THE CONTEOL OF CABBAGE BLACK-LEG. 23 



Appleton) and at New London (near Shiocton) with those taken at 

 Eacine shows that the rainy periods from May 15 to June 15 were 

 more numerous and as a rule of longer duration in the first two 

 localities. It may be inferred' from this that there was more oppor- 

 tunity for pycnospore dissemination and subsequent infection at 

 Appleton and Shiocton than at Racine. In view of the evidence 

 already presented as to the importance of rainfall in this connection, 

 it is not improbable that this factor was the critical one in deter- 

 mining the difference in development of the disease in the three 

 localities mentioned. 



These field trials justify'' the conclusion, as did those made in the 

 laboratory, that neither formaldehyde nor mercuric-chlorid treatment 

 of cabbage seed, even if carried to the point where seed injury occurs, 

 is a sure preventive of black-leg. Such seed treatment does, however, 

 greatly reduce the development of primary infection. This result, if 

 coupled with favorably dry weather during the seed-bed period, may 

 suffice to give practical disease control, but with more abundant rains 

 during this period the disease ma}'^ develop to a serious extent. It 

 should be recalled in this connection that seed treatment with corro- 

 sive sublimate is also a successful preventive of seed-borne black-rot 

 organisms {12). Since cabbage-seed treatment is a useful precaution 

 in these two possible directions and is so simple and inexpensive, it 

 is strongly recommended. In the light of our present knowledge, 

 soaking in a 1 : 1,000 mercuric-chlorid solution for 30 minutes fol- 

 lowed by rinsing in clean water is safest for general use. 



IMPORTANCE OF DISEASE-FREE SEEDS. 



The fact has been brought out that in spite of any practicable seed 

 treatment abundant rainfall may cause sufficient spread of the black- 

 leg fungus from the few primary infections appearing in the seed 

 bed to produce an epiphytotic. Since, therefore, in practice one can 

 not rely with confidence upon cabbage-seed treatment for the sure 

 elimination of the black-leg organism from infected seed, it is obvi- 

 ously important to give increased attention to the securing of seed 

 free from Phoma infection. Experience in Wisconsin has shown 

 that to accomplish this the crop must be kept clean from the outset; 

 in other words, disease-free "mother seed" must be used on clean 

 soil. A large percentage of American cabbage seed is now grown in 

 two sections, eastern Long Island, N. Y., and in the Puget Sound 

 region of western Washington. In the latter section " mother seed " 

 is usually obtained from another locality, most commonly from Long 

 Island, Denmark, or England. In the Long Island section local or 

 foreign grown " mother seed " is used. In either case the danger of 

 introducing black-leg into the seed crop through infected seed is very 

 great, since practically none of the growers practice seed treatment. 



