170 



BACTERIAL DISEASES OF PLANTS 



local appearance of the disease on the leaf-tips and signs of gen- 

 eral infection in the plant? On well-grown plants the earliest 

 signs are flagging and shriveling of the lower leaves, and ''white 

 top," i.e., the premature development and drying-out of the 

 male inflorescence. This is conspicuous at a distance (Fig. 101). 

 Watch for these signs and try to correlate them with the pres- 

 ence of bacteria in the vascular system of stem and leaf. 



Can you find any macroscopic 

 evidence of the presence of the 

 disease on the inside (surface) of 

 the leaf-sheaths? or in the ear, 

 especially on the husks? Contrast 

 with Nos. I and II. 



Look for dwarfing effects. Is 

 the plant as tall as its fellows? 

 Are the ears well filled? Are the 

 roots generally sound? Do the 

 leaves become yellow before they 

 dry out? Does the plant rot? or 

 break over? Describe the disease. 

 Histology. — Select, section and 

 stain (in Ziehl's carbol fuchsin) a 

 number of leaf-tips, some days 

 after inoculation (4 to 7 days). 

 Can you find distinct evidences of 

 infection? Have the bacteria en- 

 tered through ordinary stomata, 

 or through the water-pores? 



Cut and stain cross- and longi- 

 tudinal sections of infected stems. 

 What vascular tissues are most 

 subject to attack? Does the organism attack the phloem? 

 Do cavities occur uniformly? In maturing plants how far can 

 you trace the vascular infection? In the vessels of the stem is 

 the general movement of the bacteria upward or downward? 

 What is your reason for this belief? In large leaves begin- 

 ning to wilt or shrivel is there simply occlusion of the stem- 

 bundles which supply these leaves, or are the bacteria also 



Fig. 112. — Young sweet-corn 

 plant inoculated on tips of the 

 leaves in the seedling stage and 

 promptly destroyed by A plan o- 

 bacter stewarti. District of Colum- 

 bia, 1902. Time, 19 davs. 



