The aggressive subgroup of the Dutch elm disease pathogen Ophiostoma ulmi (Buism.) Nannf. syn. Ceratocystis ulmi (Buism.) Moreau is named as a new species, O. novo-ulmi, and is thereby separated from the ‘old’ non-aggressive subgroup, which is retained as O. ulmi. O. novo-ulmi differs from O. ulmi in colony morphology, growth rate, optimum temperature for growth, perithecial neck length, pathogenicity to elm, bark colonising ability, cerato-ulmin protein production, synnemetal and protoperithecial production, mating type frequency, protein and isozyme polymorphisms, mitochondrial DNA and nuclear DNA polymorphisms, and mitochondrial DNA size. In addition, a strong unidirectional fertility barrier operates between the two species, while their hybrids show remarkable variation, poor fitness, and many are infertile. These aspects are summarised. New information on perithecial dimensions is presented. O. ulmi is redefined and a neotype designated. The status of the Eurasian and North American races of O. novo-ulmi is currently under investigation.

Ophiostoma novo-ulmi sp. nov., causative agent of current Dutch elm disease pandemics

P lant disease epidemics resulting from introduction of exotic fungal pathogens are a well-known phenom- enon. Limited resistance in the host and excessive aggres- siveness in the pathogen (reflecting their lack of prior co- evolution) can result in an explosive outbreak of disease. Introduction events also present a window of evolutionary op- portunity for the pathogen. In its endemic location, a plant pathogen tends to be subject to routine selection constraints, favoring maintenance of a relatively stable, if fluctuating, population structure over time. When introduced into a new environment, it will often be subject to novel or episodic se- lection, reflecting sudden exposure to new biotic and abiotic influences, such as a new host population, new vectors, new competitors, or a different climate. These influences provide the potential for rapid evolution (Brasier 1995). Falling within the category of episodic selection is the sud- den contact that can occur between closely related but pre- viously geographically isolated pathogens as a result of in- troductions. Theoretically, this process presents an opportunity for rapid emergence of new or modified pathogens via interspecific gene flow (Brasier 1995). Until recently, this phenomenon has been little studied. Now Dutch elm disease, a major ecological accident of the 20th century (Heybroek 1993), is providing remarkable insights into rapid evolution of a plant pathogen outside its en- demic environment. This article describes the migratory events and unusual genetic events that have occurred in this fungus and other new examples of rapid pathogen evo- lution via interspecific gene flow, and discusses some of the wider environmental, evolutionary, and quarantine impli- cations. The Dutch elm disease pathogens Elm trees (Ulmus) are confined mainly to the temperate re- gions of the Northern Hemisphere. China and Japan have a total of about 25 elm species, while Eurasia, North Amer- ica, and the Himalayas each have about five or six species. Dutch elm disease (so called because the early seminal re- search was in The Netherlands (Holmes and Heybroek 1990)) is the elm's main enemy. It is a wilt disease, caused by ascomycete fungi of the genus Ophiostoma, that spreads within the tree's vascular system. The pathogens are trans- mitted from diseased to healthy elms by elm bark beetles of the genus Scolytus (Fransen 1935, Webber and Brasier 1984).

/pdf/rapid-evolution-of-introduced-plant-pathogens-via-1m7amdshh0.pdf

Rapid Evolution of Introduced Plant Pathogens via Interspecific Hybridization

Defence mechanisms against fungi in bark, xylem, roots, and foliage of woody angiosperms and gymnosperms are addressed in this newest addition to the "Springer Series in Wood Science". The topic is approached from an anatomical and physiological angle, with emphasis on the relationships between plant structure and function and their role in defensive reactions and wound repair. Contributions on the biochemistry of defensive reactions, in-depth treatises on specific diseases such as Dutch elm disease, and Fusiform rust of pines, as well as chapters on wound reactions in bark tissues, microenvironmental aspects of xylem decay, the role of mycorrhizal fungi in defence, physiological responses of trees to fungal symbionts of insects, and the application of tissue culture and immunocytochemistry to the study of defense mechanisms in trees are included.

Defense Mechanisms Of Woody Plants Against Fungi

Two enormously destructive pandemics of Dutch elm disease occurred in the 20th century, resulting in the death of a majority of mature elms across much of the northern hemisphere. The first pandemic, caused by Ophiostoma ulmi, occurred as this pathogen spread across Europe, North America and Southwest and Central Asia during the 1920s–1940s. The current pandemic is caused by another Ophiostoma species, O. novo-ulmi. Since the 1940s, O. novo-ulmi has been spreading into the regions previously affected by O. ulmi. It has also spread as two distinct subspecies, termed subsp. americana and subsp. novo-ulmi. This sequence of events has resulted in competitive interactions between these previously geographically isolated pathogens. This article summarizes the biological properties of the Dutch elm disease pathogens and their history of spread. It reviews the remarkable series of genetic events that have occurred during their migrations; including the emergence of genetic clones, the spread of deleterious fungal viruses within the pathogen clones, and the rapid and continuing evolution of O. novo-ulmi via horizontal gene flow. The wider role of horizontal gene flow in the evolutionary potential of migratory plant pathogens is discussed.

Rapid evolutionary changes in a globally invading fungal pathogen (Dutch elm disease)

Although both environment and genetics have been shown to affect the mycorrhizal colonization of host plants, the impacts of these factors on hosts that can be dually colonized by both ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi are less understood. We examined the influence of environment and host crosstype on the EM and AM colonization of cottonwoods (Populus angustifolia and natural hybrids) by comparing levels of colonization of trees growing in common gardens that differed in elevation and soil type. We also conducted a supplemental watering experiment to determine the influence of soil moisture on AM and EM colonization. Three patterns emerged. First, garden location had a significant impact on mycorrhizal colonization, such that EM colonization was 30% higher and AM colonization was 85% lower in the higher elevation garden than the lower elevation garden. Second, crosstype affected total (EM + AM) colonization, but did not affect EM or AM colonization. Similarly, a significant garden x crosstype interaction was found for total colonization, but not for EM or AM colonization. Third, experimental watering resulted in 33% higher EM colonization and 45% lower AM colonization, demonstrating that soil moisture was a major driver of the mycorrhizal differences observed between the gardens. We conclude that environment, particularly soil moisture, has a larger influence on colonization by AM versus EM fungi than host genetics, and suggest that environmental stress may be a major determinant of mycorrhizal colonization in dually colonized host plants.

Environmental and genetic effects on the formation of ectomycorrhizal and arbuscular mycorrhizal associations in cottonwoods

Inoculation experiments on Ulmus procera, U. glabra and U. pumila with isolates of Ceratocystis ulmi from Canada, the United States, England, France, the Netherlands and Iran confirmed earlier work on the correlation between cultural characters and pathogenicity. Inocula-

Further studies on pathogenicity in Ceratocystis ulmi

Electron microscope observations of Ceratocystis ulmi-induced tylosis development in 3 species of elm were made during the first 2 weeks after artificial inoculation. Tylosis forming from both ray and vasicentric parenchyma occluded vessels adjacent to those containing the fungus conidia within one day after inoculation. The expanding protoplast forced the intact vessel-parenchyma “pit membrane” and protective layer through the pit into the vessel lumen. Cell organization during this expansion is discussed. The observations on tylosis development for the 3 elms used were identical.

Electron microscope observations of Ceratocystis ulmi-induced tylosis development in Ulmus

Seven clones of hybrid poplar, with parents from among four sections of the genus Populus, were potted in five Iowan soils. After a year they were sampled for mycorrhizae and endogonaceous spores. Eight species of Endogonaceae were recorded, seven known to form endomycorrhizae. Two ectomycorrhizal fungi, Ccnococcum geophilum and an unidentified species were noted. One poplar clone had fewer replicates in which Glomus fasciculatum spores were found, but otherwise there were no differences among the poplars in presence of spores. A soil from a P. grandidentata woodland had fewer pots with G. mosseae spores, while an agricultural soil had more occurrences of Gigaspora gigantea than other soils. No other significant differences among spores were noted for soils. Vesicles of endomycorrhizae occurred in all clones, but one was colonized more sparsely than the others. Two clones had ecto- and ectendo-mycorrhizae, two types of the former being recorded. One of these clones had significantly more C. geophilum colonization than the other. There were indications of differences in mycotrophy among the sections of the genus Populus.

H. S. McNabb

Papers

Further studies on pathogenicity in Ceratocystis ulmi

Electron microscope observations of Ceratocystis ulmi-induced tylosis development in Ulmus

Mycorrhizal symbionts associated with hybrid poplars from Iowa, USA