIC PRESS, INC.
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United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1 7DX
Library of Corîgress Cataloging in Publication Data
Main entry under title:
Plant diseases and vectors.
Includes bibliographical references and index.
1. Plant diseases. 2. Virus diseases of plants.
3. Insects as carriers of plant disease. I. Maramorosch,
Karl. II. Harris, Kerry F.
S3731.P65 632f.3 80-28311
ISBN 0-12-470240-6
PRINTED IN THE UNITED STATES OF AMERICA
81 82 83 84 9 8 7 6 5 4 3 2 1
CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.
Moshe Bar-Joseph (35), Virus Laboratory, Agricultural Research Organization,
The Volcani Center, P. O. Box 6, Bet-Dagan 50-20, Israel.
Luite Bos (1), Research Institute for Plant Protection, Binnenhaven 12,
Wageningen, P. O. Box 42, The Netherlands.
Lloyd N. Chiykowski (105), Chemistry and Biology Research Institute,
Agriculture Canada, Ottawa, Ontario K1A 0C6, Canada.
Thomas W. Carroll (293), Department of Plant Pathology, Montana State
University, Bozeman, Montana 59717.
Luigi Chiarappa (319), Plant Protection Service, Food and Agricultural
Organization of the United Nations, Via dιlie Terme di Caracalla, 00100-
Rome, Italy.
Gustaaf A. de Zoeten (221), Department of Plant Pathology, University of
Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706.
James E. Duffus (161), United States Department of Agriculture, Science and
Education Administration, 1636 E. Alisal Street, P. O. Box 5098, Salinas,
California 93915.
Bryce W. Falk (161), Department of Plant Pathology, University of California,
Riverside, California 92521.
Steven M. Garnsey (35), Horticultural Research Laboratory, United States
Department of Agriculture, Science and Education Administration, 2120
Camdem Road, Orlando, Florida 32803.
Robert M. Goodman (181), Department of Plant Pathology and International
Soybean Program (INTSOY), University of Illinois, Urbana, Illinois
61801.
Michael E. Irwin (181), Office of Agricultural Entomology, Department of
Plant Pathology, and International Soybean Program (INTSOY),
ix
CONTRIBUTORS
χ
University of Illinois and Illinois Natural History Survey, Urbana, Illinois
61801.
C. L. Mandahar (241), Department of Botany, Panjab University, Chandigarh-
160014, India.
Pätru G. Ploaie (61), Laboratory of Phytopathology, Plant Protection Institute,
Academy of Agricultural and Silvicultural Sciences, Bucharest 71592,
Rumania.
PREFACE
This is the fourth in a five-volume series of books on vectors of plant disease
agents. The first three volumes, Aphids as Virus Vectors, Leafhopper Vectors
and Plant Disease Agents, and Vectors of Plant Pathogens, are up-to-date trea­
tises on the interactions of plant pathogens such as viruses, mycoplasmalike or­
ganisms, spiroplasmas, fungi, bacteria, and rickettsialike organisms with their
plant hosts and insect, nematode, mite, or fungal vectors. Such interactions form
the bases of vector-dependent, pathogen transmission cycles and, hence, become
integral parts of any studies aimed at elucidating the dynamics of disease spread.
In this sense, disease epidemiology was certainly not ignored in earlier books in
the series, but neither was it the topic of primary emphasis, as is the case in this
volume.
Successful transmission of vector-dependent pathogens requires that the
transmission participants, vector, pathogen, and host, interact in a manner com­
patible with pathogen acquisition, carry-over, and inoculation by the vector and
subsequent infection site development in the host. The newly emerging science
that asks how various biotic and abiotic components of the environment affect
pathogen-vector-host compatibility as measured by pathogen spread or vector
transmission efficiency might be referred to as transmission ecology. Consider­
ing the complexity of any one transmission system and the number and types of
vectors and pathogens covered in the three previous volumes, it would of course
be impossible to include all of the information available on transmission ecology
and disease epidemiology in a single volume. Therefore, we have attempted to
choose timely topics that illustrate some of the incipient overriding principles in
this challenging field of research.
The book is comprised of ten chapters representing the expertise of 13 out­
standing scientists from a total of seven different countries. The occurrence of
disease epidemics or, to state it differently, the extent to which pathogens are
dispersed by arthropod vectors, is basically a reflection of the degree of spatial
and temporal overlap between vectors and pathogen sources. Appropriately,
therefore, we begin our book with a chapter on the ecological involvement of
wild plants in plant virus pathosystems. This is followed by an equally timely
discussion of the principles and applications of enzyme-linked immunosorbent
xi
xii PREFACE
assay (ELISA) in diagnosing plant viruses and monitoring their move­
ment in the environment. Chapters three and four give detailed accounts of
the epidemiologies of diseases caused by leafhopper-borne viruses, mollicutes,
and rickettsialike organisms. Chapter five brings us abreast of the latest develop­
ments in understanding the importance of helper agents to the transmission ecol­
ogies of many aphid-borne plant viruses. And, in Chapter six, the aphid-borne
and seed transmitted soybean mosaic virus serves as an excellent model for pre­
senting an in-depth coverage of the many and varied factors that can contribute to
the epidemiology and control of a disease affecting a major agricultural crop of
the world. Chapter seven, "Early Events in Plant Virus Infection," focuses
attention on the ecological importance of virus-plant interactions and the devel­
opment of disease at the organismic level, including virus genome release and
cell ingress, host-virus recognition and virus replication in the first cell entered,
and transport of the infectious entity from cell to cell and over long distances
within the plant. A vector of plant viruses not covered in earlier volumes of the
series—the host plant itself—is dealt with in Chapters eight and nine, with dis­
cussions of the ecological and epidemiological significance of virus transmission
from one sporophyte generation to the next via virus infected or infested seed or
pollen. And, finally, man himself, yet another participant in and manipulator of
the transmission cycle, is the focus of attention in Chapter ten; especially with
respect to the epidemiological hazards he has created in major crops of develop­
ing countries around the world.
It is the editors' hope that this volume furthers our knowledge of transmission
ecology and disease epidemiology, not only by serving as a valuable supplemen­
tal textbook, reference work and bibliographical source but also by catalyzing
novel syntheses of present thinking and stimulating further research in the area.
If we have succeeded in these regards, it is thanks to our authors for their schol­
arly contributions and to the staff of Academic Press for their continuing encour­
agement and assistance in producing these volumes.
Chapter 1
WILD PLANTS IN THE ECOLOGY OF VIRUS DISEASES
L. Bos
Research Institute for Plant Protection (IPO),
Wageningen,
The Netherlands
1.1 INTRODUCTION 1
1.2 THE ECOLOGICAL MODEL 2
1.3 DEFINITION OF WILD PLANTS . 4
1.4 VIRUS INFECTIONS OF WILD PLANTS 5
1.5 VIRUS 'POLYPHAGISM' 7
1.6 IMPACT OF WILD PLANTS ON CROP VIRUSES 8
1.6.1 Virus Survival 8
1.6.2 Origin of Crop Viruses 12
1.6.3 Virus Spread 13
1.6.4 Virus Establishment 15
1.6.5 Refuges and Sources of Virus Vectors 17
1.6.6 Sources of Crop Resistance to Viruses 20
1.7 RELATIVE CONTRIBUTION OF WILD PLANTS TO VIRUS DISEASES. . . 21
1.8 IMPLICATIONS FOR CONTROL OF VIRUS DISEASES 23
1.9 NATURAL COMPLEXITY 26
1.10 REFERENCES 28
1.1 INTRODUCTION
In recent years, crops have been increasingly studied as ecosystems. No mat­
ter how artificial they may seem—as suggested by the designation 'crop industry'—
crops remain highly complex natural systems, which comprise a multitude of in­
terrelated factors. Several of these act from the natural environment at large.
Copyright© 1981 by Academic Press, Inc.
PLANT DISEASES AND VECTORS 1 All rights of reproduction in any form reserved.
ISBN 0-12^70240-6
2 L. Bos
Among these factors, viruses play an intriguing part and their involvement can
be described as an ecosystem or virus pathosystem. I have recently discussed its
general principles and outlined a scheme to survey the various factors and to clar­
ify their multicausal and dynamic interplay (Bos, 1980). It is also meant to com­
prehend ways of managing crop ecosystems (interfering with virus ecology) to
control virus diseases.
The present book on virus ecology goes into much greater detail and I have
been asked to write on the role of wild plants. Wild plants have long been pre­
sumed to contribute to virus spread and disease incidence. For earlier discussions
and general information see Hein (1953), Schwarz (1959), Break and Polâk
(1966), Heathcote (1970), Murant (1970), Duffus (1971) and Schmidt (1977).
Only recently though has wild plant involvement been described in ecological
terms. This chapter is an elaboration of a paper presented during the 3rd Inter­
national Congress of Plant Pathology in Munich, West Germany, September
1978 (Bos, 1978). It will analyze the various aspects, and illustrate these with
examples from the literature.
1.2 THE ECOLOGICAL MODEL
Figure 1 summarizes the groups of factors involved in virus ecology and surveys
the dynamic interrelationships within the virus ecosystem as developing with
time. It allows a first orientation as to the involvement of wild plants and will be
used as a basis for farther elaboration of the subject.
From a cultural point of view, man's chief concern is the crops. These have to
be protected, for instance from various viruses. Some of the viruses of plants
have been characterized, but several are still unidentified. In addition to form
and size, viruses differ in ecological relationships, so they may require completely
different ways of control.
Besides the crops and the viruses come the sources of infection. These may be
(1) similar crops, or closely or remotely related crops, or even a single infected
plant, nearby or at a distant place, (2) individual plants within the crop to be
protected, introduced by infected seed or planting stock, or (3) uncultivated
plants in or near the crop, or far away. That is where wild plants enter the picture.
The next group of factors is vectors or vehicles of spread. Plants themselves,
including wild ones, may 'actively' participate through contact or by their natural
propagation material including true seeds. Man himself, although long unaware,
has been actively engaged in spreading viruses with crop propagation material
and in germplasm, part of which is derived from wild crop relatives often from
distant parts of the world.
Growing conditions greatly influence susceptibility and sensitivity of crops
and wild plants to virus infection, as well as vector behavior. Crop micro-climate
is considerably affected by the presence of wild plants within the crop territory.
Nutrition and even infection with viruses may have a direct bearing on host pal-
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atability to insect and other vectors and even on vector behavior. Thus there
are a multitude of interdependences and I will later return to some of these in so
far as they bear on the role of wild plants.
1.3 DEFINITION OF WILD PLANTS
Webster (1976) defines 'wild' as "living in a state of nature, inhabiting natural
haunts, not tamed or domesticated, growing or produced without the aid and
care of man, not living near or associated with man." Hence in plain words, wild
plants are uncultivated plants, plants that develop without human aid, for in­
stance in natural habitats. They have long been ignored by agronomists and left
to the interest of biologists. However agricultural interest in wild plants has rap­
idly increased because wild plants, still growing in their natural habitat or assem­
bled in botanic gardens, may include species or types that offer prospects for
cultivation, either directly or for provision of genes in crop diversification. Wild
plants also include the ancestors of present-day crops. Interbreeding with related
cultivars may broaden genetic variation. Collections of wild germplasm are now
thriving. Wild plants include also crop species or their descendants that have
gone astray and run wild, such as groundkeepers and volunteer plants, and crop
plants in abandoned fields. Wild plants usually grow in places where they pre­
sent no direct threat to crop development, as in wild vegetation and on unculti­
vated land.
Weeds also are uncultivated plants, and Webster's définition at first sight in­
cludes them, except that weeds live near or in association with man. They usually
grow on cultivated land and may in certain instances depend on man. Thus,
Webster (1976) defines a weed as a "plant growing in ground that is or has been
in cultivation usually to the detriment of the crop or to the disfigurement of the
place." Duffus (1971), in referring to literature, describes weeds as "plants with
harmful or objectionable characteristics that grow where they are unwanted."
They directly interfere with crops and compete with crop plants for space, food,
water and light.
Hence, the terms weed and wild plant are not exactly equivalent but there is
much overlap. Weeds are wild plants that grow on cultivated land, but no sharp
limit exists. Likewise, there is no clear distinction of wild plants or weeds from
cultivated plants, since weeds and volunteer plants developing in a crop may later
be used for grazing cattle in a stubble. The term weed is emotive; it has a deroga­
tory connotation as something threatening and thus requiring destruction. Wild
plants, even when distant from crops but harboring pathogens including viruses
that are harmful to crops, would be weeds in the sense of Duffus (1971), and so
should be destroyed. Calling something a weed has direct ethical consequences.
This is even more apparent with the equivalents in German, 'Unkraut', and French,
'mauvaise herbe.' There is now a tendency to speak more neutrally of 'wild plants.'
For their role in the environment, these plants are now enjoying more goodwill.