2 BULLETIlsr 1029, U. S, DEPARTMENT OF AGRICULTUEE. 



source of j)rimary inoculum the following year. As he pointed out, 

 it has been the experience of many powers that the appearance of 

 this disease in epiphytotic form in a given locality is usually asso- 

 ciated with a certain lot of cabbage seed. Observations by the writer 

 have strengthened the belief that infected seed is the chief cause of 

 heavy losses from black-leg. 



Henderson {3) found that naked pycnospores of Phoma lingam 

 were killed within two or three minutes by 1 : 200 formaldehyde or 

 1 : 1,000 mercuric-chlorid solutions. Spores within pycnidia em- 

 bedded in soft cortical tissues of the host were killed after 21 minutes 

 by the formaldehyde solution, while with the other fungicide a few 

 spores were still viable after this length of treatment. Henderson's 

 study was limited by the small number of infected seeds available. 

 It was found, however, that after treatment in 1 : 200 formaldehyde 

 solution for 21 minutes or in 1 : 1,000 mercuric-chlorid solution for 

 10 minutes, infected seeds still yielded pure cultures of Phoma 

 lingam. 



In 1917 a crop of the yellows-resistant Wisconsin Hollander seed 

 was grown at Racine, Wis. A scattering of Phoma lingam developed 

 on the seed plants, and a very small percentage of the pods became 

 infected. All of this seed (approximately 25 f)ounds) was treated 

 with a 1 : 256 solution of formaldehyde, and most of it was planted 

 in the same locality the following season. Black-leg developed in 

 epiphytotic form in most of the seed beds, resulting in heavy crop 

 losses where such plants were set in the field. A typical field from 

 this lot of seed as it appeared at the end of the season is shown in 

 Plate I, A. The uniformity with which the disease developed in beds 

 from the lot of seed in question and the fact that many of these beds 

 were on soil which had not grown cabbage for many years left no 

 doubt that the fungus carried over in the seed was the source of in- 

 fection. Moreover, it was evident that despite seed treatment the 

 parasite had spread from comparatively few centers to a large per- 

 centage of the plants during their growth in the seed bed. 



This situation raised the important question as to how much re- 

 liance is to be placed upon seed treatment as a means of control for 

 black-leg. Experiments and observations reported later in this paper 

 show that a very small percentage of infected seeds may under cer- 

 tain environmental conditions cause almost a total loss of the crop. 

 Moreover, the detection of such infected seeds is very difficult, even 

 by an expert. Since a very large portion of the cabbage seed used 

 in this country is grown in a few restricted areas in America and 

 abroad, the average grower uses seed concerning the historj^ of which 

 he knows very little, and this fact makes it the more necessary either 

 that seed treatment be entirely effective or that the production of 

 Phoma-free seed be insured. 



