60 PROTECTION OF PLANTS — 1922-23 



A STUDY IN DISEASE SUSCEPTIBILITY. 

 Dr. B. T. Dickson, Macdonald College, Que. 



The photograph in Plate 1 was taken in the experimental greenhouses of 

 Macdonald College on February 10th, 1923. The plants in the bed were 

 being used in a study of tobacco mosaic and all but two were inoculated success- 

 fully in October, 1922, with mosaic. The usual symptoms appeared as the 

 plants developed until November 27th, 1922. On that day numerous small 

 spots appipared in several lower leaves of mosaic-diseased plants. The spots 

 were at first watersoaked, and about pin-head in size. They rapidly turned 

 dark brown and eventually became almost white. In many instances zonation 

 showed and on some leaves lesions coalesced between the larger v^eins. The 

 disease spread as a result of the spraying and splashing of water from diseased 

 leaves on other leaves of mosaic-diseased plants. In spite of continued 

 attempts under ideal moisture conditions, no lesion appeared in any leaf of a 

 healthy plant. It was at first thought to be wildfire from its symptoms but 

 cultures gave only a micrococcus. Further study is being made of the disease, 

 by T. G. Major, M.Sc,, the Tobacco Pathologist of the Tobacco division 

 Central Experimental Farm, Ottawa, in co-operation with the writer. 



Several points of interest are to be noted in the photograph. The dwarfing 

 of the mosaic-diseased plants is well shown. The two healthy plants are 23/^ 

 feet taller than the top of the photograph i. e they are twice as taU as, and 

 with corfespondingly larger leaves than, the diseased plants. Their unspotted 

 condition is marked in the two lower leaves, showing among the diseased 

 plants. The fact that healthy seedling are not affected is indicated in the two 

 pot plants in the foreground. Mosaic symptoms can be seen in many of the 

 leaves of affected plants. 



The photograph is by Mr. H. R. Angell, a plant pathology student of 

 Macdonald College. 



Contribution No 16, Dept. of Botany, Macdonald College, Que. 



