408 A Tomato Canker 



days after ripening. The summer of 1919-20, when most of the observa- 

 tions were made, was exceptionally dry: another season with heavier 

 or more continuous rainfall the losses would probably be far more 

 serious, and might ruin the whole crop. 



The "canker" lesions differ from those caused by other tomato 

 diseases attributed to bacteria. The wilt disease caused by Bacterium 

 solanacearum Erw. Sm.(3) is a distinctly vascular trouble and causes 

 a characteristic wilting of the plants. Aj)lanobacter michiganense is also 

 found in the vessels (2). "Streak," described by Paine and Bewley as 

 caused by Bacillus Lathyri, is characterised as the name suggests "by 

 the formation of dark brown or black sunken patches on the stem," 

 varying from small spots to long furrows or blazes (i)." On the fruit it 

 forms light or dark brown sunken patches with round or irregular 

 outline. 



The effects of the canker organism on leaf, stem and fruit are widely 

 different from any of these as will be evident from the detailed de- 

 scriptions to be given later. 



Symptoms of Disease. 



On the leaves the first indication of infection is the appearance of 

 numerous dark green, semi-translucent, water-soaked points on the 

 under surface. In cases of artificial infection in autumn weather this 

 occurred seven to eight days after inoculation, under summer conditions 

 the progress of the disease may be more rapid. The spots increase in 

 size and become round or irregular and about 2 mm. in diameter; they 

 are slightly sunken and are often present in such numbers in the neigh- 

 bourhood of the lateral veins and leaf margins, that they coalesce, and 

 produce irregular, discoloured streaks. The colour soon changes from 

 dark green to deep quaker drab 1 (Ridgway 51) or vinaceous slate (50). 

 The discolouration penetrates to the upper surface, and. the spots 

 eventually consist of a smoke grey centre, which is then membranous 

 and semi-translucent surrounded by a deep brownish drab margin. 



Where the spots are numerous the intervening leaf tissue becomes 

 dry, brown and brittle, the original lesions being still plainly visible in 

 the dead areas. In this way the affected portions of the leaf, especially 

 the edges and the tips, become dead and dry and break away, giving the 

 leaves a very ragged appearance, and many of the smaller leaflets are 



1 The numbers quoted after name of colours refer to plates in Ridgway's Colour 

 Standards and No7nenclature. 



