16 



BULLETIN 1029, U. S. DEPAETMES"T OE AGRICULTURE. 



rain was eliminated the spread of the disease was completely checked, 

 while it was enhanced where artificial sprinkling- ^as done. Al- 

 though plat 3 was exxDOsed to numerous heavy dews, these were ap- 

 parently insufficient for the dissemination of spores for any appre- 

 ciable distance. 



Tabli 



Y. — Effect of variation in rainfall on the development of black-leg on cab- 

 ijages in the seed bed at Madison, Wis., in 1919. 



Treatment. 



Extent of disease at 

 the end of the ex- 

 periment. 



Number r)i<.p„^p^ 



of plants ^'flll^ 



exam- )}'f. 



ined. '^^^^'■ 



No. 1 

 No. 2 



No. 3 

 No. 4 



Exposed to natural rainfaU 425 



jVrtificiaUy sprinkled 174 



Covered during rains .' 359 



Covered every evening and during rains 369 



28.7 

 37.9 



2.9 



This experiment was repeated in 1920. Two lots of seed, one 

 heavily infected (Xo. 2-19) and one mildly infected (Xo. 7-19), were 

 l^lanted in clean soil on May 12. Part of the plat was protected from 

 rains during the growth of the plants, water being supplied arti- 

 ficially, with care to avoid splashing. After the first appearance of 

 the disease on May 22, primary infections continued to develop on 

 the cotyledons and at the base of the hypocotyl with about eqyial 

 rapidity in the protected and unprotected portions of the plat. Pyc- 

 nidia in these lesions were first noted on May 25. By June 14 prac- 

 tically all the primarily infected plants were dead, and each bore 

 many mature pycnidia. The first secondary infections were noted on 

 June IT, and from that time on the disease developed rapidly on leaves 

 and stems of the plants in the unprotected portion of the plat. In an 

 adjoining plat healthy plants were sprayed with a suspension of 

 spores taken directly from lesions on infected plants. The disease 

 aj)peared on these plants about 15 days later. It is thus to be expected 

 that even under favorable conditions for dissemination and infection 

 the appearance of secondary lesions would not ordinarily take place 

 until two or three weeks after pycnidia appeared on plants infected 

 from the seed. The plants in the plat were large enough for trans- 

 jolanting by the end of June, when they were pulled and examined for 

 the presence of the disease. The results are given in Table YI. In 

 spite of the fact that primary infections were very numerous in the 

 case of the No. 2-19 seed very little spread to the aerial portions of 

 surrounding plants occurred where the splashing of rain was avoided. 

 Likewise, with the No. 7-19 seed the extent of the dissemination was 



