I02 Report of the Botanist of the 



diseased. Very frequently, it is necessary to remove large leaves 

 having an area of 60 to 80 square inches because of a single dis- 

 eased spot, perhaps an inch square, on the margin.^'' It might be 

 two or three weeks or even more before such a spot would enlarge 

 sufficiently to seriously impair the usefulness of the leaf. 



In the experiment made in 1902 (page 98) the plants on the 

 treated half-acre had a trimmed-up appearance due to the removal 

 of many diseased leaves. This was very noticeable. Toward 

 the close of the season many plants were entirely destitute of 

 stem leaves. As a consequence, the heads were small and it was 

 plain that the removal of the leaves had been decidedly injurious. 

 This opinion is fully sustained by the fact that the yield of the 

 treated half-acre was at the rate of 5^^ tons per acre less than that 

 of the check. (See Table V, page 101.) 



Infection occnrs through the roots as well as on the leaves. — Rus- 

 sell"*^ and Smith^2 demonstrated beyond doubt that infection very 

 frequently occurs through the water pores at the margins of the 

 leaves and, sometimes, also in leaf wounds made by insects. The 

 leaf-pulling treatment is based on the assumption that these are 

 the chief modes of infection. In fact no other mode of infection 

 has been seriously considered, although there is a popular opinion 

 among farmers that the disease is often communicated through 

 the seed. 



Field observations made in 1901 led the writers to suspect that 

 infection may also occur through the roots. This suspicion was 

 greatly strengthened by observations made on the experiment 

 field at Phelps in 1902. Within a month from the time the plants 

 were transplanted many of them were showing signs of infection 

 in the stem without any leaf symptoms whatever. It was ob- 



10 The question may be asked, Why not break off only as much of the leaf as may 

 be necessary to remove the diseased area? This is impractical for two reasons: 

 (i) It consumes too much time ; and (2) The writers have observed that in a large 

 percentage of the cases in which this has been done the broken leaf-margin has 

 promptly become reinfected, possibly from germs on the hand of the operator. 



n Russell, H. L. Loc. cit. pp. 27-30. 



12 Smith, Erwin F. Pseudomonas campestris (Pammel). The Cause of a Brown Rot 

 in Cruoiferous Plants. Central-bl. f. Bakt., Parasttenk. etc., II Abt., 8:409-413. 



