THE CONTROL OF CABBAGE BLACK-LEG. 13 
doubt whether dry heat can be used successfully on a commercial 
basis because of the wide range of susceptibility 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, however, 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 Avas 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 lingam 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 
73603°— 22— Bull. 1029 3 
