STUDIES ON CLUBROOT OF CRUCIFEROUS PLANTS 1 

 CHARLES CHUPP 



Such an extensive literature on clubroot of cruciferous plants has accu- 

 mulated that it would seem impossible for any one point to have escaped 

 careful consideration. But when a close examination is made of all the 

 data, it soon becomes apparent that only such prominent phases as 

 symptoms, cytology of the organism, and control methods, have been 

 dealt with extensively, while certain other less conspicuous features have 

 been neglected. There still remain to be satisfactorily solved the follow- 

 ing problems: (a) the part played by swarm-spores in the dissemination 

 of Plasmodiophora Brassicae Wor., the organism that causes clubroot; 

 (b) spore germination; (c) the manner in which the pathogene enters the 

 host; (d) the distribution of the organism thruout the tissues of the root; 

 (e) formation and size of the spores; and (f) the relation of bacteria to 

 the normal development of the myxomycete. It is for the solution of 

 these problems that the following investigations have been conducted. 



DISSEMINATION 



In a general way the manner in which the spores are carried is known, 

 altho two errors are often met with in popular descriptions. For example, 

 in a number of reports (Atkinson, 1889, Carruthers, 1893, an d others) 2 

 are statements implying that swarm-spores swim about in the water 

 of the soil until they reach a cabbage root. In a way this is correct, 

 but the average layman at once pictures the swarm-spores as traveling 

 from row to row of plants or even from field to field. Nothing could 

 be more erroneous, for, as far as dissemination is concerned, the motility 

 of the swarm-spore plays such a slight part that it need not be considered. 

 Its energy is not directed in a straight line, and the very minuteness of the 

 organism would preclude any effective locomotion in the time that it 

 remains alive. 



In order to test the distance to which swarm-spores may travel in the 

 soil, a box two feet square was filled with clay mixed with muck- soil, 

 and diseased roots were buried in one end. Cabbage seeds were then sown 

 in the box, care being taken not to transfer any of the soil from the place 

 where the inoculum was inserted. When the seedlings over the area 



1 Also presented to the Faculty of the Graduate School of Cornell University, September, 1916, as a 

 major thesis in partial fulfillment of the requirements for the degree of doctor of philosophy. 



ACKNOWLEDGMENT. The author gratefully acknowledges the helpful suggestions and criticisms offered 

 him by Professor H. H. Whetzel and others in the Department of Plant Pathology at Cornell University. 



- Dates in parenthesis refer to bibliography, page 451. 



4 2I 



