PLAGUE

the investigations of the indian plague commission working in conjunction with the advisory committee showed that bubonic and septicaemic plague are transmitted by the agency of fleas. highly susceptible animals are not infected by their plague-stricken fellows so long as fleas are excluded from the cages, but with the introduction of these ectoparasites the disease forthwith spreads from the sick to the healthy. subsequent experience has confirmed the main findings of the commission, and outbreaks of human plague are unquestionably mere extensions of preceding or concurrent plague affecting animals. rats (rattus rattus) are of outstanding importance as reservoirs of infection through their close associa- tion with man, but marmots (arctomys bobac), gerbilles (tatera lovengulac) and ground squirrels (citellus beecheyi) have been responsible for epidemic outbreaks in china, south africa and california respectively. mode of infection—the plague commission concluded that the disease can be transmitted by the faeces of infected fleas, but did not state that this is the usual method of spread. bacot and martin later demonstrated that the entrance to the flea’s stom- ach becomes obstructed by a mass of plague bacilli. such fleas in their persistent efforts to feed merely succeed in distending the gullet with blood, which, now contaminated, regurgitates on to the skin of the host. the organisms then enter the body either through the flea’s bite or through some abrasion of the skin. fleas as vectors —the flea most commonly incriminated as a plague-vector is xenopsylla cheopis, but several other species of rat fleas (ceratophyilus fasciatus, stivalius ahalae), the common mouse flea (leptopsylla musculi), the dog flea (ctenocephalus caitis), @ marmot flea (ceratophyllus silanticvi) and the human flea (pulex irritans), amongst others, are proven carriers of plague that are known to bite man. when the plague commission made their investigations only one species of yenopsyila (x. cheopis) had been recognised, and they were unaware that two other species of this genus commonly infest indian rats. one of these, x. astia, is incapable of conveying plague in circumstances where transmission by x. cheopis is readily effected. x. astia is the predominating species infesting rats in parts of india which have remained free from plague, or where plague when introduced has not spread. the inhospitality of x. astia to the plague bacil- lus may be the responsible factor in such immunity, as was first suggested by hirst in 1914. epidemiology.—laborious stuclies on the biology of fleas have added to our knowledge of the epideriiology of plague. these insects cannot withstand desiccation, and their survival apart from their host is proportional to the rate at which they lose moisture. bacot and martin (1924) found the water content of fleas to be 80% of their weight, and death occurred before the loss of water had reached 60° % of their weight. this goes far to ex- plain the observation that plague does not maintain itself in epidemic form when the temperature rises above 80° f., accom- panied by a saturation deficiency of over :30 of an inch; whereas bacot (1914) demonstrated that starved rat fleas (c. fasciatus) infected with plague may remain alive, and infective, for one or two months in cool weather. he also showed that the larval and pupal stages may be prolonged by natural agencies, and in consequence active adult fleas may be found, in favourable 154 situations, where there have been no hosts for long periods— 10 months in the case of x. cheopis, and 22 months in that of c. fasciatus. the seasonal variations in the prevalence of plague have been recognised for many centuries. in temperate coun- tries epidemics attain their maximum in the summer months, whereas in tropical climates the onset of hot, dry weather brings an existing epidemic to an end. in each case, the peak of the plague curve coincides with meteorological conditions most favourable to the activity and longevity of fleas. in contra- distinction to the foregoing, pneumonic plague, once established, spreads independently of animals and fleas. in outbreaks of ordinary plague, a secondary involvement of the lungs super- venes in a small proportion of cases, and if atmospheric and environmental conditions favour the survival and transmission of the bacilli an epidemic of pneumonic plague may result, the virus being transmitted from the cougher to the victim up to a range of about three feet. treaimeni.— unfortunately, there is no plague specific. serum, to be really effective, must be given before the symptoms of the disease are manifest, an impossible procedure in most cases. still, when available, serum should be administered intra- venously in doses of 80-100 c.c.’s, and repeated. some advocate injection of tincture of iodine (7-10 minims), or other anti- septics, intravenously, and also into the buboes. these measures are of doubtful benefit. buboes showing signs of suppuration should be incised. otherwise, treatment is on general lines, and cardiac stimulants should be administered early. prophylaxis—so long as members of a community are con- tent to live in association with rats and fleas correlative plague is a likely result. meanwhile, plague prophylaxis consists mainly of warfare against rats, in the hope of ultimately banishing them from human habitations and from contact with man. any general elimination of these pests over wide areas is a slow, costly and usually impracticable affair, and all that can be achicved in most cases is some degree of palliation by rat-proofing of grain stores, wharves and other places which serve as centres for the dissemination of plague. destruction of rats by trapping and poisoning is a commendable procedure, but of itself will never eradicate endemic plague, for the losses in the rat population are quickly made good if survivors are provided with food and harbourage. an important measure of personal prophylaxis is inoculation with anti-plague vaccine, which affords a good degree of protection. persons entering plague-infected houses should wear gaiters, and otherwise render their clothing as flea- proof as possible; if there is any suspicion of pneumonic plague, goggles, and masks made of several layers of fine gauze, should be worn without fail. bibliography.— journal of hygiene, plague supplements (1912-7); league of nations report, ch. 130, prevalence of epidemic disease in the far east (geneva, 1923); ministry of health, report 19, /fydro- gen cyanide for fumigation purposes (1923). planck, max (1858- ), german physicist, was born at kiel april 23 1858. he studied at the universities of munich and berlin, became an assistant in munich university, and was subsequently appointed a professor in kiel (1885) and in berlin (1889). planck devoted himself to the study of theoretical physics, and in particular to that of thermodynamics. his lec- tures on this subject appeared in book form and were translated into english, french and russian. he gained international fame by his law of radiation (1901), which asserts that the energy of radiation is emitted and absorbed in integral multiples of certain indivisible “ quanta ” of energy, which depend on the frequency of oscillation of the electrons. in 1912 planck extended this “ quantum theory ” to all kinds of energy and added the assump- tion that only emission proceeds discontinuously, in quanta, while absorption is continuous. in 1913 he somewhat altered his theory, but subsequently reinstated the old conception. al- though several physical phenomena, such as the properties of the specific heats of solids, the kinetic theory of gases, confirm planck’s theory, it is still warmly disputed. in 1918 he was awarded the nobel prize for physics. planck became editor of the annalen der phystk. planck—plant pathology among his works are: die entstehung und bisherige entwickelung der quantentheorie (1920), transl. by h. t. clarke and l. silberstein (1922); vorlesungen tiber die theorie der warmestrahlung (1921); physikalische rundblicke (1922), transl. by r. jones and d. h. w il- liams (1925); einftihrung in die theorie der elektrizitit und des magnelismus (1922); and die a bleitung des strahlungsgesetzes (1923). planquette, robert (1850-1903), french musical com- poser (see 21.725), died in paris jan. 28 1903. plant pathology.—the progress made in the theoretical and practical study of plant pathological problems during the early part of the century consisted not of any very outstanding discovery in any particular part of the subject, but of a general and considerable advance in all departments. there was first of all a much quickened interest in the subject of plant diseases on the part of the departments of agriculture in all the more progressive countries of the world. more funds became avail- able for the investigation of disease problems, and the number of workers in this branch of botanical science correspondingly increased. (see also botany; ecorocy.) in this respect the united states of america, with its varied and as yet somewhat experimental system of agriculture, stil occupies the leading place, but britain is following in the same direction, especially in connection with the development of her tropical colonies and dependencies. as a consequence of this increased activity, the period under consideration produced a vast accumulation of scientific literature; many diseases new to science and of greater or less importance have been described; more detailed knowledge on the subject of the older and better known diseases has been obtained; and as regards the purely practical aspect of the problem improved methods of dealing with plant diseases have been developed. the latter include more efficient methods of spraying, dusting, etc., for the purpose of checking the onset of disease, and, what is of perhaps greater economic importance, the breeding of varieties of economic plants which are immune or at least resistant to certain diseases. virus diseases —what are now known as the “ virus ”’ dis- eases of plants were formerly considered as physiological dis- eases, 4.¢., as being due to certain disturbances of function not correlated with the presence of a parasite. the recognition of their infectious or contagious nature, either by direct inoculation or by grafting, was a great step in advance. the general view now held is that they are due to a “ virus” which is ultra- microscopic and which will pass through the pores of certain filters. as to the nature of the virus, whether it is a kind of very small bacterium, or an enzyme, or an organism sx? generis (twort), no definite decision can at present be given. (see filter-passing microbes.) a large and ever increasing number of economic plants prove to be susceptible to diseases of the virus type. to men- tion some of the better known: the various forms of virus disease of the potato (mosaic, leaf-roll, crinkle, etc.), the mosaic diseases of tomato, tobacco, cucumber, hops, sugar-cane, various beans, clover, raspberry, dahlia, etc.; the ‘‘ curly top ” disease of sugar beet, a number of peach diseases and the phenomenon known as “ infectious chlorosis.”” to the same category prob- ably belong the well-known “ reversion” of black currants which is troublesome in england, and the ‘ spike disease ” of sandalwood in south india. in britain the most important virus diseases from the eco- nomic point of view are those occurring on the potato. these are widespread, especially in the warmer and drier parts of the country. researches have shown that virus disease is the chief cause of the deterioration, i.e., of the loss of cropping power which potato stocks suffer in the warmer parts of britain. the view held formerly was that this deterioration was due to climate ber se, the potato being considered to be a plant of cold climates and therefore not maturing its ‘‘ seed ” properly under warmer conditions. according to the latter view, deterioration is thus inevitable, the only way to keep up the cropping power of the potato being to import more vigorous “ seed ” from farther north. those who hold the “ virus ” theory, on the other hand, point out that the effect of climate is only indirect, in that it encourages the multiplication of insects (aphides) which are ce plant pathology the natural agents for spreading the disease in the field. preven- tion of deterioration is thus possible, either by control of the insects concerned, or, failing that, by the provision of disease- free stock in the first instance and avoidance of subsequent con- tamination. the prompt recognition of diseased plants and their removal (“ rogueing ”’) before the disease has been carried to neighbouring plants is the practical method of controlling virus diseases of the potato and virus disease in general. carriers —apart from their economic importance, great scientific interest attaches to diseases of the virus type. the nature of the active principle has been the subject of numerous researches (allard, doolittle and others). there exist definite carriers ” of the disease—that is, plants which on inoculation show no symptoms of disease in themselves, but are able to trans- mit the disease to susceptible plants. this feature affords an interesting parallel to what is met with in certain animal dis- eases, e.g., typhoid fever. in fact, the virus diseases of plants are in general very similar to the corresponding diseases in animals, and serve as a kind of common ground for the sciences of plant and animal pathology—two sciences which in the main are very different. diseases due to fungi.—as regards plant diseases of a purely fungoid type, the expansion of world agriculture, and especially the agricultural development of the tropics, proceeds apace, and thus economic crops are brought into contact with new en- vironments where they are liable to attack by parasites unknown in their old habitat. furthermore, the plantation method of cultivating tropical tree crops as opposed to the sporadic occur- rence of these plants under natural conditions brings with it greater dangers from parasites, as the mass formation of a plantation lends itself readily to the spread of disease once the latter has entered. new fields of investigation are thus con- stantly being opened up. even in countries with a stable and long-established system of agriculture such as england, new problems may suddenly arise, as witness the wart disease of potato (syxchvirium endobioticum). in the early years of the century this disease was almost unknown, but im the period under consideration it attained serious dimensions, so much so that its presence determined the whole development of the potato industry in england in later years. among diseases which have become of considerable impor- tance in england mention may be made of the silver leaf disease (stereum pur pureum) of plums and other fruit trees; the ameri- can gooseberry mildew (sphacrotheca mors-uvae), which is firmly established and the downy mildew (pseudoperonospora humuli), which is a constant source of danger to hop cultivation. in the united states the bark disease (endothia parasitica) of sweet chestnut, which came in from japan, and the white pine blister rust (chronartium ribicola), which is native to europe, have caused great damage. the problems of rusts and smuts in cereals have received a large amount of attention in all grain- growing countries; and the same may be said of a number of fungi which cause foot-rot, leaf stripe or barrenness of ears in various grasses (ophiobolus, hclminthosporium, gibberella). in various countries a whole group of “ wilt ” diseases, due chiefly to a species of fusurinm, has come into prominence— wilt of cotton, of banana, of various leguminous and solanaceous plants, etc. some of these, e.g., the cotton wilt, have been definitely controlled. banana wilt, on the other hand, is not yet under control, and this fact is of the utmost importance in the banana industry. the establishment of the rubber industry in the east brought in its train a whole literature of fungoid diseases (fomes, corticium), as well as the somewhat obscure brown bast disease. the rubber plantations in the i:ast have not as yet suffered from a leaf spot disease (melanopsammopsis), which has so far prevented the establishment of rubber plantations on the south american continent, the natural home of the /fcrea rubber tree. an interesting internal boll discase (vematospora) has been described from the west indies. in this case the ac- tive transmitting agents are biting insects. from present indi- cations it is not unlikely that this disease will prove to be an obstacle in the way of cotton cultivation in parts of africa. 155 diseases due to bacteria.—bacterial diseases of plants, which were at one time believed to be non-existent, are now known to be fairly common, and some have proved to be of great economic importance—for example, the angular leaf spot disease (bac- terium malvacearum) of cotton, which is especially severe on sea island and egyptian types; the citrus canker (pseudomonas citri), which has caused great damage in florida, south africa, etc.; the pear blight (baci//us amylovorus), which is serious in various parts of canada and the united states; and the wild fire disease of tobacco ( buctertum tabacum), which 1s now one of the most important diseases of tobacco in the united states and south africa. the crowngall organism (b. tumefaciens) has been much studied with special reference to its supposed resem- blance to human cancer, but this analogy has-been called in question. (see bacteriology.) breeding of immuse or resistant varieties —with regard to methods of prevention or treatment of plant diseases, the period under consideration shows considerable advances. much is now known on the subject of the inheritance of immunity or susceptibility to plant disease, and as the practical result of breeding work varieties immune or highly resistant to discase have been developed. in england many high-class varieties of potato have been developed which are entirely immune to wart- disease. wheats highly resistant to rust (and with other good commercial qualities) have been established (biffen, nilsson- ehle), varieties of hops immune to mildew (salmon), of aspara- gus resistant to rust (norton), of beans resistant to anthracnose (barrus, burkholder}, and so on. by simple selection of resist- ant individuals in the field, strains of cotton and cowpea have been isolated which are highly resistant to wilt (orton, watson). similar success has been obtained in the same way with the tobacco root rot (garner). strains of parasitic fungi—the problem of breeding im- mune or resistant varieties has been complicated by the dis- covery that many parasitic fungi exist in a number (sometimes a large number) of “ biologic forms ”’ or races, each of which has a different host range. hence a plant which is immune in one area may be susceptible in another, the strain of the parasite being different in the two areas. this feature of parasitism, which has been made out in very striking form in connection with the black-stem rust of cereals (stakman and co-workers), leads to the practical conclusion that breeding for disease resist- ance must be a regional problem. the first step in the solution of the problem must be the determination of the strain of fungus prevalent in the given area, and hence the value of the plant disease surveys which have been taken up by the agricultural departments of various countries, stakman’s work, however, while indicating unexpected difficulties in the way of obtaining immune varieties, has led toa simplification in another direction. the old view of a “ bridging species,” according to which a fungus is able to pass from a susceptible plant to one which ts otherwise immune (ward, salmon, pole-e vans), has been con- siderably discredited, so that there are now strong grounds for thinking that once an immune variety has been obtained it will remain so as long as it does not come into contact with a fungus of different parasitic power. prevention by sprays, etc.—many advances in detail with regard to the chemical control of disease by the use of sprays and washes have been made. these have been made possible to a large extent by the increased knowledge of the life-history of the various pests, so that more exact information with regard to the best time for spraying can now be given. this item makes all the difference between success and failure in spraying. the technique of preparing spray fluids has received much attention, with a view to obtaining the maximum fungicidal efficiency with the least damage to the plant. improvements have taken the form of presenting the active chemical in a finer state of division, in the perfecting of spreading reagents (soap, casein preparations) and in the use of new fungicidal chemicals, such as “ uspulum.” dusting has been much advocated in place of spraving as being more practicable, and in many cases as being equally effective. the latest development along these lines is 156, the use of aeroplanes for dusting, as in the campaign against the cotton boll weevil in the southern united states. (see cotton.) the same methods may be applied to rust control in wheat-growing regions. control by eradication.—the control of certain diseases in which the parasite passes a part of its life-history on an alternate host is illustrated by the ‘‘ barberry eradication ’’ campaign in the northern part of the wheat belt in the united states and canada, the barberry being the alternate host of the black stem- rust of wheat. a similar campaign to eradicate species of ribes, which are alternate hosts to the rust of the weymouth pine, is in progress in the eastern united states. the control of inter- nal boll disease of cotton in the west indies by the cutting down of certain malvaceous trees, on which the transmitting insects lived during the period when the cotton was not on the land, is a striking example of the same method. biological methods of control—a method of control which refers particularly to diseases of insect origin is the introduction and acclimatisation of enemies (parasites or “‘ predators ”) of the injurious insect. it frequently happens that in trying to establish a new crop in a country some insect pest by-and-by gets in, and that the latter in the absence of its natural enemies flourishes exceedingly and becomes a serious menace to the crop. by investigations in the original home of the crop it has been found possible in many cases to determine the natural enemies of the insect pest, either parasitic or predatory insects, or in a few cases bacteria or fungi, and to establish these'in the new country. many of these efforts have resulted in failure, due to faulty methods of procedure or to the intrinsic difficulty of the problem. in some cases, however, brilliant success has been achieved, as in the control of the sugar-cane leaf hopper in hawaii. strenuous attempts are made by the united states department of agriculture to control the ravages of the corn borer insect by the introduction of its natural enemies in europe, its original home. research along these lines appears to be one of the most promising outlets for entomological science for the future. legislation—a marked feature in recent years is the increas- ing amount of governmental control which is coming into being in connection with plant diseases. this development is to a certain extent the outcome of the increased transportation of plant products incidental to commerce generally and to the establishment of new crops in new countries. control methods are in many cases useless unless carried out thoroughly and simultaneously and in a specified manner over the whole area concerned. as between nations governmental control takes the form of inspection at the port of entry, of quarantine and in certain cases of actual exclusion. thus the import of goose- berry stock into england from the united states is forbidden on account of the mildew, and conversely the import of english potatoes into the united states is forbidden on account of wart disease. considerable controversy has arisen as to the efficacy of these measures, as in many cases they are applied too late. irritation is also caused by the fact that, while such regulations were primarily intended for the express purpose of excluding certain pests, they are capable of being used for the furtherance of economic profit. governmental control within the limits of any particular country takes the form of compulsory notification of certain diseases, of the prohibition of growing particular varieties in certain “‘ scheduled ” areas, of the inspection and certification of seed and stock plants and of the enforced clearing up of plant material which constitutes a source of danger to succeed- ing crops. the control of wart disease and of silver leaf disease in england is a good illustration of these methods, and in the former case at any rate very considerable success has been achieved. an interesting experiment along the same lines is in progress in british west africa, where a universal clearing up of cocoa diseases is being attempted. scientific developments.—developments in the more scientific aspects of plant pathology have dealt mainly with the physio- logical study of the interrelation between host and parasite and plant physiology with the effect of external factors on the incidence of parasitism. in the latter connection temperature and humidity have re- ceived most attention, ¢.g., the relation between parasitic attack and soil temperature (jones and co-workers), and the effect of low temperatures on fungal growth and parasitism (brooks, cooley). these investigations have a practical outlook, espe- cially with regard to the transportation and cold storage of fruit, a subject to which great attention is paid (stevens, haw- kins, kidd, west). another direction m which advance has taken place is the discovery of ‘ saltation ” or mutation in fungal organisms (crabill, stevens, brown and others). the occurrence of this phenomenon has probably a direct bearing on the existence of the many strains of pathogenic organisms already alluded to. bis_rocrapiy.—f. l. stevens and j. g. hail, diseases of eco- nomic plants (1921); o. g. anderson and f, c. roth, zusecticides, fungicides, etc. (1923); w. f. bewley, diseases of glasshouse plants (1923); e. j. butler, ‘‘ characteristics of virus diseases of plants,” science progress (1923); w. nowell, diseases of crop plants in the lesser antilles, west india committee (1923). see also current numbers of journals, phytopathology; annals of applied biology; journal of agricultural research; zeitschrift fir pflanzenkrankheiten, reports of agricultural research departments, etc. (w. a. b,)