THE VIRUS DISEASES OF PLANTS 417 



amons:st which is the " spike " disease of the sandalwood tree 

 in India. S *, '. 



It has been known for more than a century that it is some- 

 times possible to communicate variegation of the foliage of 

 plants to non-variegated plants in the same manner as that 

 mentioned in the last paragraph, namely by grafting a varie- 

 gated branch on a stock with normal green leaves. Subsequent 

 work b}^ Lindemuth,* Baur,^ and others has established the 

 existence of several examples of this communicable variegation, 

 which Baur calls " infectious chlorosis." It is characterised by 

 a deficient development of chlorophyll, but the plants are not 

 sickly, though less resistant to unfavourable conditions than the 

 normal individuals. The infectious types are not usually propa- 

 gated by seed, though certain other quite similar variegations 

 that are non-infectious are reproduced in the seedlings. Except 

 for the innocuous nature of infectious chlorosis and the fact that 

 spread to neighbouring plants does not occur in nature, so far as 

 is known, there are obvious similarities between this condition 

 and the group of diseases of which " peach yellows " is a type. 



In none of the groups mentioned above has any definite 

 lesion been detected in the tissues of the affected plants, except 

 in one form of potato mosaic (" crinkle ") and in mosaic of 

 maize, where some of the cells of the parenchyma may be 

 affected. Arrested development of tissues or whole organs, 

 localised h^^pertrophies, and malformations are not uncommon, 

 but there is usually no definite necrosis of the cells in the earlier 

 stages of the disease. In the later stages necrosis and death 

 may take place — peach yellows, for instance, is invariably fatal 

 — but this appears to be a generalised result of malnutrition, 

 and not due to the local action of any specific toxin. There is, 

 however, a group of diseases of the virus type characterised by 

 a localised necrosis of a particular tissue, of which leaf roll of 

 potato is the best known example.^ % ^ In this case there is a 



1 Barber, C. A., Report on Spike Disease in Sandalwood Trees in Coorg, 

 Indian Forester, xxix, p. 21, 1903. 



2 Butler, E. J., Report on " Spike" Disease among Sandalwood Trees, 

 ihid., Appendix Series, 1903. 



' Coleman, L. C, Spike Disease of Sandal, Bull. Dept. of Agric. Mysore 

 State, Mycol. Ser., No. 3, 1917. 



* Lindemuth, H., Studien uber die sogenannte Panaschiire, etc., Landw, 

 Jahrb., xxxvi, p. 807, 1907. 



^ Baur, E., in Ber. d. Deutsch. Bot. Gesellsch., xxii, p. 453, 1904 ; xxiv, 

 p. 416, 1906 ; XXV, p. 410, 1907 ; xxvi, p. 711, 1908. 



® Quanjer, H. M., The Mosaic Disease of the Solan aceae, its Relation to the 

 Phloem-necrosis and its Effect on Potato Culture, Phytopathology, x, p. 35, 

 1920 [Bibl.]. 



' Schultz, E. S., and Folsom, D., Leaf Roll, Net-necrosis, and Spindling- 

 sprout of the Irish Potato, Journ. of Agric. Res., xxi, p. 47, 1921. 



^ Papers by Quanjer, Cotton, Murphy, and others in Rep. Internal. Potato 

 Conference of 1921, Royal Hort. Soc, London, 1922. 



