THE COISI^TEOL, OF CABBAGE BLACK-LEG. 



17 



very small in the protected portion of the plat as compared with that 

 in the portion exposed to natural rainfall. 



Table VI. — Effect of variation in rainfall on the development of hlack-leg on 

 cabbages in the seed bed at Madison, Wis., in 1920. 



Seed lot. 



No. 2-19 



No. 2-19. 



No. 7-19. 



No. 7-19. 



Treatment. 



Extent of disease at 

 the end of the ex- 

 periment. 



Number 

 of plants 

 exam- 

 ined. 



Covered during rains 94 



Exposed to natural rainfall 117 



Covered during rains j 135 



Exposed to natural rainfall 355 



Diseased 

 (percent). 



2.1 

 81.1 



4.4 

 49.8 



It is, therefore, undoubtedly true that in regions where cabbage 

 plants are grown in open seed beds, variation in the rainfall which 

 prevails during the period between the appearance of primary pyc- 

 nidia and transplanting has a very great influence upon the develop- 

 ment of black-leg. This fact should also be of value in checking the 

 disease by avoiding the splashing of water where the plants are grown 

 in covered coldframes or in greenhouses. 



IMPORTANCE OF SPREAD IN THE SEED BED AS COMPARED WITH DISSEMINATION 



IN THE FIELD. 



The question of the importance of the spread in the seed bed as com- 

 pared with that in the field is, of course, to be considered in inter- 

 preting the develojDment of the disease in midseason or later. It is 

 natural to expect that the greatest amount of dissemination of the 

 fungus from plant to plant takes place under the crowded condition 

 in the seed bed and during the process of pulling and setting plants. 

 Henderson (3) found that when plants showing no visible signs of 

 the disease were taken from an infected seed bed and set into clean 

 soil, a high percentage of them developed typical black-leg lesions on 

 stems and roots within a few weeks. An instance is cited later in this 

 paper (see fields 1 and 2 in Table IX; also PI. II) where early and 

 late planting from the same seed bed resulted in a wide difference in 

 the destructiveness of the disease, due to the dissemination of the 

 fungus after the first plants were removed. Numerous field observa- 

 tions have been made where black-leg wilt affected alternate plants in 

 a row, as a result of the fact that one of the two droppers on the 

 transplanting machine ^ had set diseased plants. Many cases have 



2 The machine referred to is the transplanter commonly used in many sections for 

 setting out cabbage, tobacco, and tomato seedlings. The setting is done by two persons, 

 who place the plants alternately in the furrow, coincident with the release of a small 

 quantity of water from a supply tank. One row is planted at a time. 



