516 Second European Journey 



from the standpoint of human pathology, and he will realize the value 

 of such knowledge and its deficiency in plant pathology. Let him study 

 Erwin Smith's (1912) photomicrographs of crown gall. . . . 



Erwin Smith's work in crown gall was already being accepted 

 as a model for research in plant pathology. But no parasite in 

 peach yellows had ever been cultivated on artificial media and 

 the disease reproduced by reinoculation in accordance with the 

 " standards " set by his work in crown gall. Nor has this been 

 done even today. 



L. O. Kunkel in 1933 established that peach yellows is trans- 

 mitted by an insect vector, Macro ps is trhnaculata.'^^° Heat treat- 

 ments for the cure of peach yellows and other virus diseases of 

 peach have been found effective,"^ although as yet no way of 

 practically applying the knowledge in large or even small orchards 

 has been worked out. Research has been in keeping with the most 

 exact standards. But any " organized living parasite " remains 

 unproven. 



Smith attached special value to his laboratory's work in " deter- 

 mining methods of transmission of various bacterial diseases. 

 I have myself," he v/rote Paul Moore in 1923,^°' 



worked out the dissemination of two bacterial diseases by insects and Dr. 

 [Frederick V.] Rand of this laboratory has confirmed and extended my 

 work on one of the diseases (cucurbit wilt)i°3 and now recently has 

 shown conclusively that a third disease (Stewart's disease of maize) ^04 jg 

 transmitted by a beetle. This is an entirely new and extremely interesting 

 discovery of which no one had the least suspicion when he began work. 

 This makes two bacterial diseases of plants that we now know to be 

 carried in the bodies of Diabroticas. Most of the beetles of these two 

 species, Diabrotka vittata and Dicibrotka duodecempunctata, after eating 

 the diseased material, evidently soon throw off the organism but in a certain 

 small proportion it persists in their bodies for a long time and they serve 

 as the spring starters of new infections. 



^""Insect transmission of peach yellows, Contr. Boyce Thompson Inst. 5: 19-28, 



1933. 



^"^ L. O. Kunkel, Heat treatments for the cure of yellows and other virus diseases 

 of peach, Phytopathology 26(9): 809-830, Sept. 1936; also, abstract, Phytopath. 25: 



24, 1935. 



^°- Letter November 7, 1923, to Moore of the National Research Council. 



^"^ F. V. Rand, Dissemination of bacterial wilt of cucurbits, Jour. Agric. Res. 

 5(6): 237-260, Nov. 8, 1915; hh?n. 6: 4n-434, June 12, 1916; also, with Ella M. 

 A. Enlows, Bull. 828, U. S. Dep't Agric, 1920. 



^°* F. V. Rand et al.. Insects as disseminators of plant diseases. Phytopathology 

 12(5): 225-240, May 1922. 



