Feb. 24, 1919 



Angular-Leaf Spot of Tobacco 



223 



many were no larger than 0.5 mm., mere pin pricks. The spots were 

 frequently grouped on a limited area of the leaf and were often found 

 on one side of the midrib only. 



The discrepancy in size between the field spots and those developed in 

 the greenhouse is apparently due to the difference in the size and vigor 

 of the plants. The plants did not grow vigorously in the greenhouse 

 and at maturity were not more than half as large as plants in the field. 

 The largest spots in the greenhouse were always found on the most 

 vigorous plants and on the most rapidly growing leaves. The incuba- 

 tion period was also shortest with the same plants and leaves. This 

 varies between 4 and 10 days, with the average about 7 days. When 

 plants which have developed a number of leaves are inoculated, the 

 first spots are seen on one or two leaves near the top and intermediate 

 in age. Later, within a few days, spots may appear on two or three 

 older leaves immediately below these, but no spots develop on old, full- 

 grown leaves, nor on very young ones. Young leaves become infected 

 however, when the inoculum is rubbed in with the fingers or a cotton 

 plug. The young leaves are closely set with trichomes, and these seem- 

 ingly serve as a mechanical protection against inoculation with the 

 atomizer; the spray is caught and retained on them. The spots attain 

 their maximum size within a few days after their appearance. They are 

 largest on the younger leaves and may be mere pin pricks or flecks on the 

 older leaves. 



Table II. — Results from the inoculation of tobacco plants with the angiilar-leafspot 



organism 



o Leaves are numbered from the top downward. The plants bore from lo to 14 leaves. Bold-face figures 

 indicate flecks or spots that are very small and are visible only in transmitted light. 



