Interspecific Transmission of Mosaic Diseases of Plants 5 



of inoculum. From three to five plants were inoculated from each source 

 of inoculum at one time in most of the greenhouse experiments, and six 

 or seven plants were used in each experiment in the field. 



Generally, as many checks were left as there were plants inoculated 

 with each source of inoculum. The checks were usually treated in the 

 same manner as the inoculated plants, except that a leaf of the check 

 plant itself was substituted for the inoculum. With potatoes, the pro- 

 cedure was different. In planting, a tuber was cut in several (from 4 to 

 10) pieces and planted in as many pots, each being given the same number 

 but a different letter. All the plants except one were used in inoculation 

 experiments. This one was saved as a check for freedom from disease of 

 the whole unit. For the most part, these were the only checks used for 

 potatoes. The second vegetative generation was always grown when 

 tubers could be obtained. In most of the inoculations made on potatoes, 

 the plants included in any one experiment were taken from different 

 tuber units. 



methods of inoculation 

 Rubbing 



The method of inoculation used in most of the earlier experiments was 

 as follows: A mosaic leaf was crushed in the hands into a little ball. A 

 leaf of the plant to be inoculated was held in the left hand and vigorously 

 rubbed with the crushed tissue held in the right hand. Sometimes the 

 inoculation was made with one hand only, rubbing the leaf and the in- 

 oculum together between the thumb and the forefinger. The rubbing 

 was vigorous enough to break the epidermis of the leaf and was repeated 

 on several leaves of each plant inoculated. Under these conditions the 

 method was found very effective. The hands were freed from infective 

 material by washing with soap, as recommended by Allard (1917). 



A variation of this method consisted in the wearing of rubber gloves. 

 The gloves were disinfected by soaking in 10-per-cent formalin for about 

 five minutes. This method was soon abandoned. 



The method first described was likewise abandoned early in the course 

 of the experiments, in favor of the following: Two pairs of forceps were 

 used, one with curved points and one with broad flattened points. The 

 forceps were flamed and then used to fold the leaf several times. The 

 inoculum was then held in the curved forceps and rubbed on the leaf to 

 be inoculated, which was supported from behind by the flat-pointed 

 forceps. 



In the later experiments the inoculum was sometimes ground in a mortar, 

 which had been disinfected by standing in sulfuric acid and rinsed in 

 water. In this case a knife blade was generally substituted for the flat- 

 pointed forceps, as a support for the inoculated leaf. 



