December 29, 1923] 



NATURE 



955 



Virus Diseases of Plants. 



AN interesting discussion upon "Virus Diseases 

 of Plants" was held during the meeting of 

 the British Association at Liverpool between the 

 Sections of Botany and Agriculture. These obscure 

 maladies, which are of great economic importance, 

 affect a great variety of cultivated plants and have 

 itely received much attention from plant patho- 

 logists. Formerly these diseases were attributed to 

 'general physiological degradation, notably in the 

 jotato, but since they have been shown to be markedly 

 ifectious, they are usually considered to be caused 



Ity organisms of ultramicroscopic size, which are dis- 



^Seminated largely by means of insects. 



The discussion was opened by Dr. Paul Murphy, 

 who first described the symptoms of these diseases 

 in general and compared them briefly with certain 

 diseases of animals of somewhat similar type. He 

 then dealt specifically with the " leaf -roll " and 

 " mosaic " diseases of potatoes, both of which cause 

 enormous losses in yield. In discussing " leaf-roll " 

 of potatoes. Dr. Murpliy maintained that the abnormal 

 accumulation of starch in the leaves, which led to 

 rolling, preceded the degeneration of the phloem, 

 which is also a marked symptom of this disease. In 

 potato " mosaic," characterised first by a mottling 

 of the foliage and later by marked degradation of 

 the whole plant, he stated that this disease sometimes 

 masked other " virus " diseases of the potato such 

 as " stipple-streak " and " crinkle." He had also 

 demonstrated that certain varieties might act as 

 " carriers " of this disease, in which the symptoms 

 remained dormant, although infection could still 

 be spread from these plants. As instancing the 

 rapidity with which degeneration caused by such 

 diseases might occur. Dr. Murphy said that on a 

 farm at Ottawa potatoes had been grown healthily 

 for seventeen years, but that, after this period, 

 marked degeneration set in during the course of a 

 single season, which had affected all potatoes sub- 

 sequently grown on that farm. He considered, how- 

 ever, that there still remained a certain reduction in 

 yield attributable to non-pathogenic causes when 

 the same healthy stock was grown in different but 

 apparently suitable localities. 



Prof. H. M. Quanjer, of Wageningen, Holland, who 

 has made a special study of these diseases in the 

 potato, then gave an account of his own researches 

 on these maladies. In regard to " leaf -roll " he 

 combated the view of Dr. Murphy that the seat 

 of the disturbance lay in the abnormal accumulation 

 of starch m the leaves, maintaining that the primary 

 effect of disease was the necrosis of the pliloem con- 

 sequent upon the entry of the " virus " through 

 insect agency. He pointed out that infection by 

 aphides during May and June first resulted in rolling 

 of the upper leaves during August. Prof. Quanjer 

 claimed that the real seat of these " virus " diseases 

 was the phloem, and suggested therefore that they 

 should be called " phloem diseases " rather than 

 " virus " diseases, although he admitted there was 



no visible degeneration of the phloem in "mosaic" 

 diseases. In this connexion also it must be conceded 

 that there are other diseases of phloem tissues which 

 do not fall into the category of " virus " diseases. 

 Prof. Quanjer emphasised the role played by insects, 

 especially aphides, in the dissemination of these 

 diseases, but pointed out that in some " mosaic " 

 diseases transmission was possible through mechanical 

 abrasion of the leaf hairs. 



Dr. W. B. Brierley exhibited lantern slides which 

 showed in a striking manner, by reference to American 

 statistics, the losses caused by these diseases in crop 

 plants. With regard to sugar-cane " mosaic," he 

 stated that varieties resistant to the disease had 

 recently been discovered which would probably 

 prove the salvation of the cane industry in certain 

 districts. 



Mr. T. Whitehead classified " virus " plant diseases 

 into four categories, of which the following are 

 examples : 



(i) Infectious chlorosis, which is transmissible only 

 by grafting. 



(2) Spike disease of the sandal-wood tree, in which 

 there is neither abnormal starch accumulation nor 

 phloem necrosis. 



(3) Leaf-roll of potatoes, in which abnormal starch 

 accumulation accompanies phloem necrosis. This 

 disease is transmissible by insects, but not by ex- 

 pressed sap alone. 



(4) Potato mosaic, in which there is neither 

 accumulation of starch nor phloem necrosis, although 

 the sugar content may be unusually high. 



This disease is transmissible by the sap alone, 

 without insect agency. Mr. Whitehead appealed 

 for more accurate methods in diagnosing this group 

 of diseases, and gave striking evidence for the trans- 

 mission of potato " leaf -roll " through the soil. He 

 suggested that these diseases could be best controlled 

 by raising resistant varieties and by establishing 

 special beds of potatoes for seed purposes, which 

 could be rogued effectively and lifted early. 



Mr. Holmes Smith expressed the view that leaf- 

 roll was by far the most serious of the " virus " 

 diseases of the potato in this country. Unfortunately 

 manurial treatment had no effect upon it, although 

 this was somewhat beneficial in potato mosaic. 



Dr. R'. N. Salaman pointed out that although this 

 year he had taken the trouble to spray his seedling 

 potatoes, planted in old garden soil, with nicotine 

 at frequent intervals in order to control aphides, 

 infection by mosaic and leaf-roll had been more 

 serious than ever before, although seedlings planted 

 in remote plots in other crops had remained healthy, 

 Solanum nigrum appeared to be of no importance 

 as a " carrier " of these diseases. Dr. Salaman 

 expressed the view that " virus " diseases of the 

 potato were probably not congenital, and that 

 susceptibility to leaf-roll was transmitted independ- 

 ently of susceptibility to mosaic disease. 



F. T. Brooks. 



Australian Railway Development : a Study in Political Geography. 



A I i<. (). H. T. RISHBETH read a paper on this 

 ■^ * ^ subject to Section E (Geography) of the 

 British Association at Liverpool. Railway systems 

 typify the humanised as opposed to the purely 

 physical environment, and in so far as they reflect the 

 higher social and political mentality of the people, 

 contribute most useful data for the human geo- 

 grapher. In Europe the system of national states, 



NO. 2826, VOL. I 12] 



with their semi-geographical basis, was evolved before 

 the railway era. The railway systems superimposed 

 on a well - defined national background share the 

 intense individualisation of the continent. Europeans 

 brought to Australia this tradition of individualism 

 and exclusivene.ss. 



Mr. Rishbeth maintained that Australia is a clear- 

 cut geographical unity and that its interstate bound- 



