8 BULLETIN 1038, U. S. DEPARTMENT OF AGRICULTURE. 



in natural and artificial infections. Diastase activity in the healthy- 

 leaves was found to be almost double that in the spiked leaves. The 

 mesophyll tissues are hypertrophied and the leaves increased in 

 thickness and the vascular tissues become reduced in advanced stages 

 of the disease. Chemical analysis (24) shoves a higher percentage of 

 nitrogen and of most of the ash constituents in healthy leaves. Since 

 young, healthy leaves pass through a stage comparable in chemical 

 composition to that of spiked leaves, it seems probable that in the 

 latter case development has for some reason been checked in the early 

 stages of growth. 



In a large percentage of Coleman's grafting experiments (24) 

 spike was successfully transmitted to originally healthy stocks, and 

 in almost every case the disease first appeared in the stock on branches 

 closest to the point of grafting and spread from these regions to the 

 other parts of the stock. A considerable time always elapsed before 

 external signs of disease appeared. In all cases examined, infection 

 had spread to the roots and resulted in the death of the root ends 

 and haustoria. 



The occurrence of spike disease is not dependent on the fertility 

 of the soil, nor does injury to the roots have anj^ relation to the dis- 

 ease. Venkatarama (81) found experimentally that isolation of the 

 trees from all possible hosts by digging trenches did not cause signs 

 of spike even after two years under observation. Cutting the root 

 connections and removing the haustoria, injecting the lateral roots 

 with strong sulphuric acid, and girdling to the heartwood did not 

 cause the disease. Thousands of trees previously growing under a 

 heavy covering of vines were exposed to the light, but the increased 

 loss of water resulted in nothing resembling spike disease. Fischer 

 (34) states that the spike disease spreads from a center, not appear- 

 ing simultaneously over considerable areas. 



In tobacco mosaic the yellow and green mottling of the leaves is a 

 prominent sign. Not only leaves but also the calyx may be mottled, 

 and the corolla becomes flecked with red and white blotches instead 

 of exhibiting the normal even red or white color. The light areas of 

 the leaves are usually slower growing than the green areas, thus often 

 resulting in distortions which may become extremely marked in 

 young leaves. Often, however, such leaves almost recover from these 

 malformations as they mature (1). Sometimes the laminae are al- 

 most suppressed, and in other cases a long, sinuous, ribbonlike leaf 

 is produced. In many cases abnormally dark-green blisters develop 

 on the immature leaves. 



Koning (47) and Heintzel (40) reported a separation of the cells 

 in diseased foliage which often leaves spaces nearly as large again as 

 the cell itself. The chlorophyll bodies become distributed in irregular 

 groups, chlorophyll disappears, the cell walls disintegrate, and finally 



