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R. LOCCI Chair of Mycology, Department of Biology Applied to Plant Defence, University of Udine, Via delle Scienze 208, 33100 Udine, Italy
Streptomyces scabies represents an emblematic example of the taxonomic status of actinomycetes. The causal agent of potato scab was isolated more than a century ago (Thaxter, 1891,1892). Considering that no patent interests or priorities were involved, apart from the obvious transfers at the generic level, first from Oospora to Actinomyces and then to Streptomyces, major taxonomic problems would not reasonably be expected for this taxon (Locci, 1994). The story however is exactly the opposite. Even in 1926 Millard and Burr wondered "(a) whether more than one species of Actinomyces could excite scab, and (b) what is the relation (if any) between the specific organism and the type of scab produced,,. Fifty years later the taxonomic status of S. scabies was invalidated because of the availability of "many taxonomically different reference strains,, (Pridham and Tresner, 1974).
It might seem a paradox but, notwithstanding the overwhelming preponderance of saprophytic actinomycetes, the first actinomycete discovered was an animal pathogen, Actinomyces bovis (Harz, 1877). S. scabies and S. ipomoeae, the agent of soil rot of sweet potato (Halsted, 1890; Person and Martin, 1940), were described just a few years later. In comparison to other bacteria, actinomycetes apparently play a relatively minor role as inducers of plant diseases. In fact they are major pathogens of certain crops in particular areas and, under special conditions, affect quality, palatability and market value of agricultural products, contribute to 'soil sickness' and produce phytotoxins (Locci, 1994). With regard to scab inducers, development of pathology itself has been restrained by taxonomic problems, essentially imputable to the lack of reliable criteria for species recognition, with consequent considerable confusion as to the causal agents. One item in particular seems to have constantly bothered researchers in the last century: the pathogenicity - taxonomic status dilemma. This has affected both the systematics of the organisms, as well as the identification of the agents responsible for the pathological syndrome induced in the host plant, leading to a state of chaos which is only beginning to be cleared up. At this point some historical comments appear pertinent.
THE SCAB INDUCERS
Thaxter (1891,1892) examined a large number of strains and included all the ones able to induce scab in the new species Oospora scabies (Bradbury, 1986). They were characterised as melanin producers with grey spores borne in spiral chains. No type culture was maintained and the species was subsequently transferred to the genus Actinomyces by Gussow (1914). Wollenweber in 1920 described several species, i. e. , Actinomyces aerugineus , A. albus, A. albus var. cretaceus, A. albus var. ochroleucus, A. annulatus (sic), A. intermedius, A. nigrificans, A. roseus, A. tricolor, as potential agents of potato scab. In fact the 1920s mark the beginning of a tendency to attribute to different actinomycetes the role of scab inducers. Millard and Burr (1926) question the inadequacy of Thaxter s description and carry out a thorough study (morphology of aerial mycelium, cultural characteristics on ten different media, tyrosinase reaction, starch hydrolysis, growth at 37.5 C, etc.) of isolates from indigenous (UK) and imported tubers and from soils. The strains are tested for pathogenicity and eleven species out of twenty are found capable of reproducing scab symptoms in potato tubers (Table 1). The authors stress the existence of different types of scab (ordinary, pimple, pitted, stud, superficial and tumulus) and the occurrence of scab inducers in virgin soils. The trend continues. Cocchi (1933) in Italy isolates three organisms which he identifies as A. clavifer and A. flavus, sensu Millard and Burr; the third strain does not seem similar to any of the previously described species. During the following fifty years the build up of the number of true or putative inducers of scab continues and an illustrative, though very reductive example, of this trend is represented by the papers by Jermoljev and Sethofer (1949), Bubentsov (1956), Kravtsova (1958), Corbaz (1964) and Krassilnikov et al. (1970). Dey et al. (1981) reports S. collinus as a new scab agent in India. Daulbaev (1966) isolates some seven specific taxa (albus, candidus, globisporus, griseus, rectus, uiolaceus, uiridis), but in no case succeeds in isolating S. scabies. On the other hand, following investigations in Germany on 183 samples of scabbed tubers, Hoffmann ( 1958) argues that S. scabies should be regarded as the sole inducer of scab, though he admits that some other species might be slightly pathogenic . In the meantime Actinomyces scabies is transferred to the genus Streptomyces and renamed S. scabies (Waksman and Henrici, 1948). To add further confusion Waksman (1961) redescribes the species, erroneously designating strain IMRU 3018 as the neotype strain. In fact the strain lacks spiral spore chains and does not produce melanin (Shirling and Gottlieb, 1968). It is not necessary to go into any further detail following the laborious work carried out in 1986 by Bradbury. He lists all the up to date described species, subspecies and varieties of potato scab associated actinomycetes, reports their taxonomic status and summarises their characteristics . The Guide to Plant Phytopathogenic Bacteria depicts the situation in 1986 in the following terms. Under the old generic denomination of Actinomyces 18 species, subspecies and varieties are still listed (aerugineus, albus, albus var. ochroleucus, anulatus, bicolor, canulus, cinereus, citrioflavescens, clavatosporus, flavus, globisporus sub sp. scabies, globisporus var. roseum (sic), nigrificans, scabies subsp. xanthostroma, sporostellatus, toxicus, violaceus subsp. incanescens and violaceus subsp. chromogenes). Obviously these taxa are not valid and they are not included in the Approved Lists of Bacterial Names (Skerman et al., 1980). As for the genus Streptomyces fifteen species are included in the Approved Lists of Bacterial Names (aureofaciens, clavifer, collinus, fimbriatus, globisporus, griseus, griseus subsp. cretosus intermedius, longisporus, praecox, rimosus, sampsonii, setonii, tricolor violaceus), while some other (candidus carnosus, coroniformis, cratifer, gracilis, levoris, loidensis, maculatus, marginatus, praefecundus, roseus, salmonicolor, scabies, tenuis, tumuli, viridogenes, vulgaris, wedmorensis) are not(Bradbury(1986). This brings up to total to more than fifty subgeneric taxa of Streptomycessus, involved as potential inducers of potato scab. Obviously the list is an optimistic one by defect, for example A. spiralis (Millard and Burr, 1926) escapes Bradbury s attention. At any rate at this stage the situation is to say the least chaotic and needs some clarification. THE SCAB SYNDROME Suggestions of possible correlations between potato scab symptom expression and diversity of the etiological agents can be found very early on in the literature. Researchers realise immediately that scab might be a collective term covering complex pathologies (see, e.g., Millard and Burr, 1926). ------------------------------------------------------------ -------
Species Sporophore Tyrosinase Virulence Type of Scab
Morphology Reaction
------------------------------------------------------------
A. carnosus (R-RF) - - -
A. gracilis (R-RF) - - -
A. maculatus - - - -
A. praecox (RF)-SP - - -
A. praefecundus (R-RF) - - -
A. salmonicolor - - - -
A. sampsonii (RF) - - -
A. spiralis SP - - -
A. tenuis (R-RF) - - -
A. clauifer (RF) + 1 IND
A. cratifer (RF) - 1 IND
A. coroniformis (RF) - 2 ORD
A. loidensis (RF) - 2 SUP
A. marginatus (R-RF) - 3 ORD
A. wedmorensis (RF) - 3 PIM
A. setonii (R-RF) - 4 PIT
A. uiridis (SP) - 5 PIT
A. fimbriatus SP + 6 ORD-PIT
A. flauus (RF) - 6 TUM
A. scabies SP + 12 PIT
-------------------------------------------------------------------
Table 1. Morphology, tyrosinase reaction, virulence and type of scab, produced as a result of artificial inoculation, of the species isolated by Millard and Burr (1926). Sporophore morphology: R = rectus; RF = rectus flexibilis; S = spira; designations in brackets are inferred from the authors, drawings. Tyrosinase reaction: - = negative; + = positive. Virulence: - = negative; numbers from 1 to 12 indicate increasing virulence. Types of scab: - = no scab; IND = indeterminate; ORD = ordinary; PIM = pimple-like; PIT = pitted; SUP = superficial; TUM = tumulus. ------------------------------------------------------------------- ---
In the early 1980s the situation appears complex and confusing even to plant pathologists, who are confronted by an extremely diverse scab symptomatology. Symptoms range from roundish to star shaped lesions with cracked and torn edges. Cracks or furrows, 3-4 mm deep, may cut into the tuber when severe lesions coalesce. Affected tissues vary from light tan to brown, consisting of a superficial cork-like reticulated layer (russet scab), an erumpent or cushion-like, 1-2 mm high, formation (raised scab), or an extension into the tuber (pitted scab) of various depths, up to 7 mm (Lapwood, 1973; Hooker, 1983). However even more disturbing are some eco-pathological facts. Common scab prospers in dry soils with neutral to alkaline pHs and is generally controlled by irrigation and by maintaining the soil pH below 5.2. However acid or uncommon scab, first detected in Maine in 1953 (Manzer et a1., 1977), occurs in soils with pH values as low as 4.5. On the other hand the agents of russet scab thrive under moist soil conditions. In addition European russet scab, or "netted scab,,, differs from the North American one for cultivar susceptibility, root attack and optimum soil temperature (Scholte and Labruyere, 1985). These observations imply consequences of practical importance, since they interfere directly with the control of the disease.
NEW TAXONOMIC APPROACHES
In the early 1980s actinomycete taxonomy is at a standstill. Traditional systems of classification based on hierarchical treatment of microbial features have exhausted their potential and repeatedly shown their pitfalls and limitations. The way out is represented by the adoption of procedures for overall evaluation of reciprocal similarities and dissimilarities between microorganisms (Williams et a1., 1983a). The philosophy and practical application of numerical taxonomy are well known. Strains are grouped into clusters on the basis of shared similarities. The procedure is in sharp contrast to traditional practices as clusters are recognised and defined using many equally weighted features. One of the advantages of the new approach is that it overcomes the possibility of a single feature, especially at higher levels, of overturning the whole system. In short the adoption of numerical or adansonian taxonomy (Sneath and Sokal, 1973) in the early 1980s revolutionises actinomycete systematics. The logical subsequence to classification is identification (Sneath, 1978). Probabilistic identification is the natural end product of numerical taxonomy, interestingly enough it has taken quite a time for this relationship to be accepted. Relatively few numerical classifications have been developed into probabilistic identification systems (Williams et a1., 1983b). On the other hand the application of numerical taxonomy to practical purposes represents the turning point between self complacency at having devised a nice tool and real use of the utensil. It is the identification scheme which is of particular value to non-taxonomists, providing them, ideally, with a relatively rapid but informative system for comparing new isolates with existing taxa. Numerical phenetic data provide an ideal basis for the construction of computerised identification matrices. According to Sneath (1989) "the numerical process allows to know to some order of magnitude the certainty that an identification is correct. The results are not greatly affected by an occasional aberrant property of the unknown, or an occasional experimental mistake in performing the tests. Furthermore, the system is robust toward missing information, and quite good identifications can be obtained if only a moderate proportion of the tests have been performed.,, The first large scale applications of probabilistic identification of actinomycetes were carried out on the important aerobic sporeforming taxa Streptomyces (Williams et a1., 1983b) and Streptoverticillium (Williams et a1., 1985; Locci et al., 1986).
AGENTS AND SYMPTOMS
Sneath (1978) points out that many areas of microbiology, such as ecology and environmental studies, are seriously hampered by lack of good identification systems. As stressed before, this statement fully applies to the inducers of potato scab. To what extent have the new approaches affected the scab problem?
Common Scab. Following the erroneous designation of strain IMRU 3018 as neotype strain of S. scabies (Waksman, 1961) and the dismissal from the 8th edition of Bergey's Manual of Determinative Bacteriology (Pridham and Tresner, 1974) as species incertae sedis, Elesaway and Szabo (1979) propose, as an alternative neotype, strain ATCC 33282 corresponding to the original description. The authors expand the diagnosis of the species (including ultra-microscopic and carbohydrate utilisation features) and suggest the reevaluation of the taxon. The species name however does not appear on the Approved Lists of Bacterial Names published the following year (Skerman et a1., 1980). Ten years later Lambert and Loria ( 1989a) investigate agents of common scab isolated in Canada and USA. They also include in the study the above mentioned strains, IMRU 3018 and ATCC 33282, together with non-pathogenic isolates from scab lesions plus a number of reference strains such as S. griseus, S. griseus subsp. scabies, S. sampsonii, S. setonii and S. tendae. By a phenotypical analysis of 42 characters, the authors differentiate the inducers of common scab from atypical pathogenic strains, saprophytic streptomycetes isolated from scab lesions and reference strains. The strains form an homogeneous group characterised by smooth grey spores borne in spiral chains, melanin production and utilisation of all ISP (International Streptomyces Project, Shirling and Gottlieb, 1966) sugars. Most S. scabies isolates do not degrade xanthine and are susceptible to 25ug/ml of oleandomycin, 10IU/ml of penicillin G, 20ug/ml of streptomycin, 10ug/ml of thallium acetate and 0.5ug/ml of crystal violet. The type strain of S. scabies proposed is ATCC 49173.
Acid Scab. A similar analysis is carried out at the same time and by the same authors (Lambert and Loria, 1989b) on streptomycetes inducing acid (or uncommon) scab. These agents had been previously isolated and described by Bonde and McIntyre (1968) and by Manzer et a1. (1977), who found differences between typical and acid scab strains in pigment, spore chain morphology, raffinose utilisation and tolerance of low pH. The organisms causing acid scab appear distinct from the agents of common scab in phenotype and ecology and a new species, S. acidiseabies, is therefore proposed (Lambert and Loria, 1989b). In culture S. acidiscabies differs from S. scabies, having flexuous spore chains, a spore mass colour ranging from white to salmon-pink (depending on the medium), a red or yellow pH-sensitive pigment and no melanin. S. acidiscabies grows on laboratory media at pH 4.0 (versus pH 5.0 for S. scabies), does not utilise raffinose as a carbon source, and tolerates higher concentrations of crystal violet (0.5 ug/ml), thallium acetate (10 ug/ml), streptomycin (20 ug/ml), oleandomycin (25 ug/ml) and penicillin G (10 IU/ml) than S. scabies. The level of similarity to S. scabies is only 64% and no strong similarities can be found with the major Streptomyces groups of Williams et a1. (1983a), S. griseoruber being most similar (67%) to S. acidiscabies. The type strain proposed is ATCC 49003.
Russet Scab. Russet scab had been reported in Europe and America since the beginning of the century (Harrison, 1962). The disease is characterised by corky reticulations on the tuber surface. The infection is generally restricted to the skin, affecting the quality of the crop (Faucher et a1., 1993). In contrast to common scab, the incidence of russet scab increases in moist soil. At the moment two types of russet scab are recognised, the American russet and the European (or netted) scab. Actinomycetes causing American russet scab (Faucher et al., 1993) are different from S. scabies (as they form pigmented mycelium, flexuous spore chains and no melanin) and from S. acidiscabies (on the grounds of spore mass colour and inability to grow at pH 4.5). They show a bright yellow substrate mycelium, which then turns brown. The aerial mycelium is grey, spore chains are flexuous and the spore surface is smooth. The organisms do not produce melanin, but degrade xanthine and xylan. Most strains utilise L-arabinose, D-fructose, D-glucose, D-mannitol, raffinose, rhamnose, sucrose and D-xylose and grow in the presence of NaCl (5%), tellurite (100ug/ml), penicillin (100ug/ml), phenol (0.1%) and oleandomycin (25ug/ml). Growth however is inhibited by thallium ( 10ug/ml) and streptomycin (20ug/ml). By comparison with selected cluster groups of streptomycetes, as defined by numerical taxonomy (Williams et a1., 1983a), the russet scab-inducing organisms show the highest similarity level (78%) with Group 14 suggesting a possible inclusion in the S. aureofaciens cluster. With reference to the agents of common and acidic scab, the similarity level is 73% and 66% respectively. European russet scab is apparently a distinct disease from the American one (Sundheim, 1968; Bang, 1979; Scholte and Labruyere, 1985) with respect to cultivar susceptibility, root attack and optimum soil temperature. Scholte and Labruyere (1985) proposed the name of "netted scab" for the former. No specific taxonomic investigations have so far been carried out on the causal streptomycetes and therefore their specific identity remains uncertain.
Potato Scab in Israel. At this stage (early 1990s) the relationship taxonomic status - pathogenicity appears to be at least partly clarified. The three (or four, considering the as yet unclarified case of russet scab) main patterns of potato scab are categorised, both in terms of etiology and symptomatology. Apparently, since black holes still seem to exist and some of them will be discussed in the Conclusions. The results of a recent study on scab inducing agents in Israel (Doering-Saad et al., 1992) however deserve immediate attention. Up to the early 1980s in southern Israel scab is successfully controlled by irrigation. However deep-pitted scab becomes a problem after 1981, even in well irrigated fields, and appears to be incited by different organisms. On the basis of a numerical study carried out on some thirty scab inducing organisms potato scab in Israel seems to be caused by a complex and diverse population of actinomycetes (Doering-Saad et al., 1992). The majority of indigenous pathogenic isolates, analysed according to the classification scheme of Kampfert et al. (1991), are assigned to S. violaceus (57%) and S. griseus (22%), the remaining ones are allocated to S. exfoliatus, S. rochei and to single-member clusters. Reference strains of S. scabies and S. acidiscabies, received from R. Loria, are similarly recovered in different clusters and subclusters. For some of them however the probabilistic identification, carried out following the scheme of Kampfert and Kroppenstedt (1991), is admittedly ambiguous. According to the authors "it appears likely that genes, required for pathogenicity are spread by mobilization elements among different Streptomyces species within their natural habitat". This conclusion is more than acceptable, however it cannot be ruled out a priori that the scab inducing population might be a composite one, possibly identifiable, by alternative methodologies, with previously described scab agents in other parts of the world (see also Conclusions).
RELATIONSHIPS AMONG SCAB INDUCERS
In addition to numerical taxonomy, alternative approaches allow the evaluation of mutual relationships between scab inducers. Using DNA-DNA hybridisation Healy and Lambert (1991) could establish that most S. scabies strains exhibit greater than 70% relatedness to the type strain, although values as low as 21% were obtained. The levels of homology between . S. scabies and nonpathogenic type strains range from 10 to 42%. Strains which exhibit <60% relatedness are significantly more likely (P<= 0.05) to differ in secondary phenotypic traits. Pathogenic strains that differ from typical S.scabies only in sugar utilisation patterns have been isolated and this may be another indicator of species diversity. Of the strains that exhibit low relatedness to the type culture of S.scabies, one was obtained from a russet (superficial) type of lesion, while another differed in the violet tinge of its grey spore colour. In addition, not all pathogenic strains can be differentiated from nonpathogenic strains on the basis of DNA relatedness data. These results are confirmed by more recent studies (Paradis et al., 1994). The work of Paradis and collaborators also stresses the fact that neither fatty acid nor whole-cell protein analysis are capable of discriminating pathogenic strains . At this stage it is not possible to determine whether pathogenicity is an old trait which preceded divergence of these streptomycetes or whether pathogenicity is transferable among related organisms by some genetic mechanism. In contrast to S. scabies, relatedness between S. acidiscabies isolates and their type strain is quite high. Levels of relatedness with all other species are <20% confirming the lack of close relationships between this species and other scab pathogens. This is consistent with the results obtained by Arias and Loria (1992) with monoclonal antibodies. The group, including S. sampsonii, S. griseus and the atypical common scab pathogens, appears to be as diverse on the basis of DNA relatedness as on that basis of comparison. The pathogens belonging to this group are poorly understood.
PATHOGENICITY FACTORS AND TAXONOMY
One final item, with taxonomic implications, the pathogenicity inducing mechanism in the host, must be considered. Enzymatic activities of different nature (tyrosinase, esterases, cutinases, etc.) have been thought ton be responsible, notwithstanding the fact that as early as 1926 Fellows makes an observation which goes unregarded for several years. He notes the darkening of cells in advance of colonisation and suggests that this could be a response to the action of a toxin or enzyme secreted by the scab organism. Some twenty-five years later Shoemaker (1952), with a series of experiments, which may however be criticised in strict microbiological terms (Lawrence et al., 1990), succeeds in confirming the validity of Fellows, hypothesis. Finally in the early 1990s Lawrence and collaborators reproduce the typical symptoms of common scab on aseptically grown potato minitubers with cell free extracts of scab lesions (Lawrence et al. , 1990). Isolation and fractionation of the active components in tissue extracts yield two active compounds, thaxtomin A and B, characterised as unique 4-nitroindol3-yl containing 2,5-dioxopiperazines (King et al., 1989; Lawrence et al., 1990). The active principles fulfil the basic requirements of vivotoxins. More recently from S. scabies, grown on surface sterilised potato slices (King et al., 1992) and on oatmeal agar and broth media (Babcock et al., 1993), some other three thaxtomins have been isolated and characterised. The role of thaxtomins in the pathogenesis of S. scabies is supported by the correlation between susceptibility to the actinomycete and sensitivity to the phytotoxins (Delserone et al., 1991; King et a1. , 1991) even in molecular terms (Bukhalid and Loria, 1993). Both S. scabies and S. acidiscabies produce the vivotoxin thaxtomin. S. acidiscabies is also pathogenic for the other crop species attacked by S. scabies, a further indication that thaxtomin production may represent a primary pathogenicity mechanism in these two species. The toxin is unusual, and its production by unrelated species is an interesting example of either evolutionary convergence or genetic transfer (Healy and Lambert, 1991).
DISCUSSION and CONCLUSIONS
The systematics of scab inducing actinomycetes has definitely improved during the last five years and taxonomy-etiology patterns are emerging, a century after the isolation of the first causal organisms. However the situation is far from being completely elucidated. A series of problems exists on both facets of the issue and await clarification. At the moment the state of the art seems to consist of a few emerging facts: - potato scab is a complex phytopathology involving diverse symptomatologies and different causal agents - the disease inducers, so far identified, operate optimally under diverse environmental conditions - the taxonomy of scab inducers has recently been clarified, but should not be in any way considered as definitive.
In plant pathology causal agent, host and environment are the three interacting components of the pathogenetic process. In the case of the various forms of potato scab however further complications are represented by the fact that the potential agents are frequent components of the soil mycoflora (where the phenomenon takes place), even in substrates where the crop has never been planted before, and that the organisms can survive without problems in soil as saprophytes. In fact S. scabies is a saprophytic pathogen able to survive for long periods on decaying plant parts in the soil and possibly on roots of living plants, in old feed lots and in fields heavily manured with animal wastes (Hooker, 1983). This could have immediate practical implications connected with the possibility of biological control of the disease. Because of the habitat of actinomycetes, this strategy should be confronted with caution since the disease inducers do not only inhabit the soil (where biological control is not easy) but are also opportunistic pathogens. Data on suppressive and conducive soils (Menzies, 1959), on disease decline (Deacon, 1983) and on antagonistic streptomycetes (Hayashida et al., 1989; Liu and Anderson, 1992) are available. Still in connection with phytopathology, other minor problems defy immediate understanding. For example in radish (Raphanus sativus L.) scab is an economically important disease in commercial production of the crop. The organisms isolated from affected tissues differ morphologically and culturally from S. scabies. A scab potato isolate, used for comparison, was shown to be pathogenic for radish, while radish scab inducers were not able to affect potato tubers. The case obviously deserves further investigation (Levick et al. , 1985). The Israeli situation mentioned earlier on (Doering-Saad et al., 1992) is intriguing and prima facie could only be explained by admitting the existence in that habitat of a peculiar streptomycete population. However this assumption must be refuted on the grounds of the behaviour of the reference strains subjected to the same taxonomic treatment. In fact in this case the obvious conclusion would be a complete revision of the pathological patterns recently established in other areas of the world. Some alternatives however are worth considering. Historically (Doering-Saad et al., 1992) the population of scab inducers appears to have evolved in time from forms kept under control by irrigation (as it is the case of common scab) to organisms able to thrive under moist soil conditions (like the agents of russet scab). This is confirmed by the extreme diversity of the micromorphological (spore chain form, spore ornamentation, etc.) and cultural (spore mass colour) characteristics of the isolates. This notwithstanding none of these features is taken into account either in the numerical study or the probabilistic identification. The correctness of this approach (exclusive use of physiological characters) could at least be argued, since it is against the principles of numerical taxonomy (Sneath and Sokal, 1973) in that it excludes a priori entire groups of features. A more comprehensive approach might show alternative relationships between the scab strains. Again further studies are needed to clarify this problem. Turning now to the taxonomy of scab inducers, undoubtedly progress has been made in the last few years. The vicissitudes that lead to the invalidation of the taxon S. scabies have been previously reviewed. More recently however some patterns correlating specific etiology and symptomatology have emerged with the consequent revival of S. scabies, the proposal of a new species (S. acidiscabies as agent of acid scab) and the recognition of the role of other actinomycetes as inducers of specific forms of potato scab. It should be kept in mind that this revision is still in progress and should by no means be considered as definitive. In fact taxonomy is by definition a dynamic process. It closely reflects the development of knowledge and consequently is bound to continuously develop. By nature, man looks down on the past as on something preliminary to his present accomplishments. It is not easy to accept that it is just a matter of time before others will look down condescendingly on the naivety of their predecessors. Admittedly this is a pessimistic view but a quick look at the history of science will show that this happens over and over again. Man needs to establish some fixed landmarks and think of them as definitive. However this is not so in terms of knowledge. With reference to scab inducers, the situation emerging at the moment is in a fluid state with some reference marks more or less well defined. This assumption appears to be supported by a series of observations which need to be categorised and placed in the right perspective and by the existence of grey areas still waiting elucidation. It is most probable that less virulent streptomycetes exist and their pathogenic role has yet to be clarified. Potato scab inducers reside in soil and are part of a numerous and active actinomycete community. The transfer of pathogenicity genes between different "species,, is not a remote possibility, thus representing a further complication in the pathoecology of the organisms. Thaxtomin production, at the moment correlated with pathogenicity, is similarly not a unique feature of S. scabies and the alternative hosts of the latter species are paralleled by those of S. acidiscabies (Locci, 1994). These and other elements seem to point out that, for example, within the streptomycetes that are phenotypically consistent with S. scabies, pathogenicity would appear to occur in a subset of strains which have more genetic diversity than a conventional species (Healy and Lambert, 1991). Thus it is likely that the major host-pathogen patterns so far established will maintain their individuality (or possibly increase in number) and that the species, identified at the moment as responsible for each pattern, could act as aggregation centres for additional scab inducers.
REFERENCES Arias, I. & R. Loria (1992). Production of monoclonal antibodies to Streptomyces scabies (Abstr.). Phytopathology, 82: 241 Babcock, M. J., E. C. Eckwall & J. L. Schottel (1993). Production and regulation of potato-scabinducing phytotoxins by Streptomyces scabies. J. gen.Microbiol., I39: I579-1586 Bang, H. (1979). Studies on potato russet scab. 1. A characterization of different isolates from northern Sweden. Acta Agric. Scand., 29: 145-150 Bonde, M. R. & G. A. McIntyre (1968). Isolation and biology of a Streptomyces sp. causing potato scab in soils below pH 5.0. Am. Potato J., 45: 273-278 Bradbury, J. F. (1986). Guide to Plant Pathogenic Bacteria. CAB Int.Micol.Inst., Kew Bubentsov, S. T. (1956). Actinomycosis of potatoes in central Kazakhstan and its control. Trud. resp. Sta. Zashch. Rast. Kazakh. , 3: 192-212 (in Russian) Bukhalid, R. A. & R. Loria (1993). Cloning of a pathogenicity factor from Streptomyces scabies. (Abstr. ). Phytopathology, 83: 1360 Cocchi, F. (1933). Ricerche sulla 'scabbia, delle patate. Boll. R. Staz. Pat. Veg. , N.S. , 13: 74-l39 Corbaz, R. (1964). Etude des streptomycetes provoquant la gale commune de la pomme de terre. Phytopath. Z. , 51: 351-360 Daulbaev, E. A. (I966). Isolation of actinomycetes, the causal agents of potato scab. Izv. Akad. Nauk Kazakh. SSR, Ser. Biol. , 3: 43-47 (in Russian) Delserone, L. M. , R. Loria & I. Arias (1991). Correlation between susceptibility of potato cultivars to Streptomyces scabies and sensitivity to thaxtomins (Abstr. ) Phytopathology, 81: 1193 Deacon, J. W. (I983). Microbial Control of Plant Pests and Diseases. Am. Soc. Microbioi. , Washington Dey, S. K. , N. Singh & H. Singh (I981). Streptomyces collinus Lindenbein-a new species causing scab of potato. Curr. Science, 60: 830 Doering-Saad, C. , P. Kampfer, S. Manulis, G. Kritzman, J. Schneider, J. Zskrzewska-Czerwinska, H. Schrempf & I. Barash (1992). Diversity among Streptomyces strains causing potato scab. Appl. Environ. Microbiol. , 58: 3932-3940 Elesaway, A. A. & J. M. Szabo (1979). Isolation and characterisation of Streptomyces scabies strains from scab lesions of potato tubers. Designation of the neotype strain of Streptomyces scabies. Acta Microbiol. Acad. Sci. Hung. , 26: 311-320 Faucher, E. , B. Otrysko, E. Paradis, N. C. Hodge, R. E. Stall & C. Beaulieu (1993). Characterization of streptomycetes causing russet scab in Quebec. Plant Disease, 77: 1217-1220 Fellows, H. (1926). Relation of growth in the potato tuber to the potato scab disease. J. Agric. Res. , 32: 757-781 Gussow, H. T. (1914). The systematic position of the organism of the common potato scab. Science, 39: 431-432 Halsted, B. D. (1890). Some fungous diseases of sweet potato. N. J. Agric. Exp. Stn. Bull. 76 (after Person & Martin, 1940) Harrison, M. D. (1962). Potato russet scab, its cause and factors affecting its development. Am. Potato J. , 39: 368-387 Harz, C. O. (1877). Actinomyces bovis ein neuer Schimmel in dem Geweben des Rindes. Deutsche Zeitschrift fdr Thiermedizin, 5: 125-140 Hayashida, S. , M. Y. Choi, N. Nanri, M. Yokoyama & T. Uematsu (1989). Control of potato common scab with an antibiotic biofertilizer produced from swine feces containing Streptomyces albidoflavus CH-33. Agric. & Biol. Chem. , 53: 349354 Healy, F. G. & D. H. Lambert (1991). Relationships among Streptomyces spp. causing potato scab. Int. J. Syst. Bacteriol. , 41: 479-482 Hoffmann, G. M. (1958). Untersuchungen zur Atiologie pflanzlicher Actinomykosen. Phytopath. Z. , 34: 1-56 Hooker, W. J. (1983). Common scab. In: W. J. Hooker (ed. ) Compendium of Potato Diseases. Am. Phytopath. Soc. , St. Paul, pp. 33-34 Jermoljev, E. & V. Sethofer (1949). Results of potato scab resistance tests. Ochr. Rost. , 22: 204-225 (in Russian) Kampfert, P. & R. M. Kroppenstedt (1991). Probabilistic identification of streptomycetes using miniaturized physiological tests. J. gen. Microbiol. , 137: 1893-1902 Kampfert, P. , R. M. Kroppenstedt & W. Dott (1991). A numerical classification of the genus Streptomyces and Streptoverticillium using miniaturized physiological tests. J. gen. Microbiol. , 137: 1831-1891 King, R. R. , C. H. Lawrence, M. C. Clark & L. A. Calhoun (1989). Isolation and characterization of phytotoxins associated with Streptomyces scabies. J. Chem. Soc. (London), 13: 849-850 King, R. R. , C. H. Lawrence & M. C. Clark (1991). Correlation of phytotoxin production with pathogenicity of Streptomyces scabies isolates from scab infected potato common tubers. Amer. Potato J. , 68: 675-680 King, R. R. , C. H. Lawrence & L. A. Calhoun (1992). Chemistry of phytotoxins associated with Streptomyces scabies, the causal organism of potato common scab. J. Agric. Food Chem. , 40: 384-387 Krasuilnikov, N. A. , A. I. Korenyako, Yu. I. Shneider & A Kh. Khashkhozhev (1970). New species of actinomycetes, the causal agents of potato scab. Mikol. i Fitopatol. , 4: 430-445 Kravtaova, G. P. (1958). The role of tuber infection in the development of potato common scab. Sborn. Inst. prikl. Zool. Phytopath. , 5: 237-244 (in Russian) Lambert, D. H. & R. Loria (1989a). Streptomyces scabies sp. nov. , nom. rev. Int. J. Syst. Bacteriol. , 39: 387-392 Lambert, D. H. & R. Loria (1989b). Streptomyces acidiscabies sp. nov. Int. J. Syst. Bacteriol. , 39: 393-396 Lapwood, D. H. (1973). Streptomyces scabies and potato scab disease. In: G. Sykes & F. A. Skinner (eds. ) Actinomycetales: Characteristics and Practical Importance. Academic Press, London, pp. 253-260 Lawrence, C. H. , M. C. Clark & R. R. King (1990). Induction of common scab symptoms in aseptically cultured potato tubers by the vivotoxin, thaxtomin. Phytopathology, 80: 606-608 Levick, D. R. , T. A. Evans, C. T. Stephena & M. L. Lacy (1985). Etiology of radish scab and its control through irrigation. Phytopathology, 75: 568-572 Liu, D. & N. Anderson (1992). Biological control of potato scab with suppressive isolates of Streptomyces (Abstr. ). Phytopathology, 82: 1108 Loeei, R. (1994). Actinomycetes as plant pathogens. Eur. J. Plant Pathol. 100: 179-200 Locci, R. , S. T. Williams, G. M Schofield, J. C. Vickers, P. H. A. Sneath & A. M. Mortimer (1986). A probabilistic approach to the identification of Streptoverticillium species. In: G. Szab6, S. Bir6 & M. Goodfellow (eds. ) Biological, Biochemical and Biomedical Aspects of Actinomycetes. Akademiai Kiado, Budapest, pp. 507-516 Manzer, F. E. , G. A. McIntyre & D. C. Merriam (1977). A new potato scab problem in Maine. MaineAgric. Exp. Stn. Bull. ,85: 1-24 Menzies, J. D. (1959) Occurrence and transfer of a biological factor in soil that suppresses potato scab. Phytopathology, 49: 648-652 Millard, W. A. & S. Burr (1926). A study of twenty-four strains of Actinomyces and their relation to types of common scab of potato. Ann. Appl. Biology, 13: 580-644 Paradis, E. , C. Goyer, N. C. Hodge, R. Hogue, R. E. Stall & CBeaulieu (1994). Fatty acid and protein profiles of Streptomyces scabies strains isolated in Eastern Canada. Int. J. Syst. Bacteriol. , 44: 561-564 Peraon, L. H. & W. J. Martin (1940). Soil rot of sweet potato in Louisiana. Phytopathology, 30: 913-926 Pridham, T. G. & H. D. Tresner (1974). Genus I. Streptomyces Waksman and Henrici 1943, 339. In: R. E. Buchanan & N. E. Gibbons (eds. ) Bergey s Manual of Determinative Microbiology. Williams & Wilkins, Baltimore, 8th ed. , pp. 748-829 Scholte, K. & R. E. Labruyere (1985). Netted scab: a new name for an old disease in Europe. Potato Res. , 28: 443-448 Shirling, E. B. & D. Gottlieb (1966). Methods of characterization of Streptomyces species. Int. J. Syst. Bacteriol. , 16: 313-340 Shirling, E. B. & D. Gottlieb (1968). Cooperative description of type cultures of Streptomyces. III. Additional species description from first and second studies. Int. J. Syst. Bacteriol. , 18: 279-392 Shoemaker, R. A. (1952). Streptomyces scabies and its relationship to common scab of potato. Ph. D. Thesis, University of Toronto
Skerman, V. B. D. , V. McGowan & P. H. A. Sneath (1980). Approved Lists of Bacterial Names. American Society for Microbiology, Washington, D. C. Sneath, P. H. A. (1978). Identification of Microorganisms. In: Norris & Richmond (eds. ) Essays in Microbiology. J. Wiley, Chichester, pp. 1-32 Sneath, P. H. A. (1989). Numerical Identification. In: Williams, S. T. , M. E. Sharpe & J. G. Holt (eds. ) Bergey s Manual of Systematic Bacteriology. Williams & Wilkins, Baltimore, p. 2324 Sneath, P. H. & R. R. Sokal (1973). Numerical Taxonomy: The Principles and Practice. W. H. Freeman & Co. , San Francisco Sundheim, L. (1968). Streptomyces sp. causing russet scab of potatoes in Norway. Eur. Potato J. , 11: 194 Thaxter, R. (1891). The potato scab. Conn. Agric. Exp. Stn. Rept. , 1890: 81-95 Thaxter, R. (1892). Potato scab. Conn. Agric. Exp. Stn. Rept. , 1891: 153-160 Waksman, S. A. & A. T. Henrici (1948). Family II. Actinomycetaceae Buchanan and family Streptomycetaceae Waksman and Henrici. In: R. S. Breed, E. G. D. Murray & A. P. Hitchens (eds. ) Bergey's Manual of Determinative Microbiology. Williams & Wilkins, Baltimore, 6th ed. , pp. 892-980 Waksman, S. A. (1961). The Actinomycetes. Volume II. Classification, Identification and Description of Genera and Species. Williams & Wilkins, Baltimore Williams, S. T. , M. Goodfellow, G. Alderson, E. M. H. Wellington, P. H. A. Sneath & M. J. Sackin (1983a). Numerical classification of Streptomyces and related genera. J. gen. Microbiol. 129: 1743-1813 Williams, S. T. , M. Goodfellow, E. M. H. Wellington, J. C. Vickers, G. Alderson, P. H. A. Sneath, M. J. Sackin & A. M. Mortimer (1983b). A probability matrix for identification of streptomycetes. J. gen. Microbiol. , 129: 1815-1830 Williams, S. T. , R. Locci, J. Vickers, G. M. Schofield, P. H. A. Sneath & A. M. Mortimer (1985). Probabilistic identification of Streptoverticillium species. J. gen. Microbiol. , 131: 1681-1689 Wollenweber, H. W. (1920). Der Kartoffelschorf. Arbeiten des Forschunginstitut fur Kartoffelbau, Heft 2, 102 pp. Copyright 1994 C. E. T. A.
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