CANKER OF TOMATO: NON-NUTRITIONAL ENVIRONMENT 215 



is it the peptone? or the sarco-lactic acid of the beef juice? 

 Test its growth in diluted steamed tomato juice or potato 

 juice, and in the same with very small quantities of lactic 

 acid added (the first days are the important ones for observa- 

 tion). What are your conclusions? Can you devise an agar 

 medium on which it will grow readily? Determine effect of 

 drying, of insolation, of germicides. What causes loss of 

 virulence? What is the best medium for its retention? 



For the disease. Signs. — ^Period of incubation. Time 

 between appearance of the disease on the inoculated leaf and gen- 

 eral infection of the plant. 

 What are the most conspi- 

 cuous external indications of 

 his disease? Why are the 

 leaves not reflexed as in case 

 of No. IV? Note especially 

 the swollen whitish lines 

 followed by corrosion and 

 cracking open of stems, pet- 

 ioles, etc., and the irregular 

 wilting of the leaflets on at- 

 tacked leaves. Write a de- 

 scription of it. 



Histology. — Cut free- 

 hand sections of leaves, 

 stems, roots and fruits, and 

 determine: a, extent of 

 movement of the bacteria in the plant; 6, tissues attacked 

 (xylem, phloem, parenchyma) ; c, movement of the bacteria to 

 the surface of the plant; d, evidence of bacterial entrance through 

 stomata. Make stained preparations from paraffin-embedded 

 material, showing the bacteria in these various tissues (stain 

 with Ziehl's carbol fuchsin). How are the cavities formed? Is 

 cellulose destroyed? 



Variability. — How long does an attacked plant live? Are 

 young plants more susceptible than half-grown or old plants? 

 Search in the field and hothouse for resistant varieties, and if 

 you see indications of resistance, make experiments. If you 



Fig. 158. — -Detail showing infection 

 of a small bundle in a tomato leaf sprayed 

 with Aplanohacter michiganense. Time, 21 

 davs. 



