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Trans. Brit. trrycol. Soc. 45 (2), 282-288 (1962).

REVIEWS

Plant Pathology. An Advanced Treatise. Vol. III. The Diseased Population:Epidemics and Control. Edited by J. G. HORSFALL and A. E. DIMOND.

(New York and London: Academic Press, 1960.) Pp, x +675,illustrated. Price: $22.00.

The general plan of this treatise is that volumes 1 and II should deal with the individualorganism-with host and with parasite, respectively-and volume III with disease at thecrop level. This distinction is somewhat artificial for, apart from the mere matter ofscale, there is little essential difference between what happens to the plant and whathappens to the crop; the only new point that arises in crop pathology concerns the spreadof disease from plant to plant, whether from an outside source, or from a focus or fociwithin the crop. If one looks back to chapter 4 of volume I, which is largely concernedwith the assessment of disease within a crop, one finds that this distinction has alreadybroken down, and it will appear, furthermore, that some chapters of this volume havetheir closest relatives in the earlier ones.

Following a Prologue, there is a discussion of' Inoculum potential', a term that hasbeen defined variously: from' energy of growth of a pathogen available for infection ofa host at the surface of the host organ to be infected' in this chapter (p. 28), to 'numberof independent infections that are likely to occur in a given situation in a populationof susceptible healthy plants' (Prologue, p. 3). In the narrower sense adopted by theauthor, inoculum potential is strongly influenced by access to nutrient material ('foodbase'), and synergistic effects arising from size of inoculum may playa part at somestage of the invasive process. In these respects' Inoculum potential' is clearly related tochapters 5-7 and 9 of volume II. In connexion with soil-borne parasites, it describes,and emphasizes the importance of, mycelial aggregates (strands, rhizomorphs) by whichthe fungus is able to spread through the soil, and thereby to invade fresh tissue, which,incidentally, is one type of autonomous dispersal.

Of the four chapters (3-6) allocated specifically to dispersal, the first is entitled, Autonomous'. It is not easy to see what the editors had in mind for the substance ofthis chapter. One is aware of only three kinds of autonomous dispersal as far as plantdisease is concerned: by mycelial growth as discussed in the preceding chapter; by somemethods of spore discharge, as described in chapter 5; and by locomotory movementsof zoospores and eelworms, which are of great interest physiologically, but could hardlysuffice to fill a chapter of a treatise on plant pathology. As it is, the author gives us aclear picture of the various ways in which infective material is carried from place toplace, more or less unwittingly in the normal practice of crop husbandry (on implements,in farm-yard manure, on or in farm produce, etc.), There is no question ofthe importanceof these practices in the spreading of disease, even though none of them can be calledautonomous.

'Dispersal of inoculum by insects and other animals, including man' covers part ofthe same ground but deals especially with viruses and their vectors, enphasizing theimportance of a knowledge of insects' habits: migration, food, preferences, method offeeding, etc., and the effect on these of seasonal and weather conditions. Then follows'Dispersal by air and water-the take-off', covering the variety of arrangements wherebyfungus spores are sent upon their journey-from relatively unspecialized forms that relyupon rain splashing, to the wide range of explosive mechanisms for shooting the sporesup into the turbulent air layers, as in many Ascomycetes, and the elevated sporophoresof Basidiomycetes which achieve the same result in a different way. Both these chapterscontain a great mass of clearly arranged detail.

The last chapter of this group 'Dispersal by air and water-the flight and landing'is outstanding for its mathematical exposition of the problem of air dispersal of smallparticles, plant pathology being merely one of the many fields in which this problemarises. A formula is derived for the average course of a particle (or in other words, for

Reviewsthe' centre of gravity' of the cloud of particles) in terms of rate of sedimentation in stillair, wind velocity and vertical exchange due to turbulence and heating. From this, andother formulae, the progress of the cloud of particles can be described: time of flight,concentration at any point, rate of deposit, etc. The main query raised by this chapteris whether a treatise on plant pathology is a suitable medium for a mathematical displayof this kind. It is a foregone conclusion that, of the plant pathologists who happen toopen the book at about p. 180, not one in a hundred will venture to read further, andso a distinguished piece of analysis can hardly expect anything better than decent burial.This is all the more unfortunate as the chapter is written (by a German) in excellentEnglish, apart from occasional difficulties with the prepositions and with the more orless compulsory order of arrangement in English-e.g. in the words (p. 177) 'the distancetravelled without the mixture at which it was from the surface', which are puzzling inEnglish, but would be quite clear in the German version.

Three chapters are concerned with epidemics. 'Analysis of epidemics' is anothermathematical paper, but on a more modest scale and any difficulties that arise are notfrom the formulae but from a tendency to fancifulness in the text, as in the sentence'The previous history of the epidemic, every previous fluctuation, is remembered in themultiplication rate, and this memory factor is the special contribution of an incubationperiod to the concept of compound interest'. There is a note of challenge, not only inthe text but also in the doctrine, e.g, in the so-called 'law of lesion size', according towhich the' highest potential rates of multiplication are with small lesions ', For purposesof this law a systemically infected plant is called a lesion, so that an apple tree with virusis a larger lesion than a potato plant similarly affected. An apple tree is normallyfarther away from its immediate neighbours than is a potato plant and for that reason,other things being the same, there would be a slower transmission of disease through theapple crop. To that extent the law is plausible, but it would be remarkable if otherthings were in fact the same, and there are lesions of the more accepted kind (e.g, of applescab, apple canker) to which the law does not seem to apply at all. The reader thereforemay feel that if a twist has been put to the meaning of 'lesion', there is a comparabletwist to the meaning of 'law'.

The author of' Forecasting epidemics' says, somewhat ruefully but realistically, that, If the ratio of benefit to cost is large, the rational grower will apply controls whetherthe disease is forecast or not. If the ratio is small, no amount of urging will induce therational grower to act.' Between these extremes there are many opportunities for usefulforecasting, and the various factors that are taken into consideration in preparing theseforecasts are set out fully in this chapter.

'Biological interference with epidemics' ranges into many biological fields: action ofantagonists, of predators, of hyperparasites, and so on. While it is undoubted that suchactivities tend to the maintenance of a rough long-term balance between organisms innature, it has proved difficult (apart from some instances) to transfer laboratory successesto the more variable and complex conditions of the field.

The remainder of the volume is dedicated to control measures, in one form or another.First of all, in 'Quarantines' there is a fully documented account of an intricate andcontroversial subject, the difficulties of which do not always have a plant pathologicalbasis. 'Cultural practices in disease control' is a long and diffuse chapter. Even on thenarrower (and customary) definition of cultural practices, the topics would be rathermiscellaneous, and when the author uses a wider definition which allows a section ongenetic resistance (to quote only one example), the scope is virtually unlimited. Muchof the text is superfluous, inasmuch as it skims over a whole range of topics that are dealtwith more fully and more appropriately in other parts of the treatise.

Two chapters are concerned with the chemical control of disease. 'Soil treatment'gives a valuable discussion of chemical and physical interactions between soil and biocide(fungicide or nematocide), with much up-to-date information on the uses of specificsubstances and on the technique of their application. 'Performance of fungicides onplants and in soil-physical, chemical and biological considerations' is a parallelchapter, but with main emphasis on foliar applications and on reactions occurring atthe surface of the host plant. These accounts are complementary to those of chaptersEI-14 of volume II and many plant pathologists, finding it more and more difficult tokeep abreast of the flow of literature on this rapidly advancing and highly technical sideof their subject, will particularly welcome these five chapters.

Transactions British Mycological SocietyThe broad lines of 'The problem of breeding resistant varieties' are to be found in

many text-books, but here we have a full and authoritative account that runs smoothlythrough all the historical phases of the problem, though there is necessarily a great massof illustrative detail, this is so handled as never to obscure the general picture of theprogress that has been made and of the work that still lies ahead. This is the final chapterof the treatise and it is certainly one of the best.

Looking back over the three volumes, one cannot escape from the feeling that this isnot a treatise in the accepted sense of that word-viz. an account which runs in an evenlybalanced and authoritative manner over the full range of the subject. To do this, on thescale aimed at in this book, would clearly be beyond the capacity of anyone author, andhence the multi-authorship. The treatise thus comes to be a collection of essays, eachwritten by an expert in a certain field of investigation, which mayor may not be clearlydefinable. But what is gained in authority may be somewhat offset by loss in balance andby the danger of undue overlapping.

The reader cannot fail to be impressed by the authoritative treatment found in manyof the chapters, but he will probably also feel that there is too much repetition. Part ofthis arises no doubt from its multi-national authorship which puts time and distancedifficulties in the way of co-ordinating the several texts, but the main cause of it lies inthe major partitioning of the volumes according to a scheme that seems to be logical andclear-cut but is in practice unrealistic. This shows up, for example, when one compareschapter 5 of volume I (' Tissue is disintegrated ') with chapter 7 of volume II (' Chemicalability to breach the host barriers'). Though they start from opposite angles-the oneof the host, the other of the parasite-they very soon find themselves on the same ground,and it would have been better if the two had been dovetailed into one chapter, underjoint authorship. There are other examples of the unnatural separation of closely relatedsubjects and some of these have already been noticed. None of the three volumes is self­contained-which may be annoying to the intending purchaser.

A disadvantage, even if a minor one, of composite authorship is that each contributionhas its own preamble. Forty-four chapters means forty-four separate preambles. Whilemost of them are short and to the point, others are not so, and here and there one comesupon some very elementary statements that could well be dispensed with in an 'advancedtreatise'. Then there is the inevitable diversity among authors in presentation-fromthose who load their text with what seems to be irreconcilable detail and who give thereader little guidance in the matter, to those who place the details in perspective anddevelop a clear line of thought from start to finish. There are many good examples ofthe second type in this treatise.

All the chapters give extensive literature references and each volume has its ownauthor index and subject index. The subject indexes are remarkably detailed and musthave involved an immense amount of labour. For teacher and research worker alike,the special value of this book lies in its more or less co-ordinating the largest body so farassembled of the scattered literature of plant pathology, and this value is much enhancedby the ease and rapidity with which any desired piece of information is made available.

w. BROWN

A World Monograph of the Genus Pleospora and its Segregates. By L. E.WEHMEYER. (Ann Arbor: University of Michigan Press, 1961.)Pp. ix+451, 135 figures, 12 plates. Price: $15.00.

Following his well-known monograph of the genus Diaporthe and its segregates, Prof.Wehmeyer has studied another 'difficult' group, Pleospora and related forms. Hisaccount is based on over 1200 collections which he has personally examined, includingtype specimens of about 400 species. The examination of these specimens represents asample of a large population and as a result' relationships have been revealed throughestablishing long series of overlapping variations between species and individual collec­tions'. In an introductory chapter the morphology and taxonomy of the group arereviewed, in the light of developmental studies of the ascostromata, and especially onthe structure and the development of the ascospores, where a number of distinct linesof development are distinguished. Wehmeyer comes to the conclusion' that the develop-