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The place of mycology in the teaching of Plant Pathology
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THE PLACE OF MYCOLOGY IN THE TEACHING OF PLANT PATHOLOGY by R.N. Hilton Botany Department, University of Western Australia With the British Mycological Society currently holding a symposium on Mycological Education, it seems appropriate to consider the place of Mycology in a Plant Pathology course. Once Mycology was effectively Plant Pathology - rather to the detriment of both subjects. Now in some universities the two courses are in danger of being separated altogether, partly because of agricultural students questioning the relevance to Plant Pathology of much of the Mycology, and partly because of staff seeking to drop subjects from overcrowded time-tables, or at least to make them optional. The remarks which follow have been based on the experience of some years' teaching Plant Pathology to agriculture students for whom a compulsory part of their course was attendance at Mycology for science students where both courses were given by the same person; but when different teachers are involved justification of the Mycology component becomes even more essential. One must start with a knowledge of the plant and the injuries it may suffer from non-parasitic agencies. It may then be taken as axiomatic that the basis of Plant Pathology is host parasite relations. From a foundation study of this subject can be built introduction to fungicides, applicators, and legislation - subjects that are much akin to chemistry, engineering and even sociology, but must have their place in a plant pathology course. By the time that agriculture students have reached their fourth year they have acquired a considerable knowledge of the host side of the relationship. So much so, that they may be unequipped to deal with the pathogen side. The host is basically one type of organism - a seed plant; because of this a student who starts plant pathology with no pathogen background will tend to equate parts with flowering plant equivalents: the seed may be equated with the bacterial spore, the zoospore, the conidium, the ascospore, the virus particle - and with a whole gamut of pathogen propagules that in fact differ from each other in enzyme equipment, genetic make-up, and structure quite as much as they all do from a seed. The hyphae from an agaric sporophore may be seen as roots, and the colony in a dish as some sort of disorganised whole plant. A view of host parasite relations derived from such concepts will be a travesty of the true situation. It might be a sufficient background against which to set technological information: such as how to recognise the symptoms of a disease and respond with some appropriate treatment. But there will be no understanding of the nature of the relationship. The person who goes on in plant pathology may eventually correct this view of things but teachers must be concerned with the majority who will never do so and whose ignorance will not only hamper their professional activity, but deny them experience of a new dimension of knowledge. Our first step must be to correct the imbalance between knowledge of host and knowledge of potential parasite. Not just what these parasites might be in terms of name, but the sort of aggressive equipment in terms of genetics and enzyme systems that parasites have. In the fungi we have not only what is still the major group of plant pathogens, but we have numerous pathogens that can be grown independently with great ease. They are perfect teaching material-and not not only for plant pathology, as teachers of genetics and biochemistry will confirm. However important viruses, plant pathogenic bacteria, or nematodes are in
67
practice, one could not advocate basing a plant pathology course on them. These above concepts will probably be accepted by any teacher of plant pathology or microbiology: dispute lies in how much mycology is essential to offset the emphasis on the host that has come from the rest of the agriculture course. That is, how much mycology as against other subjects vieing for timetable space. Too little can be a waste of time. The introductory chapters to several otherwise excellent texts in the realm of fungal ecology and pathogenicity would best have been omitted and the space so made available devoted to more of the main topic. I suggest, from my experience of teaching mycology, that 18 lectures and 9 three hour practical classes is the minimum. Even this depends on good living material to look at, as poor material and slides need longer explanations. We must ask ourselves what basic information we want our students to have about the potential pathogen before we start talking of host pathogen relations. A few items are listed as follows. This listing is not, of course, an outline of a series of lectures in Plant Pathology. 1. Taxonomy. A student must have a framework into which he can fit the pathogenic organism that his teacher is talking about. One does not want to interrupt a theme on the inoculum potential of zoospores to explain terms like chytrid, biflagellate, Oomycete, Aphanomyces etc. When he comes up against plant disease a student must have some clue as to what any associated organism is. If he can identify a few common saprophytic fungi he is less likely to waste his own and others' time. If he can identify the pathogen he may open up the literature on that particular disease for himself. If he can't identify it, at least he should know the books to consult and the specialist to whom it might be sent. Such information is out of place during a plant pathology course, as information, say, on the identification of fusaria might come up as-a dozen points during' the course In a mycology course there is a logical place for it. 2. Patterns of sexuality in the Fungi (as distinct from Genetics of Fungi). In contrast with the consistently diploid host the pathogen may be diploid (possibly all Oomycetes, a piece of information omitted by the plant pathology texts), haploid, dikaryotic, or parasexual. This can be understood only by looking at specific examples. An understanding of it is fundamental to anyone preparing to face inconsistencies in disease pattern even when host variation and environment remain constant. J.R. Raper summarises the position (The Fungi II, Chapter 15), but it presupposes some familiarity with the organisms mentioned. 3. Dispersal in Fungi. Plant Pathology is concerned with media of dissemination and agents of dissemination, but this is a dull study unless one knows how the propagules are presented for dissemination. Professor Ingold's books on dispersal are a revelation if students see the organisms that he is writing about. In a good mycology course students see the puffing of an apothecium, the way in which the thread-like spores of Claviceps eject, the twisting of the Peronospora sporangiophore, the vanishing of the basidiospore from the basidial sterigma. 4. Macrofungi. Not only are a number of plant pathogens large fungi, but so are many of the saprophytes that break down biological substrata. This is particularly true of the parasites of woody crops. A plant pathologist must have some idea of how to recognise major saprophytes and he won't be able to pick up this information by supplementing his plant pathology with, say, microbiology. A new interest in agarics is arising in connection with mycorrhiza and root disease control. 5. Longevity of propagules. One can rarely speak of the longevity of a fungus, only of the longevity of some specified propagule of the fungus. As knowledge of longevity is fundamental to a discussion of plant disease control, from the use of fallow crops to plant quarantine, one cannot discuss these matters without some information on whether a fungus is likely to produce sclerotium, chlamydospore, conidium, or ascospore, or all four, or something else.
68 6. Scale of structure. It takes time for a student to realise that a plant parasitic nematode may be as small as a fungus spore, that a Drechslera spore may be so large that individual spores can be positioned on a leaf with a hand-held needle, that a Tilletia spore may be one hundred times the volume of an Ustilago spore. Yet a sense of scale (regrettably absent from the widely-used textbook by Alexopoulos, incidentally) is fundamental to an understanding of even such a subject as determining the volume median diameter of an agricultural sprayer. One must visualise the spores on the leaf in relation to the deposited droplets between which they may germinate. One might ask whether determination of VMD is a part of an undergraduate Plant Pathology course anyway, but there is little point in teaching about fungicides if one says nothing about how they are most effectively applied on at least the level of small-scale field experiments. 7. Enzyme systems. Whether a pathogen can tackle a living plant or not, whether it can tackle wood, or whether it has to live obligately on the host, is not entirely a random matter and some prediction of habit can be made if one knows to which group the pathogen belongs; Rusts, Powdery Mildews and Downy Mildews, as obligate parasites for example ; the Polypores in the Basidiomycetes and the Xylariacaeae in the Ascomycetes as wood-attackers. The approach to plant pathology using prominent symptoms is an excellent one, but best if one has in one's mind the systematic position of the organisms quoted. Yet one doesn't want the taxonomic approach alone, as it detracts from the principles of the subject. 9. The Zoospore. One of the most remarkable of all fungal structures, the plant pathogenic significance of which has emerged so forcibly with the investigations on Phytophthora cinnamomi in California and Australia. The strange evanescent yet virulent properties of the zoospores are realised only after a student has tried to handle them . At first students mistake for zoospores germinating spores or particles showing Brownian movement. Every elementary text book has II picture of the zoospores of Pythium being discharged into a vesicle. How many students (or teachers, for that matter) have actually seen this feature? Often a couple of connected catenulate zoosporangia are interpreted as zoosporangium with accompanying vesicle. Perhaps this does not matter in itself - though it could waste a lot of the time of a research student who emerged from his undergraduate days with this misconception. What one does fear is the cumulative misconceptions of a student who learns only from background reading and generalisations. He might get A marks in a paper on plant pathology but his conceptions actually have little to do with reality. There is one unifying system of knowledge for this information. It is Mycology presented as Systematic Mycology. One starts with the basic fungus e.g. Olpidium with motile gametes cum zoospores. One looks at more chytrids, particularly Synchy trium, from which one sees obligate parasitism early on, and the connection of plant parasitism with the adoption of the terrestrial habit - the same themes as one sees in land plants and in mammals, although only the fungi carry lessons in Plant Pathology with them. On from the chytrids to the Oomycetes, where biflagellate zoospores dominate the scene. The uniqueness of the Oomycetes amongst the fungi, with their cellulosic walls and the possible diploid life cycle. Sporangium to conidium. The Zygomycetes, singularly inept as plant pathogens but prominent in the soil mycoflora. Foreshadowing the Ascomycete life cycle. The sporangium to conidium theme again. Then Taphrina, the Ascomycete so like Exobasidium amongst the Basidiomycetes, and so similar from the plant pathological point of view - causing serious disease of leaves, distorting them; and responding to control by copper sprays. The Ascomycetes proper introduce the Fungi Imperfecti and the ideas of dry spores and slime spores, sexuality and parasexuality. The concept of the Form Genus. The Basidiomycetes and the continuing concept of the imperfect fungus in the
69 Uredinales Imperfecti and the conidial Hymenomycetes. The Hymenomycetes with their haploid mycelium so ineffective parasitically until combined with another to form dikaryons, illustrating in a simple way the sort of versatility that parasexuality brings to a heterokaryotic fungus. The mushrooms and toadstools, and how they fall into natural families that the ordinary student can hope to learn to recognise (the genus is a more specialist matter, but at least the student can collect agarics in an intelligent manner and recognise where differences are due to stage of development rather than to species). We have had the view that Mycology and Plant Pathology are identical - the days of the "Imperial Mycologist" (Sir Edwin Butler's title as chief plant pathologist for India), the IMI, and the Review of Applied Mycology. Then has come a reaction against this with the advent of the microbiologists, plant bacteriologists, plant breeders, virologists, and nematologists. They have reacted against the old mycology of their student days with its lists of fungi labelled pathogen or saprophyte and often studied as a shrivelled up bit in the laboratory, and the seemingly effete study of "mushrooms and toadstools", the classification of which appeared to rest on no consistent principles. There has been a revolution in Mycology and Mycology teaching in the last 20 years (that is since the writer's student days). Not to recognise this, and not to make use of the ready-made mycology course that will be in progress in any University Botany Department or School of Biology, is to throwaway the finest introduction that one could devise, an introduction that combines logical development with practical application and inherent beauty and fascination. But students have to be made aware of this. Perhaps we should do more to justify courses before we start them rather than let the-message emerge hopefully as the course progresses.
THE PLACE OF MYCOLOGY IN THE TEACHING OF PLANT PATHOLOGY by R.N. Hilton Botany Department, University of Western Australia With the British Mycological Society currently holding a symposium on Mycological Education, it seems appropriate to consider the place of Mycology in a Plant Pathology course. Once Mycology was effectively Plant Pathology - rather to the detriment of both subjects. Now in some universities the two courses are in danger of being separated altogether, partly because of agricultural students questioning the relevance to Plant Pathology of much of the Mycology, and partly because of staff seeking to drop subjects from overcrowded time-tables, or at least to make them optional. The remarks which follow have been based on the experience of some years' teaching Plant Pathology to agriculture students for whom a compulsory part of their course was attendance at Mycology for science students where both courses were given by the same person; but when different teachers are involved justification of the Mycology component becomes even more essential. One must start with a knowledge of the plant and the injuries it may suffer from non-parasitic agencies. It may then be taken as axiomatic that the basis of Plant Pathology is host parasite relations. From a foundation study of this subject can be built introduction to fungicides, applicators, and legislation - subjects that are much akin to chemistry, engineering and even sociology, but must have their place in a plant pathology course. By the time that agriculture students have reached their fourth year they have acquired a considerable knowledge of the host side of the relationship. So much so, that they may be unequipped to deal with the pathogen side. The host is basically one type of organism - a seed plant; because of this a student who starts plant pathology with no pathogen background will tend to equate parts with flowering plant equivalents: the seed may be equated with the bacterial spore, the zoospore, the conidium, the ascospore, the virus particle - and with a whole gamut of pathogen propagules that in fact differ from each other in enzyme equipment, genetic make-up, and structure quite as much as they all do from a seed. The hyphae from an agaric sporophore may be seen as roots, and the colony in a dish as some sort of disorganised whole plant. A view of host parasite relations derived from such concepts will be a travesty of the true situation. It might be a sufficient background against which to set technological information: such as how to recognise the symptoms of a disease and respond with some appropriate treatment. But there will be no understanding of the nature of the relationship. The person who goes on in plant pathology may eventually correct this view of things but teachers must be concerned with the majority who will never do so and whose ignorance will not only hamper their professional activity, but deny them experience of a new dimension of knowledge. Our first step must be to correct the imbalance between knowledge of host and knowledge of potential parasite. Not just what these parasites might be in terms of name, but the sort of aggressive equipment in terms of genetics and enzyme systems that parasites have. In the fungi we have not only what is still the major group of plant pathogens, but we have numerous pathogens that can be grown independently with great ease. They are perfect teaching material-and not not only for plant pathology, as teachers of genetics and biochemistry will confirm. However important viruses, plant pathogenic bacteria, or nematodes are in
67
practice, one could not advocate basing a plant pathology course on them. These above concepts will probably be accepted by any teacher of plant pathology or microbiology: dispute lies in how much mycology is essential to offset the emphasis on the host that has come from the rest of the agriculture course. That is, how much mycology as against other subjects vieing for timetable space. Too little can be a waste of time. The introductory chapters to several otherwise excellent texts in the realm of fungal ecology and pathogenicity would best have been omitted and the space so made available devoted to more of the main topic. I suggest, from my experience of teaching mycology, that 18 lectures and 9 three hour practical classes is the minimum. Even this depends on good living material to look at, as poor material and slides need longer explanations. We must ask ourselves what basic information we want our students to have about the potential pathogen before we start talking of host pathogen relations. A few items are listed as follows. This listing is not, of course, an outline of a series of lectures in Plant Pathology. 1. Taxonomy. A student must have a framework into which he can fit the pathogenic organism that his teacher is talking about. One does not want to interrupt a theme on the inoculum potential of zoospores to explain terms like chytrid, biflagellate, Oomycete, Aphanomyces etc. When he comes up against plant disease a student must have some clue as to what any associated organism is. If he can identify a few common saprophytic fungi he is less likely to waste his own and others' time. If he can identify the pathogen he may open up the literature on that particular disease for himself. If he can't identify it, at least he should know the books to consult and the specialist to whom it might be sent. Such information is out of place during a plant pathology course, as information, say, on the identification of fusaria might come up as-a dozen points during' the course In a mycology course there is a logical place for it. 2. Patterns of sexuality in the Fungi (as distinct from Genetics of Fungi). In contrast with the consistently diploid host the pathogen may be diploid (possibly all Oomycetes, a piece of information omitted by the plant pathology texts), haploid, dikaryotic, or parasexual. This can be understood only by looking at specific examples. An understanding of it is fundamental to anyone preparing to face inconsistencies in disease pattern even when host variation and environment remain constant. J.R. Raper summarises the position (The Fungi II, Chapter 15), but it presupposes some familiarity with the organisms mentioned. 3. Dispersal in Fungi. Plant Pathology is concerned with media of dissemination and agents of dissemination, but this is a dull study unless one knows how the propagules are presented for dissemination. Professor Ingold's books on dispersal are a revelation if students see the organisms that he is writing about. In a good mycology course students see the puffing of an apothecium, the way in which the thread-like spores of Claviceps eject, the twisting of the Peronospora sporangiophore, the vanishing of the basidiospore from the basidial sterigma. 4. Macrofungi. Not only are a number of plant pathogens large fungi, but so are many of the saprophytes that break down biological substrata. This is particularly true of the parasites of woody crops. A plant pathologist must have some idea of how to recognise major saprophytes and he won't be able to pick up this information by supplementing his plant pathology with, say, microbiology. A new interest in agarics is arising in connection with mycorrhiza and root disease control. 5. Longevity of propagules. One can rarely speak of the longevity of a fungus, only of the longevity of some specified propagule of the fungus. As knowledge of longevity is fundamental to a discussion of plant disease control, from the use of fallow crops to plant quarantine, one cannot discuss these matters without some information on whether a fungus is likely to produce sclerotium, chlamydospore, conidium, or ascospore, or all four, or something else.
68 6. Scale of structure. It takes time for a student to realise that a plant parasitic nematode may be as small as a fungus spore, that a Drechslera spore may be so large that individual spores can be positioned on a leaf with a hand-held needle, that a Tilletia spore may be one hundred times the volume of an Ustilago spore. Yet a sense of scale (regrettably absent from the widely-used textbook by Alexopoulos, incidentally) is fundamental to an understanding of even such a subject as determining the volume median diameter of an agricultural sprayer. One must visualise the spores on the leaf in relation to the deposited droplets between which they may germinate. One might ask whether determination of VMD is a part of an undergraduate Plant Pathology course anyway, but there is little point in teaching about fungicides if one says nothing about how they are most effectively applied on at least the level of small-scale field experiments. 7. Enzyme systems. Whether a pathogen can tackle a living plant or not, whether it can tackle wood, or whether it has to live obligately on the host, is not entirely a random matter and some prediction of habit can be made if one knows to which group the pathogen belongs; Rusts, Powdery Mildews and Downy Mildews, as obligate parasites for example ; the Polypores in the Basidiomycetes and the Xylariacaeae in the Ascomycetes as wood-attackers. The approach to plant pathology using prominent symptoms is an excellent one, but best if one has in one's mind the systematic position of the organisms quoted. Yet one doesn't want the taxonomic approach alone, as it detracts from the principles of the subject. 9. The Zoospore. One of the most remarkable of all fungal structures, the plant pathogenic significance of which has emerged so forcibly with the investigations on Phytophthora cinnamomi in California and Australia. The strange evanescent yet virulent properties of the zoospores are realised only after a student has tried to handle them . At first students mistake for zoospores germinating spores or particles showing Brownian movement. Every elementary text book has II picture of the zoospores of Pythium being discharged into a vesicle. How many students (or teachers, for that matter) have actually seen this feature? Often a couple of connected catenulate zoosporangia are interpreted as zoosporangium with accompanying vesicle. Perhaps this does not matter in itself - though it could waste a lot of the time of a research student who emerged from his undergraduate days with this misconception. What one does fear is the cumulative misconceptions of a student who learns only from background reading and generalisations. He might get A marks in a paper on plant pathology but his conceptions actually have little to do with reality. There is one unifying system of knowledge for this information. It is Mycology presented as Systematic Mycology. One starts with the basic fungus e.g. Olpidium with motile gametes cum zoospores. One looks at more chytrids, particularly Synchy trium, from which one sees obligate parasitism early on, and the connection of plant parasitism with the adoption of the terrestrial habit - the same themes as one sees in land plants and in mammals, although only the fungi carry lessons in Plant Pathology with them. On from the chytrids to the Oomycetes, where biflagellate zoospores dominate the scene. The uniqueness of the Oomycetes amongst the fungi, with their cellulosic walls and the possible diploid life cycle. Sporangium to conidium. The Zygomycetes, singularly inept as plant pathogens but prominent in the soil mycoflora. Foreshadowing the Ascomycete life cycle. The sporangium to conidium theme again. Then Taphrina, the Ascomycete so like Exobasidium amongst the Basidiomycetes, and so similar from the plant pathological point of view - causing serious disease of leaves, distorting them; and responding to control by copper sprays. The Ascomycetes proper introduce the Fungi Imperfecti and the ideas of dry spores and slime spores, sexuality and parasexuality. The concept of the Form Genus. The Basidiomycetes and the continuing concept of the imperfect fungus in the
69 Uredinales Imperfecti and the conidial Hymenomycetes. The Hymenomycetes with their haploid mycelium so ineffective parasitically until combined with another to form dikaryons, illustrating in a simple way the sort of versatility that parasexuality brings to a heterokaryotic fungus. The mushrooms and toadstools, and how they fall into natural families that the ordinary student can hope to learn to recognise (the genus is a more specialist matter, but at least the student can collect agarics in an intelligent manner and recognise where differences are due to stage of development rather than to species). We have had the view that Mycology and Plant Pathology are identical - the days of the "Imperial Mycologist" (Sir Edwin Butler's title as chief plant pathologist for India), the IMI, and the Review of Applied Mycology. Then has come a reaction against this with the advent of the microbiologists, plant bacteriologists, plant breeders, virologists, and nematologists. They have reacted against the old mycology of their student days with its lists of fungi labelled pathogen or saprophyte and often studied as a shrivelled up bit in the laboratory, and the seemingly effete study of "mushrooms and toadstools", the classification of which appeared to rest on no consistent principles. There has been a revolution in Mycology and Mycology teaching in the last 20 years (that is since the writer's student days). Not to recognise this, and not to make use of the ready-made mycology course that will be in progress in any University Botany Department or School of Biology, is to throwaway the finest introduction that one could devise, an introduction that combines logical development with practical application and inherent beauty and fascination. But students have to be made aware of this. Perhaps we should do more to justify courses before we start them rather than let the-message emerge hopefully as the course progresses.