302 



THE AGEICULTUKAL NEWS. 



September 22, 1917. 



PLANT DISEASES. 



THE PORTO RICO CANE DISEASE: 

 COMPARISONS. 



The curious disease of sugar-cane which has appeared in 

 Torto Kico, an account of which was given in the latest issue 

 of this Journal, affords for consideration a subject of more 

 than urdinarj- interest. According to the information supplied 

 the infection is steadilj' extending, is transmitted in all the 

 cuttings made from affected plants, and is transferred to 

 plants of healthy origin brought into the area of its occupation. 

 Yet no parasitic organism has been found to which its 

 origin can be attributed. 



A comparison with the sereh disease of sugarcane of 

 the Eastern Tropics is at once suggested. Sereh was first 

 definitely reported in Java in 1882, and in the period 

 1882-96 it extended steadily and regularly from west to east 

 of the island, which it occupied with the exception, still 

 remaining, of certain highland regions. The disease is now 

 avoided partly by the establishment of a .system under which 

 cuttings i.re regularly imported from these districts to the 

 plain.s, and partly by the propagation of more resistent varie- 

 ties. The transfer of cuttings from an infested to a non-infested 

 district was found to give rise to a new centre of sereh, even 

 though the plants so introduced were not further propagated. 

 Plants raised from healthy cuttings from the disease-free 

 districts become infested in their first year in the plains. No 

 specific parasite has been found to explain the transmission 

 thus implied. 



So far the parallel with the Porto Piico disease appears 

 to be a close one, and there is a further resemblance in that 

 the primary diseased plants may show very little sign of 

 of the disease. The characters which are revealed in 

 the second generation are however quite diffeient from 

 those of the Porto Puco disease. In sereh the canes have 

 very shor" joints, numerous aerial roots, and buds 

 which sprout prematurely and so give the resemblance to 

 lemon-gra.ss from which the disease derives its name. In the 

 third generation of diseased plants a whole cane field may 

 look like a field of lemon-grass with here and there a cane 

 stalk arising from the clusters. Ratoons are almost invariably 

 badly attacked. 



Another disease which occurs on sugar cane in Java, the 

 yellow stripe disease, apparently belongs to the same class as 

 sereh and the Porto llico di.^ease, and has in common with 

 the latter, the character of atl'ecting the colour of the leaves. 

 It is characterized in most varieties, according to the descrip- 

 tion given by Dr H. M. Quanjcr, 'by the development of 

 yellow stripes on the leaves and red stripes on the stem; the 

 internodes do not exhibit their full growth; in severe cases 

 the plants are slender und the yield may be reduced to nearly 

 half the normal. The disea.se has been known for more than 

 twenty years in Java; it occurs on all types of soils and in 

 all varieties except the wild ones. The di.seased plants 

 appear scattered in the plantation; organisms could not be 

 demonstrated to be its cause.' Diseased cuttings invariably 

 give diseased jilants. Mealthy stocks give a greater or less 

 percentage of diseased oti'spring. 



Perhaps the mo.st instructive comparison that can be 

 suggested, since it refers to an affection which has been the 

 subject of many-sided investigation and has a voluminous 

 literature, is with the leaf-roll disease of the common potato. 

 Tl I- recent lucid exposition of its characters by Quanjer will 

 be followed in the ensuing renjarks. The principal symptoms 

 are rigidity, discoloration, and upward rolling of the leaves, 

 slow growth, shortening of the internodes, and the production 

 of small tubers. Plants may become primarily affected 



during the course of the growing season, in which case 

 they may show the effects of the disease slightly or not 

 at all. As in sereh, however, the next generation follow- 

 ing the beginning of the affection consists of plants 

 of which the greater part are diseased from the out.set. 

 The second, third, and subsequent generations show the 

 .same type of secondary disease in all the plants. A spon- 

 taneous disappearance of the symptoms in a long series 

 of succeeding generations has never been observed, nor, on 

 the other hand, has the extinction of the progeny occurred. 



By a series of experiments Quanjer has shown that the 

 disease is definitely contagious. He indicates the difficulty of 

 learning this fact from general observation, and expresses 

 the belief that it is for this reason that the communicability 

 of sereh has never been definitely proved. His results may 

 be briefly set out as follows: (a) Tubers from healthy 

 'elite plants' of two strains, planted where leaf-roll was 

 prevalent, showed late signs of the disease in the first 

 summer. Parallel plantings on the original farm remained 

 normal, (b) In grafting experiments the disea.se was com- 

 municated from diseased stocks to healthy scions and vice 

 versa, (c) Attempts to introduce the disease into healthy 

 plants by injecting them with the sap of diseased 

 plants or by transplanting tissue from diseased stalks were 

 unsuccessful. Similarly in transplanting pieces from one 

 tuber to another, infection did not occur where union 

 was not secured, (d) Union of opposed halves of infected 

 and non-infected tubers resulted in the issue of diseased 

 stalks from the eyes of the latter, (e) Infection may start 

 from the soil in which diseased plants have grown, and in 

 one instance was shown to take place after five years interval 

 in soil rich in humus and untiiled during the period, (f) The 

 disease is transferred to adjacent plants, (g) It is as yet 

 uncertain whether the disease can be transmitted through 

 sexual reproduction. 



According to Quanjer, the essential character of the 

 disease, from which the external symptoms arise as secondary 

 effects, is necrosis of the phloem, i e. decay of those strands in 

 the vascular bundles whose main function is conduction of 

 the elaborated product,=! of the activity of the leaves. No 

 causative fungus or bacterium has been found, so that the 

 occurrence of contagion leads to the hypothesis that a virus 

 is concerned which is either a fluid or composed of organisms 

 too small to be rendered visible by the microscope. 



The mosaic disease of various Solanaceous plants is 

 known to be communicable by means of a virus which has 

 resisted all attempts at filtration, and of which the infective 

 efficiency is not reduced by dilution of the juice of the 

 diseased plant to 1 part in 1,000 and still exists in a dilution 

 of 1 in 10,000, although no more is introduced in making 

 an inoculation than is carried on the point of a needle. 

 Since the result of such an infection is conveyed to all parts 

 of the plant and may be increased indefinitely by further 

 transmission to other plants the conclusion that it is in some 

 way propagated in the plant is irresistible. 



No .such infective virulence has been found in the 

 potato leaf-roll disease, in sereh, or as it appears, in the 

 Porto liico disease, but the idea that the existence of a con- 

 tagious virus may account for the recorded facts of the trans- 

 mission of these diseases is supported by the knowledge of 

 the possibilities in this direction which the more definite 

 properties of the virus of mosaic disease have permitted to be 

 demonstrated 



It will be interesting to learn if phloem necrosis, which 

 has been asserted (and denied) to be an essential character 

 in several so-called physii 'logic il diseases, occurs in the 

 affected Porto Pico canes. 



W.N. 



