214 CHAPTER XII 



kinds of tobacco, for in cases of very rank jrrowth, where the 

 leaf is thick and coarse, or in any dry tobacco markedly 

 deficient in mineral constituents, there is a decided tendency 

 to burn with a flame, whereas the capacity for glowing is 

 lacking. 



" The organic constituents of the leaf immediately in 

 advance of the burning area are undergoing the process of 

 dry distillation, in which the volatile products for the most 

 part escape and appear in the smoke. It is this process which 

 gives rise to the aroma. 



" (1) The fire-holding capacity is dependent primarily on 

 the content of potash combined with organic acids. (Potas- 

 sium nitrate in excess causes the tobacco to burn explosively.) 



" (2) Lime in general does not affect the fire-holding 

 capacity to any great extent, but is an essential factor in the 

 production of a good ash. How^ever, tobacco containing exces- 

 sive amounts of lime gives an ash which, although it is very 

 light in colour, lacks cohesion or ' flakes.' The potash salts, 

 more especially the organic compositions, yield an ash which 

 is firm and compact but dark in colour. Potash and lime 

 combined in the proper proportion are essential to a firm, light- 

 coloured ash."* 



Diseases and Pests. — Probably the worst fungous and 

 bacterial diseases occurring in the Union are Wildfire (Bacil- 

 lus solariacearum) , Leaf Spot (Cercospora nicotiancE), White 

 Bust (Macrosporium tabacinum), and Damping-off Disease 

 (Corticum vagum). Of these Wildfire is undoubtedly the most 

 serious at present, the industry in the Eustenberg area being 

 severely threatened by its ravages. 



To combat these diseases the best measures are to be 

 found in sterilising seed-beds, disinfecting seed, thorough cul- 

 tivation, rotating crops, and refraining from growing tobacco 

 on land known to be badly infected. 



" Mosaic " or " Calico " disease causes the leaf to grow 

 more rapidly near the veins than elsewhere, and thus causes 

 it to become wrinkled and corrugated. No causal organism 

 has been isolated, and at present it is held to be a physiological 

 disorder, due to defective nutrition. 



Root Gallworm (Hcterodfra radicicolo) is at times a dan- 

 gerous pest, particularly on land which has grown potatoes. 

 The nematode causes a thickening of the tissues of the root 

 which interferes seriously with the functions of the plant, 

 causing a feeble growth, wilting, and frequently death of the 



