INOCULATIONS WITH BACTERIA CAUSING PLANT DISEASE 293 



tion may be induced if a fine abrasive material (e.g., carborundum 

 powder, 600 mesh) is lightly dusted on the leaf before it is rubbed. 

 Some plant viruses are highly infectious. (Usually washing with soap 

 and water is sufficient to remove infectious material from the tech- 

 nician's hands.) When the mechanical methods and insect vectors fail, 

 two possibilities are left. 



Budding or another form of grafting may be employed and is some- 

 times the only successful means of virus transmission. When grafts 

 are made, special precautions are necessary to prevent desiccation of 

 the grafted parts before union has been accomplished. This may be 

 achieved by providing high air humidity, by suitable wrappers, or by 

 spraying the scions with one of the commercial wax emulsions. 



Some investigators use dodder to carry certain viruses not other- 

 wise transmitted from one plant to another. 



TREATMENT WITH BACTERIAL PRODUCTS 



The metabolic products found in bacterial cultures are prepared and 

 employed in a variety of ways which are not yet well worked out. Per- 

 haps the least change occurs in the bacterial cells if they are centrifuged 

 from a liquid culture and dried while frozen. The culture filtrate may 

 be concentrated under reduced pressure at a little above room tempera- 

 ture and then " lyophilized " if desired. 



A fermented culture or an aqueous extract may be sterilized and 

 placed in a small container. If leaves with petioles or growing tops are 

 removed from the host plant and are placed with the cut surfaces in 

 such liquids, they commonly show injury within one day or two if much 

 toxic material is present. Bacterial contamination may be reduced if 

 the pH is approximately 4.0. A rigid control of temperature and rela- 

 tive humidity is essential to repeat the results. Care is necessary while 

 interpreting such injury because many constituents of media may be 

 toxic, e.g., ammonia in alkaline material or mineral salts. Likewise, 

 many nonparasitic as Avell as parasitic fungi produce toxic substances 

 in culture that are not necessarily the reason for pathogenicity. 



The metabolites are sometimes^ applied either in liquid form or in a 

 paste made with inert material, like lanolin, a polymer of ethylene gly- 

 col, or flour. The paste has the advantage of furnishing a continuous 

 supply of material over a longer period with relatively less desiccation. 

 It is commonly applied to a wound. The liquid can be introduced into 

 the vascular system of a potted plant by placing cut roots extending 

 from the base of the pot or a cut petiole into a container of the mate- 

 rial. Likewise, a cup can be made from a rubber stopper and sealed 

 on a plant stem with vaseHne. The cup is filled with liquid, under 



