The Rhizoctonia Damping-Off of Conifers 



23 



/o 



20 



30 40 SO 



ON VEGETABLES 



Figure 1. cross-inoculation tests with thirteen strains of rhizoctonia 



The strains from conifers are designated by a plain circle, those from vegetables by a cross within a 

 circle. The vertical scale represents the average difference between inoculated and check pots for each 

 strain tested on conifers, this difference expressed in percentage of seed sown. The horizontal scale simi- 

 larly represents the results of the tests on vegetable seedlings. (Data from last column of tables 5 and 6) 



beef-peptone agars. Of these media, Czapek's, malt-extract, potato- 

 dextrose, and cornmeal agars were selected as the standard media for later 

 studies. 4 The twenty-three strains of Rhizoctonia already referred to 



4 Formulae of the four media: 

 Czapek's synthetic-solution agar 

 (Waksman, 1916, p. 113) 



Distilled water 1000 cc. 



M g S04 0.50 gm. 



IV2HP04 1.00 gm. 



KC1 0.50 gm. 



FeS04 0.01 gm. 



NaNOi 2.00 gm. 



Cane sugar 30 . 00 gm. 



Agar 15.00 gm. 



Malt-extract agar (Baxter, 1925, p. 528) 



Distilled water 1000 cc. 



Malt extract 25 gm. 



Agar 25 gm. 



Potato-dextrose agar 



Distilled water 1000 cc. 



Pared potatoes 200 gm. 



Dextrose 10 gm. 



Agar 15 gm. 



Potatoes cooked until soft, and broth 

 filtered through cheesecloth 



Cornmeal agar 



Distilled water 1000 cc. 



Cornmeal 30 gm. 



Agar 15 gm. 



Maintained in water bath at 58° C. for 

 one hour before adding agar 

 All media were boiled until the agar was dissolved, followed by a thirty-minutes heating in an Arnold 

 sterilizer and slow cooling to permit settling and removal of sediment. Prior to pouring the plates, 

 melting was accomplished by another thirty-minutes exposure to live steam. Determinations made by 

 the electrometric method after the agars had finally solidified, indicated the following pH values for the 

 different media: Czapek's, -6.82; potato-dextrose, -5.75; malt-extract, -4.87; cornmeal, 5.86. 



