266 A Case of Recovery from Mosaic Disease of Tomato 



withered leaves which had been thrown away earlier were carefully 

 collected, and those which had been attacked by various saprophytic 

 fungi rejected. The few remaining portions were thoroughly and 

 rapidly washed in sterile water, and then the moist fragments ground 

 to a pulp in a watch glass and extracted with 0-25 c.c. of sterile water. 

 This was filtered twice through two thicknesses of finest filter-paper to 

 eliminate as many bacteria as possible and any fungal spores. The 

 filtrate was then inoculated into the leaves, buds and stems of twelve 

 seedling tomato plants, and as a control twelve other seedlings were 

 similarly treated with sterile water and the pots placed on the same 

 shelf in a glass house. Of the sixteen plants which survived Black 

 Neck (Phytophthora sp. (omnivoraty), frost and the undue proximity 

 of hot pipes, five out of nine inoculated with "virus" showed mosaic 

 disease, whilst this was completely absent from the seven remaining 

 controls. Considering the old withered condition of the original leaves, 

 the treatment they received and the dilution of the "virus" derived from 

 them, the fact that five out of nine plants should be calicoed, removes 

 any doubt that the original plant suffered from this disease. As a 

 further precaution new healthy seedlings were inoculated from the 

 experimental plants and the mosaic condition obtained a second time, 

 with again a total absence in the control plants. 



The second criticism — that the "virus" is not absent, but merely 

 latent in the new apparently healthy shoots— is again a question for 

 experimental evidence only. The expressed sap of one of the shoots 

 was inoculated into seedling plants, and controls were similarly inocu- 

 lated with normal plant juice. No trace of mosaic disease was obtained 

 in either series of plants ; and a second set of seedlings inoculated from 

 the first gave a similar negative result. There can therefore be no 

 doubt that the new shoots arising from the calicoed stock were absolutely 

 free from mosaic disease. 



A further point of extraordinary interest arises out of the above 

 experiments, — are the new shoots growing from the calicoed stock 

 immune to the "virus" of the disease; i.e., have they acquired 

 immunity as the result of one attack ? 



To test this the two remaining shoots were inoculated with "virus," 

 but unfortunately the plant was killed outright by frost ten days later, 

 at this time showing no sign of disease. As the incubation period of 

 the malady is variable, and rarely less than one week, this, perhaps 

 the most interesting experiment, was a Failure. 



