Mayis, I920 Halo- Blight of Oats 145 



The white colonies of halo-producing organisms from natural infections 

 were all alike on beef-peptone agar, but on potato agar two only of the 

 many isolations gave colonies of a slightly different character, like that 

 designated in this paper as "stock. " 



On potato agar most of the isolations gave raised, umbonate colonies 

 of a butyrous consistency with thin margins, entire or slightly undu- 

 late. This was the usual type of colony isolated. The two varying 

 isolations were from a leaf lesion from Lafayette, Ind., and a glume lesion 

 on Wisconsin No. 14 oats in an experimental plot. (See PI. 31 , C; 32, A.) 

 The colonies were thicker and of an equal thickness out to the margin ; 

 the margin was slightly undulate, and the consistency of the colony 

 was like that of boiled starch or gelatin. They gave a more rapid 

 and abundant growth on potato agar than the common type. This 

 second type of colony is the same as an isolation made in 191 6 by Mr. 

 Reddy from a halo lesion on oats and kept as a stock culture at Madison, 

 Wis. 



The pathogenicity of each of these 28 isolations from natural infec- 

 tions was tested and proved by one or more inoculation experiments. 

 Mr. Reddy's oat stock culture and isolation No. 36 (the common form) 

 from a leaf lesion from Wooster, Ohio, were used as representatives of 

 the two types of white colonies in the inoculation and cultural work and 

 are designated respectively as "stock" and "36." 



INOCULATION EXPERIMENTS 



I. Inoculation experiments were carried on at Madison, Wis., in 

 experimental plots out of doors and in the greenhouses. The plants in 

 the field were in various stages of development, from half grown to fully 

 headed; and those in the greenhouse were from 4 to 8 inches high. The 

 uninjured plants were sprayed with water suspensions of organisms from 

 agar slants 2 days to i week old. The greenhouse plants were then 

 placed in damp chambers for 48 hours. Plants sprayed in the field were 

 covered with water-proofed translucent (glassine) bags for the same 

 length of time. Control plants were sprayed with sterile water and 

 treated in the same manner. Oat plants of Wisconsin No. i, Wis- 

 consin No. 5, and Wisconsin No. 14 were used for greenhouse inocula- 

 tions. Wisconsin No. 14 was used more often than the others because 

 it proved to be more susceptible than any other variety. Occasionally 

 halo lesions appeared at the end of the first 48 hours, when the plants 

 were removed from the damp chamber; but usually none appeared until 

 3 to 4 days after inoculation. On young plants the lesions were often 

 so numerous that centers of infection appeared in rows where the organ- 

 isms had entered the stomata. The halolike discolorations around 

 these points of sunken tissue were at first only slightly lighter green 

 than the normal tissue but quickly became more marked until about a 

 week after inoculation, when the tissue was a distinct yellowish green 



