THE CONTEOL OF CABBAGE BLACK-LEG. 13 



doubt whether dry heat can be used successfully on a commercial; 

 basis because of the wide range of susceptibilit}^ to injury in different 

 lots of seed and because of the difficulty of application. The experi- 

 ments conducted with hot water have not shown it to be superior to 

 the chemical fungicides ; so in view of the awkwardness of its appli- 

 cation it is not at present considered suitable for general use. Fur- 

 ther comparative trials, hoAvever, should be made. With the chemical 

 fungicides, treatment stronger than a 1 : 256 or 1 : 240 solution of for- 

 maldehyde and a 1 : 1,000 solution of mercuric chlorid for 30 minutes, 

 followed by rinsing, is unsafe for general use. Of these two treat- 

 ments the mercuric chlorid seems to be slightly superior in eradi- 

 cating the fungus. It is true, however, that many lots of seed will 

 stand much more severe treatment, especially with mercuric chlorid, 

 but in the event of such treatment preliminary tests should always 

 be made. 



FIELD TRIALS WITH TREATED SEEDS. 



The results from seed treatment showed that complete elimination 

 of seed infection is impracticable. It was evident, however, that the 

 more effective treatments, such as 1 : 1,000 and 1 : 500 mercuric-chlorid 

 solution and 1 : 256 formaldehyde solution for 30 minutes destroyed 

 or inhibited so much of the Phoma as greatly to reduce the percent- 

 age of seedling infection. The question arose, therefore, as to 

 whether any of the foregoing treatments would control the disease 

 for practical purposes. In general, infected seeds were less resistant 

 than the fungus within them. It was reasonable to expect, then, that 

 treatment of commercial seed bearing an ordinary amount of infec- 

 tion would reduce the number of germinable diseased seeds to a very 

 low percentage. 



Field observations have shown that the rapidity with which the 

 disease develops and spreads from primary centers in the seed bed 

 varies greatly from year to year. Comparison of climatic conditions 

 with the occurrence of the disease in different localities and in the 

 same locality in successive years has indicated that the amount of 

 rainfall and the atmospheric humidity are the chief factors in caus- 

 ing these variations. McAlpine {6) has pointed out that the disease 

 in Australia thrives best in wet weather followed by heat or when the 

 plants are forced by excessive watering. This view is further 

 strengthened by the fact that a good supply of moisture is necessary 

 to bring about the discharge of pycnospores of Phoma. liivgam and 

 that they are at the same time best adapted to dissemination by spat- 

 tering rain or surface drainage water. 



DEVELOPMENT OF THE DISEASE IN THE SEED BED. 



As described by Henderson {3) , primary infection of germinating 

 seedlings takes place at some point on the cotyledon or at the base 

 7.3603°— 22— Bull. 1029 3 



