122 PROTECTION OF PLANTS — 1923-24 



Series 5. — The two sets of experiments outlined below demonstrate the 

 difference in the length of incubation period depending apparently on the 

 temperature. Although one set of experiments were conducted in the field and 

 the other in the greenhouse, I think that they are nevertheless comparative 

 inasmuch as the inoculations were made at blossoming time — when the plants 

 were in as near the same conditions physiologically as the differences in envi- 

 ronment would permit. There is, of course, the possibility of a difference in 

 virulence or in amount of inoculum. 



Experiment I. — By Dr. Dickson: Four plants in the field were inoculated 

 by rubbing with diseased tissue from greenhouse plants on July 20th, 1 23. 

 On August 1st, lesions appeared in young leaves, petioles and stems. That is, 

 there was an incubation period of 11 days. 



Experiment 2. — In an experiment conducted by the writer, four plants in 

 the greenhouse were inoculated by rubbing with diseased tissue on February 

 22nd, 1924. The greenhouse was kept at a temperature of 48-50"F. 



Results- One plant showed first symptoms of streaking on stems and petio- 

 les and specking on the young leaves 18 days after inoculation. Plant 2, showed 

 first symptoms 22 days after inoculation; plant 3, 25 days after, and plant 4, 

 27 days after. Checks remained healthy. 



From these experiments it might be inferred that a temperature as low- 

 as 50°F lengthens the incubation period as compared with summer temperature. 



Series r>. — When healthy tobacco plants are inoculated by rubbing with 

 striped tomato plants, only mosaic symptoms are produced; that is, no 

 specks or splashes develop. In this connection the following experiment* 

 are of interest. 



1. A healthy tomato plant was inoculated by rubbing with crushed leaves 

 of a tobacco plant having mosaic. This plant contracted the stripe disease, 

 the lesions being very small, however. 



2. Stripe disease was also produced on tomatoes in the greenhouse 

 rubbing with mosaic tissue from Resistant White Burley tobacco. It must, 

 however, be noted that the tomato plants in both experiments were uncovered, 

 so that the possibility of transmission of the disease from one of the other dis- 

 eased plants in the same field or greenhouse, was possible. Therefore, a repetition 

 of this experiment under controlled conditions is necessary. This is being carried 

 out at the present time. 



It is interesting to note that Clinton (3), in 1907, performed the following 

 experiment amongst others : — 



"Exp. 28. Nov. 11, tomato plant was crushed after crushing fresh leaves 

 of calicoed tobacco in hands. Nov. 25, young leaves and stem showed elongated, 

 irregular, discoloured streaks or burns, like a bacterial disease, but no true 

 calico-like appearance. Check plants showed no similar trouble. Jan. 8, plant 

 dead of wilt, but before dying, some leaves showed signs of true calico. The 

 bacterial-like burn of the leaves, however, was the conspicuous trouble." 



