jan. 3,1921 Fusarium-Wilt 0} Tobacco 519 



ently healthy. If, now, the diseased stalk, roots, suckers, midribs, or 

 veins of the leaves are cut either in cross section or longitudinally (PI. 

 65, B) the vascular system will be found to be brown or distinctly black, 

 but upon pressure no "ooze" appears. The vascular decay is distinctly 

 "dry." 



On young plants in the greenhouse where the writer has had an oppor- 

 tunity to note the symptoms of the disease more carefully th'ey are essen- 

 tially the same so far as the vascular system is concerned, but the leaves 

 first lose their chlorophyll, becoming yellow and somewhat wrinkled but 

 distinctly turgid and "brittle," as compared with healthy leaves. This 

 condition may obtain for some time previous to wilting unless excep- 

 tionally high transpiration occurs. The leaves, of course, finally dry up 

 as they do in the field (PI. 63, C). In the greenhouse the symptoms are 

 most likely to appear first on the youngest leaves, and this may be 

 more or less characteristic in the field. 



So far as has been noted the parasite is not able to cause any rotting 

 of the living parenchymatous tissues of the plant. In heavily infested 

 soil where the cortical layers of the plant have been severely wounded or 

 a leaf petiole has been broken off below the surface of the soil, the para- 

 site may enter the vascular system readily and cause the death of the 

 aerial portion without in any way affecting the parenchyma of the stem 

 or roots at or below the surface of the soil. 



Histological studies of the disease were made by various methods, 

 but best results were secured by killing and fixing young tissue in Gilson's 

 fixative, imbedding in paraffin, and staining with the Pianese stain, as 

 described by Vaughan (12). Transverse sections of infected stems or 

 midribs of leaves (PI. 66, A) showed that all the vessels in local areas of 

 the vascular ring were more or less invaded, sometimes almost completely 

 "clogged" with mycelium. Longitudinal sections (PI. 66, B) showed in 

 an even more striking manner the general occurrence and the "bunch- 

 ing" of mycelium in the vessels. Nevertheless, from the behavior of the 

 diseased plants, especially with regard to yellowing and early turgidity, 

 it is not believed that death of the plants is due to clogging of the vessels 

 but rather to toxic materials formed by the parasite or as a result of 

 the parasitic action on the host. 



ISOLATION AND INFECTION EXPERIMENTS 



In the first isolations pieces of the discolored portions of the stem, 

 together with some surrounding healthy tissue, were cut out and treated 

 with 1 to 1,000 mercuric chlorid for 30 to 120 seconds, rinsed in sterile 

 water, and placed on hard potato agar in Petri dishes. Growth of fun- 

 gus mycelium from the diseased tissue was slow and not uniform. Pure 

 cultures of Fusarium, however, were secured. Isolations were later 

 made by cutting off the cortical layers with a hot blade and cutting out 



