CHAPTER 9 Plant Diseases and Plant Ecology Nikolaos E. Malathrakis and Dimitrios G. Georgakopoulos CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 Effect of Diseases on the Structure of Plant Communities and Plant Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Age Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Spatial Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Plant Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Temporal Structure-Succession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Competition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Diversity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Diversity within Plant Communities . . . . . . . . . . . . . . . . . . . . . 189 Diversity within Plant Populations . . . . . . . . . . . . . . . . . . . . . . . 189 The Effect of Pathogen Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Type of Dispersal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Wind Dispersal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Rain Dispersal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Insect-Transmitted Inoculum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Virulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Pathogen Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Effect of the Type of Epidemic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Some Major Plant Epidemics: Ecological Aspects . . . . . . . . . . . . . . . . . . . . . 195 Dutch Elm Disease [Ophiostoma (Ceratocystis) ulmi] . . . . . . . . . . . . . 195 Chestnut Blight [Cryphonectria (Endothia) parasitica] . . . . . . . . . . . . . 195 Dieback Caused by Phytophthora cinnamomi. . . . . . . . . . . . . . . . . . . . 196 Potato Late Blight (Phytophthora infestans) . . . . . . . . . . . . . . . . . . . . . 196 Tristeza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 Other Pandemics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 183 0-8493-0904-2/01/$0.00+$.50 © 2001 by CRC Press LLC 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 183 184 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Weed Control with Fungal Pathogens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Epilogue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 INTRODUCTION Plant pathogens are among the main biotic factors of any ecosystem and may play a major role in its dynamics. However, over the greater part of the history of plant ecology it has been convenient to assume that the structure and composition of plant communities is mainly determined by macro- and microclimate, soil conditions, and interactions among the plants themselves (Harper, 1990). In natural ecosystems the role of plant pathogens has tended to be neg- lected. Recently, however, attention has been paid to the importance of plant pathogens and the relevant diseases on the pattern of plant com- munities (Dobson and Crawley, 1994). Dinoor and Eshed (1984) number sev- eral reasons for the growing interest in diseases in the wild. The dramatic eco- logical impact of several plant pandemics, such as Dutch elm disease, was probably the most important. However, there are other diseases with less evi- dent, but no less important, effects on plant communities that merit great attention. In agroecosystems, on the other hand, there has always been a great deal of concern about plant diseases, but they were mostly considered from a directly economical viewpoint. Well-known examples of destructive diseases in agricultural systems include the great potato famine, which devastated the population of Ireland from 1846–1851, and the 1943 great famine in Bengal due to rice blast (Strange, 1993), but we know much less about the impact of these, and several other epidemics, on the ecology of their hosts. This is prob- ably due to the difficulty in studying this aspect of the consequences of dis- ease and man’s interference, which jeopardizes the potential interactions of plants and plant diseases. Harper (1990) questioned the existence of convincing evidence with respect to the role of pests and pathogens on plant communities and posed fundamental questions which should be answered. Although those questions are far from being answered, several publications, which have appeared since then, provide increasing evidence that plant diseases may affect plant ecology through the innumerable interactions taking place in any plant com- munity and plant population. The present chapter approaches the following aspects of the subject: (1) the effect of diseases on the structure of plant communities, (2) the contribu- tion of some major pathogen attributes and the type of epidemics, (3) the eco- logical impact of selected plant pandemics, and (4) the effect of weed control by pathogenic fungi on regulation of weed populations. 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 184 PLANT DISEASES AND PLANT ECOLOGY 185 EFFECT OF DISEASES ON THE STRUCTURE OF PLANT COMMUNITIES AND PLANT POPULATION Studies to elucidate plant-disease interactions and their effect on plant ecology are few. Data from such studies supporting the potential effect of plant pathogens on several aspects of the structure of plant communities and plant populations are presented below. Age Structure In the wild, it is assumed that newly established populations are more susceptible to diseases than are older ones (Harper, 1970). Carlsson et al. (1990) carried out comparative studies of many populations in areas where population age can be estimated to test this assumption. They compared dis- ease incidence caused by three host-specific systemic fungal pathogens on host plant populations of Valeriana sambucifolia, Trientalis europea, and Silena dioica. They found that in all three pathosystems, disease incidence was higher during an early-intermediate phase of population development. Populations of individual species with an estimated age of over 50, 400, and 300 years respectively showed low disease incidence (Ͻ10%). In other pathosystems, natural infections depended upon environmental conditions. Armillaria spp., for instance, is a well-known group of root rot-inducing pathogens world- wide. They may cause both primary infections of healthy trees as well as sec- ondary infections of stressed trees. Primary infections tend to diminish with stand age of over 20–30 years. Since only a small proportion of the total pop- ulation is usually infected, aged trees may prevail in infected areas. However, in drier, inferior forests, continuing mortality in all age classes is common in many stands (Kile et al., 1991). Trees infected by Dutch elm disease may sur- vive for some years after infection. Elms planted in the areas where the dis- ease is prevailing usually survive for less than 20 years. Given the destructive effect of the disease, old trees in such stands should be rare. In agroecosystems, plant diseases affect the aged structure of standing crops in two ways. First, an established plantation is maintained as long as it is healthy enough to produce a good yield. Second, in several cases early cul- tivars are grown in order to avoid plant diseases. The theory behind this is to reduce the time of the epidemic’s progress, as shown in Vanderplank’s equa- tion describing disease progress, X ϭ X 0 e rt (where X denotes the amount of disease at time t, X 0 the amount of initial inoculum, and r the rate of disease progress), and keep infection at a low level. Spatial Structure The effect of abiotic factors on the spatial structure of plants within plant communities is much more evident than is the effect of plant pathogens. It is 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 185 186 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT well known, for instance, that soil acidity and soil salinity, among others, are major factors that determine the spatial structure of plant communities. The role of plant pathogens is sometimes hidden under the effect of the disease that may be exacerbated by abiotic factors. For instance, in soils with high pH, potatoes often fail to thrive not because of the soil pH per se but because potato scab, which is favored by such soil conditions, becomes a production constraint. Mal secco disease of citrus, caused by Phoma tracheiphila, is deter- mining which citrus species are grown in several areas in the Mediterranean. Lemon, the most susceptible species, is grown only in the least windy areas where infections of wind-damaged shoots are fewer. In the wild, one of the most extensively documented cases is the disaster caused in Australian forests by the fungus Phytophthora cinnamomi. It pro- duces a typical epidemic which may worsen over approximately five years, but, about three years after infection, field-resistant species colonize the floor of the diseased forest, thus completely changing the spatial pattern of plants in the community (Weste and Mark, 1987). Due to its wide host range, the invasion of this pathogen in an area exerts a definite regulatory effect on an entire set of plants which may be the main component of the local flora. Other plant diseases with a large range of hosts may play a similar role. Xylella fas- tidiosa, a xylem-limited bacterium, is the principal factor preventing the development of high quality Vitis vinifera and V. labrusca grapes in the south- eastern U.S. where it is endemic (Hopkins, 1989). It is assumed that because of its very large host range of cropped and wild plants favored by the same environmental conditions, the structure of entire plant communities in the same area may be affected (Newhook and Podger, 1972). Plant Density In natural systems, plant density is the result of the established interac- tions of all the biotic and abiotic factors. Generalizations about patterns of density-dependent mortality and reproduction are a subject of plant popula- tion ecology (Harper, 1977). However, there is limited information on the effect of plant pathogens on the relevance of these generalizations for plant populations growing in the presence as opposed to absence of different plant pathogens (Mihail et al., 1998). Dense stands contribute to increased disease infection because of the establishment of microclimatic conditions such as high relative humidity, which favor pathogen infections, increase root con- tacts that enhance transmission of root diseases, etc. (Burdon and Chilvers, 1982), indicating the regulatory effect of diseases on host populations. Several fungal pathogens and all bacteria require free moisture to produce disease, while infections by most other fungal pathogens are favored by high relative humidity (Harrison et al., 1994). The spread of root diseases, such as white rot of onions caused by Sclerotium cepivorum, is positively correlated with plant 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 186 PLANT DISEASES AND PLANT ECOLOGY 187 density and can be controlled by spacing the host to eliminate root contact between adjacent plants (Scott, 1956). Mihail et al. (1998) reported that Rhizoctonia solani and Pythium irregulare reduced the number of plants and the total biomass of the annual legume Kummerowia stipulacea. Reduction was higher in plots with higher sowing densities. Burdon (1978) claims that “the interaction between plant density and disease has certain features of a self- regulatory feedback system and as such has special interest in the considera- tion of all plant communities. Thus, because of its faster rate of dissemination, a pathogen is likely to kill more plants at high than at low plant densities. This death of plants reduces plant density and this in turn tends to curb the pathogen through its effect on transmission from plant to plant.” In some pathosystems, thinning has been adapted as a standard practice for disease management, indicating the regulatory effect of pathogens on plant density. For example, thinning of high risk trees (over 50% girdle) is recommended for management of infected mature plants of southern pines infected by fusiform rust caused by Cronartium quercuum f.sp. fusiforme in the U.S. (Powers et al., 1981). Evidence of the regulatory effect of diseases on host population can be found in several other studies (Augspurger, 1988). Ingvarsson and Lundberg (1993), using a mathematical model to study the effect of Ustilago violacea on the population density of Lychnis viscaria, found three different outcomes of this interaction: (1) extinction of the fungus, (2) a stable coexistence between plant and fungus, and (3) extinction of both plant and fungus. Virus particle numbers may decrease with increasing host density due to the difficulties of insect vectors in spreading disease in dense stands (Boudreau & Mundt, 1997). Hence, it appears that diseases are an important regulatory factor for plant densities, but their effect seems to be disease specific. Temporal Structure-Succession Succession is the process whereby one plant community changes into another. Although the deterministic concept with respect to succession in plant communities was initially accepted, the role of randomness in succes- sion is rather universally adopted now (Crawley, 1994). Stemming from this new concept, the role of plant epidemics, which appears as an accident rather than as a sequence of events, could also be considered. It is reported that dur- ing primary succession in areas where no life pre-existed, the first colonists are cryptogams (Crawley, 1994). However, we are not aware of any report that plant pathogens may interfere in primary succession. There are several models on pathways of secondary succession, but all have an intrinsic deter- ministic concept. Models based on the facilitation of succession of one organ- ism by another, such as the replacement of fast growing species by slower growing ones, etc., nearly predetermine plant succession in the community on the basis of plant characteristics and available resources. None of these 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 187 188 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT models consider the possible effect of plant pathogens. Nevertheless, several recent publications regard disturbances mediated by host-specific pathogens as underlining factors that determine successional relationships in a commu- nity. Holah et al. (1997) studied the effect of Phellinus weirii, a native root rot pathogen of Pseudotsuga menziesii (Douglas fir), an early species during suc- cessional development of infected forests in the lower Cascade and Coast ranges of western Oregon. They found that the presence of P. weirii in these sites appears to push changes towards the late successional species, Tsuga het- erophylla, Thuja plicata, and Taxus brevifolia. At least in the Cascade mountain sites, not only was there an increase of the late successional species within infection centers, but the trajectory along which disease had “pushed” within these sites was common to all three areas studied. Competition Nutritional resources are the most studied factors affecting competition in plant communities (Tilman, 1994). However, there is increasing evidence of plant pathogen interference on interspecific competition among plants in the wild. The main evidence is the flourishing of species introduced into areas in the absence of their pathogens. Chondrilla juncea, a common but not dangerous weed in Mediterranean countries, became a noxious weed throughout Australia. As soon as the fungus Puccinia chondrillina, a pathogen of this plant in its origin, was introduced, C. juncea populations declined (Hassan, 1988). Several other reports indicate that rust fungi and other biotrophic pathogens reduce the ability of infected plants to compete with healthy ones. Burdon and Chilvers (1977) found that mildew reduced the competitive ability of barley when grown in mixtures with wheat. Paul and Ayres (1987) noticed reduced competition of Senecio vulgaris infected with the species-specific rust fungus Puccinia lagenophorae over lettuce (Lactuca sativa) when grown in mixtures. Finally, Paul (1989) studied the effect of the same fungus on the competitive behavior of S. vulgaris versus the weed Euphorbia peplus and found that infected S. vulgaris was less competitive than the healthy. There are fewer, but not less important, examples from soil-borne diseases. Van der Putten and Prters (1997) found strong evidence that when Ammophila arenaria was exposed to its soil-borne pathogens, it was out-com- peted by Festuca rubra spp. arenaria, especially under nutrient limitation. The main issue is how pathogens affect the competitive ability of infected plants. Many factors are involved, such as the number of competing geno- types, pathogen type, infection time, and environmental factors, making it difficult to draw an overall conclusion. Reduction of seed production due to pathogen infection might reduce the competitive ability of the infected plant. In S. vulgaris infected by P. lagenophorae, seed production decreased by 60% over that of the healthy plants (Paul and Ayres, 1986). It seems that in each pathosystem the reaction is different and not always easy to identify. 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 188 PLANT DISEASES AND PLANT ECOLOGY 189 Diversity Diversity within Plant Communities The role of plant pathogens in plant community diversity, neglected for a long time, has recently been recognised both for aerial (Alexander et al., 1996; Burdon, 1987) and root-infecting pathogens (Bever et al., 1997; Burdon, 1987). Peters and Shaw (1996) executed an experiment on plots of rough grassland dominated by Holcus lanatus. Plots were cleared of vegetation in three successive years and allowed to regenerate. One third of plots was left untreated, one third of plots was regularly sprayed with propiconazole to reduce fungal diseases, and the last third was inoculated with urediospores of Puccinia coronata f.sp. holci on the second and third years of the study and with conidia of the leaf-spotting fungus Ascochyta leptospora in the third year. Vegetation cover and disease severity were regularly monitored. The authors concluded that, in communities dominated by grasses, foliar pathogens tended to decrease the abundance of perennial herbs and, therefore, decreased the diversity in regenerating plots by favoring grasses. Mills and Bever (1998) assume that soil community as a whole can con- tribute to the maintenance of diversity within plant communities. They claim that negative feedback occurs when the presence of a plant alters the soil microbial community in a manner resulting in growth reduction of that par- ticular plant species relative to other species, with the potential interference of soil-borne pathogens. Assuming that the negative feedback was related to the species-specific soil pathogens, they tested the effect of Pythium spp. on the growth of plant species in which negative feedback through soil commu- nity had previously been observed. Their results suggest that accumulation of species-specific soil-borne pathogens could account for this negative feed- back and conclude that soil pathogens may themselves contribute to the maintenance of plant species diversity. Diversity within Plant Populations The effect of plant-pathogen interactions on pathogen populations has been well studied in great detail in a number of agricultural and natural pathosystems. Little work has been done, however, on the long-term effect of disease on plant populations, although this situation has started to change with the use of modern molecular genetic techniques, such as the various electrophoretic methods for detection of DNA polymorphism or allozyme analysis. Plant resistance to pathogens has long been explained by the gene- for-gene theory (Flor, 1971), where a single plant resistance gene interacts with a matching pathogen avirulence gene to produce a resistance reaction. This type of resistance was based on specific interactions between certain plant cultivars and pathogen races. Several plant resistance and pathogen 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 189 190 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT avirulence genes have now been cloned and sequenced, although their mode of action still remains to be explained. Although the gene-for-gene theory was initially based on an agricultural pathosystem, it has been well docu- mented in wild plant pathosystems as well (Thompson and Burdon, 1992). Single gene plant resistance, however, is not the only type of resistance in nature. A broad and quantitative type of resistance to pathogens is also very common, but it has been less studied, perhaps due to its inherent complexity. This type of resistance also exists in natural plant populations, but its long- term effect has not been elucidated. In natural plant populations, plant resistance genotypes have co-evolved with pathogen virulence genotypes, interacting in a perpetual “arms race” where selection of resistance plant genotypes is followed by the reciprocal selection of pathogen virulent genotypes. Although this procedure is greatly influenced by environment (Paul, 1990) and spatial features of the surrounding vegetation (Morrison, 1996), a few cases have been documented in which dis- ease altered in time the composition of host plant genotypes in a population. Murphy et al. (1982) examined the competitive ability of five oats (Avena sativa) multilines in a mixture over four consecutive years in the presence and absence of infections by the crown rust pathogen Puccinia coronata. Each year, plants were inoculated with a mixture of five P. coronata races and were either treated with fungicide during the growing season to prevent infection or left untreated. During the course of the experiment, the frequency of certain mul- tilines in the population started to rise while others were reduced, but no sta- tistically significant difference was observed in treated and untreated plants. It would be interesting to see whether this trend would be maintained if the experiment was continued for a number of years. This study generates the hypothesis that disease has the potential of reducing genotypic variability in a population of plants over time. A recent study on the effect of oak wilt epidemic caused by Ceratocystis fagacearum is in accord with the former assumption. The genetic structure of oak trees before and after an epidemic wave was determined with allozyme analysis of wood samples (McDonald et al., 1998). Post-epidemic trees were survivors of a 20-year epidemic. Allozyme analysis indicated that genetic diversity of post-epidemic oak trees was lower than pre-epidemic diversity for two out of the four allozyme loci tested. Data analysis considered the effect of spatial distribution of trees and suggested that disease was the major factor driving this shift in oak forest genetic structure. A hypothesis proposed by Clay and Kover (1996) similarly suggests that systemic plant pathogens may sometimes promote host plant genetic unifor- mity. Several systemic plant pathogens are known to induce asexual repro- duction of their host or enforce self-fertilization, thus reducing genetic recombination. This provides the pathogen a selective advantage, because a susceptible genotype is perpetuated in a plant population and the pathogen can be vertically transmitted with seed. Direct experimental data are needed to support this hypothesis. 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 190 PLANT DISEASES AND PLANT ECOLOGY 191 Shifts in host plant genotypes effected by disease have been observed in a number of cases. In Australia, an attempt to stop the spread of the compos- ite weed Chondrilla juncea was undertaken by using the rust pathogen Puccinia chondrillina as a biocontrol agent. Plants belonged to three pheno- typically different genotypes, one of them being the most abundant. After nine years of biocontrol a complete shift in genotype composition was recorded, with the formerly most important genotype reduced to extinction in most areas and the two other genotypes prevailing and becoming the new target weeds for control (Burdon et al., 1981). THE EFFECT OF PATHOGEN ATTRIBUTES Plant pathogens share some attributes, such as type of dispersal and vir- ulence. Each of them, alone or in combination, clearly affects the interaction of plants and diseases and finally their effect on plant ecology. The effects of some of these attributes are briefly discussed below. Type of Dispersal Dispersal of pathogens or their carriers is closely related to the spread of any epidemic and plays a major role on disease appearance in new areas (for reviews see Fitt et al., 1989; McCartney, 1989). Pathogens are spread in sev- eral ways but for simplicity we mention only wind dispersal, rain dispersal, and insect transmission of inoculum. Wind Dispersal Airborne spores may travel intercontinentally and cause disease thou- sands of miles away from the original infection. For instance, spores of wheat stem rust are transferred each year from Mexico to the U.S. and Canada as well as from India to Scandinavia. Coffee rust, caused by the fungus Hemileia vastatrix, is also transferred via airborne spores. It was discovered early in 1970 in Bahia, Brazil, and four years later it had spread in South America over an area equivalent to the size of Central America. Coffee rust possibly came to Brazil from Angola with trade winds across the Atlantic in 5 to 7 days (Schieber, 1975). For long distance pathogen migration by air currents, propagules should reach high altitudes in the atmosphere by eddy diffusion. Otherwise they remain in the lower atmospheric layers and disperse over rather short dis- tances. Studies for dispersal of Cronartium ribicola, the causal agent of white- pine blister rust indicated that it is spread about 0.4 km away from infected Ribes. Other wind-borne pathogens, such as Venturia inaequalis (apple scab), follow the same pattern (Meredith, 1973). 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 191 192 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Rain Dispersal Bacterial plant pathogens, as well as fungi producing mucilaginous spores, are dispersed by rain splash since mucilage prevents dispersal by wind alone. Distance of dispersal depends on the size of rain drops and rarely exceeds 1 m. Insect-Transmitted Inoculum The majority of viral diseases and many bacterial and fungal diseases are transmitted by insects or other animals, such as nematodes. Many of the known catastrophic pandemics are animal-dispersed. Chestnut blight is spread by birds and insects, Dutch elm disease by the beetle Scolytes spp. and tristeza by several aphids such as Toxoptera citricida (Agrios, 1997). The dis- tance of animal-dispersed diseases depends on many factors, including ani- mal activity, type of crop, plant species and pathogen strain. T. citricida, for instance, is 25 times more efficient in transmitting the tristeza virus than Aphis gossypii. Although some strains of the virus are more easily transmitted by A. gossypii than others, their transmissibility, by either species, is markedly affected by the source plant used for acquisition feeding (Raccah et al., 1978; Bar-Joseph, 1989). Most viruses spread within crops and cause diseases of the “compound interest” type. However, the ultimate proportion of infected plants and the rate at which new infections appear vary widely among dif- ferent viruses and for different crops. Viruses that infect annual crops spread more rapidly than those of trees and shrubs. In a typical orchard in California, the citrus tristeza virus spreads to an average of two citrus trees a year for each infected one already present. By contrast, cauliflower mosaic virus spreads from a single infected plant to as many as 131 in one season. Invariably, viruses such as citrus tristeza, cacao swollen shoot, and plum pox take several years to spread throughout plantations. Nevertheless, their ecological impact is important since trees are far larger and take longer to grow (Thresh, 1974). Long distance transport of several wind-borne diseases is one of their main characteristics with respect to their epidemiology and their effect on plant ecology. Coffee rust, a wind-borne disease, spread to South America within four years, 1971–1974 (Schieber, 1975), but it took approximately two decades for Dutch elm disease, another fast-spreading insect-borne disease, to spread across Europe (Gibbs, 1978; Ingold, 1978), while chestnut blight spread in the U.S. at a rate of about 37 km/year (Anagnostakis, 1987). Man himself also acts as the main long distance transporting agent of many diseases. Several pathogens have been transferred to Europe from the New World during the last century and changed the structure of several crops as well as of natural plant communities. For example, potato late blight and downy mildew of grapes were introduced in Europe around 1845 and 1875, respectively, from America (Strange, 1993; Agrios, 1997), and chestnut blight was introduced in the U.S. probably from Japan or China. Citrus 920103_CRC20_0904_CH09 1/13/01 10:59 AM Page 192 [...]... virus diseases (Collmer and Howell, 199 2) Galliteli et al ( 199 1) reported a twofold increase of tomato yield in plants treated with a satellite containing a mild strain of CMV over the nontreated ones, as well as a slowed spread of disease in untreated plants 92 0103_CRC20_ 090 4_CH 09 194 1/13/01 10: 59 AM Page 194 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT in the same field Similar... ascospores and conidia Insects and animal vectors may also play a role in dispersal The chestnut blight epidemic expanded at 92 0103_CRC20_ 090 4_CH 09 196 1/13/01 10: 59 AM Page 196 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT a rate of about 37 km/year, and by 195 0 most of the chestnut trees (C dentata) had been destroyed It also appeared in Italy during the 193 0s, and within 25 years... structure of a Texas live oak population Can J Bot., 76: 190 0 – 190 7 92 0103_CRC20_ 090 4_CH 09 202 1/13/01 10: 59 AM Page 202 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Meredith, D.S., 197 3 Significant of spore release and dispersal mechanisms in plant disease epidemiology Annu Rev Phytopathol., 11:313–342 Mihail, J.D., Alexander, H.M and Taylor, S.J., 199 8 Interactions between root-infecting... from Asia (Maloy, 199 3; Strange, 199 3) 92 0103_CRC20_ 090 4_CH 09 198 1/13/01 10: 59 AM Page 198 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT WEED CONTROL WITH FUNGAL PATHOGENS Several fungi have been used for weed control For example, P chondrillina has been used against C juncea (skeleton weed), Entyloma compositarum against Ageratina riparia (hamakua pamakani), and Colletotrichum gloeosporioides... Crawley, M.J., 199 4 Plant Ecology Blackwell Scientific Publications, Oxford Dinoor, A and Eshed, N., 198 4 The role and importance of plants in natural plant communities Ann Rev Phytopathol, 22:443–446 Dobson, A and Crawley, M.J., 199 4 Pathogens and the structure of plant communities TREE, 9: 393 – 397 Fitt, B.D.L., McCartney, H.A and Walklate, P.J., 198 9 The role of rain in dispersal of pathogen inoculum Annu... outside Mexico eliminates the ecological effect of the disease in the agroecosystems of the large potato producing areas (Strange, 199 3; Agrios, 199 7) Tristeza Citrus tristeza virus (CTV) is one of the most important citrus diseases for the last 60 years Since its first appearance in Argentina in 193 0, it has 92 0103_CRC20_ 090 4_CH 09 1/13/01 10: 59 AM PLANT DISEASES AND PLANT ECOLOGY Page 197 197 destroyed... Infection by pathogens and population age of host plants J Ecol., 78(4):1 094 –1105 Clay, K and Kover, P., 199 6 Evolution and stasis in plant-pathogen associations Ecol., 77 :99 7 –1003 Collmer, C.W and Howell, S.H., 199 2 Role of satellite RNA in the expression of symptoms caused by plant viruses Annu Rev Phytopathol., 30:4 19 442 92 0103_CRC20_ 090 4_CH 09 1/13/01 10: 59 AM PLANT DISEASES AND PLANT ECOLOGY Page... 197 6) According to Sea ( 197 5; referenced by Weste and Marks, 198 7), in 197 5 the area of jarrah forest affected by P cinnamomi was estimated at 282,000 ha increasing by 20,000 ha per year In this area, several Pinus and Eucalyptus species were growing Since its introduction in Australia in 192 0 and until 198 7 it destroyed 50 to 75% of the jarrah forest’s flora of western Australia and other areas of this... for information We are particularly grateful to Dr M Karandinos for critical evaluation of the manuscript and to Dr J Peters for substantial contributions 92 0103_CRC20_ 090 4_CH 09 200 1/13/01 10: 59 AM Page 200 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT REFERENCES Agrios, G., 199 7 Plant Pathology Academic Press, London Alexander, H.M., Thrall, P.H., Antonovics, J., Jarosz, A.M and. .. 134: 295 –307 Podger, F.D., 197 2 Phytophthora cinnamomi a cause of lethal disease in indigenous plant communities in Western Australia Phytopathology, 62 :97 2 98 1 Powers, H.R., Schmidt, R.A and Snow, G.A., 198 1 Current status and management of fusiform rust on southern pines Annu Rev Phytopathol., 19: 353–371 Raccah, B., Bar-Joseph, M., and Loebenstein, G., 197 8 The role of aphid vectors and variation in . a slowed spread of disease in untreated plants 92 0103_CRC20_ 090 4_CH 09 1/13/01 10: 59 AM Page 193 194 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT in the same field. Similar results. epidemic expanded at 92 0103_CRC20_ 090 4_CH 09 1/13/01 10: 59 AM Page 195 196 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT a rate of about 37 km/year, and by 195 0 most of the chestnut. Other wind-borne pathogens, such as Venturia inaequalis (apple scab), follow the same pattern (Meredith, 197 3). 92 0103_CRC20_ 090 4_CH 09 1/13/01 10: 59 AM Page 191 192 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS