j'rksidential address section c. 6') 



Organisms Pathogenic to Plants. 



The micro-organisms causing plant diseases belong for the 

 most part to the family Bacteriaceae as defined by Migula ; they 

 are rod-like forms, with or without flagella, and there is no well- 

 established case of a plant disease caused by a coccus form. 



Dr. Erwin F. Smith (51), the most eminent of phytobac- 

 tcriologists, on the grounds of priority, uses the name Bacterium 

 for rod-like organisms with polar flagella (Migula's genus 

 Psendomonas) and proposes the name Aplanobactcr for the nc^" ■ 

 motile forms. 



This terminology is employed in the greater part of the 

 I'terature on plant diseases, although not in general use by 

 bacteriologists, and is adopted in this paper. 



A large group of plant pathogens belonging to the genus 

 Bacterium (i.e., rod-shaped organisms with polar flagella, as 

 defined above) form a slimy yellow growth on potato and other 

 media, and are so similar in culture as to be almost indistinguish- 

 able. Among these may be mentioned B. citri, causing citrus 

 canker; B. Juglandis, the cause of the walnut blight; B. tnalva- 

 cearum, which is responsible for the angular leaf spot of cotton, 

 B. campestre, causing the black rot of cruciferous plants, and 

 many other parasites of greater or less importance. All these, 

 however, causes specific diseases, and cross inoculations from one 

 host to another have invariably failed. 



Although, as a rule, a parasite only infects plants of one 

 genus, or of closely allied genera, there are certain organisms 

 which have a much wider range. One of the best known of 

 tliese is the crown gall organism, B. tumefaciens, which is of 

 great economic importance on account of its inroads on fruit 

 trees belonging to the family Rosace ae, but which also infects 

 willows, poplars, and other plants belonging to widely separated 

 families (Compositae to Salicaceae) . 



Members of nine different families are now known to be 

 attacked by Bacterium solanacearum (66) which was originally 

 described as causing a wilt of potatoes and tomatoes. Well 

 defined injury and wilting occurs in plants belonging to the 

 Leguminosae, Tropaeolaccae, Euphorbiaceae, Balsaminaceae, 

 Verbenaceae and Pedaliaceae, when inoculated with this orga- 

 r.ism ; and in several other plants the organism multiplies in the 

 vessels but does not cause serious injury. 



Certain soft rot organisms, e.g.. Bacillus caruiovorus also 

 have a wide range of hosts. 



Several of the green fluorescent species are jxithogenic Vj 

 plants, including Bacterium aptahim (8) causing a disease of 

 sugar beet and nasturtium leaves, and Schuster's potato rot 

 organism, B. Xanthochlorum (47). 



One of the most destructive parasites belonging to the genus 

 Bacillus is the pear blight organism B. am-ylovorus, which is for- 

 tunately not known to occur in this country. 



Of the species of Actinomyces, a genus of doubtful imsition 



