The Rhizoctonia Damping-Off of Conifers 19 



Gratz (1925); the lobelia strain, isolated by S. C. Teng; the beet strains, 

 isolated by Dewey Stewart; and the pepper strain, isolated by Dr. H. E. 

 Thomas. 



Cross-inoculation tests 



Thirteen strains of Rhizoctonia, including seven isolated from conifers 

 and six from vegetables, were used in cross-inoculation studies. The 

 species on which they were tested represent a selection from the more 

 important species of plants commonly attacked in the seedling stage by 

 Rhizoctonia. With two exceptions, white pine and lobelia, they include 

 all of the suscepts from which the strains were originally isolated. 



The methods employed were similar to those already described for the 

 1927-28 inoculation tests on conifers. The temperature of the greenhouse 

 was maintained at approximately 65° F. throughout the course of the 

 experiment. In testing each species of vegetable, three five-inch pots were 

 inoculated with a given strain of the fungus, while an equal number were 

 left as checks. The tests on conifers were conducted as previously 

 described. The beet seed-balls were pasteurized at 60° C. for ten minutes 

 on two successive days, in order to avoid infection from seed-borne fungi. 

 It was not possible to carry out all of the tests simultaneously, but special 

 efforts were made to maintain fairly uniform conditions throughout the 

 course of the experiment. 



The results of the cross-inoculation tests are found in tables 5 and 6, 

 where the average difference between check and inoculated pots is given 

 for each strain, this difference being expressed in percentage of seed sown. 

 For sugar beets, however, it was found that in the check pots approximately 

 one-third more seedlings appeared than the number of seed balls planted. 

 Differences in final stand are therefore based on this expected stand, since 

 it is impossible to know the exact number of seeds contained in a given 

 number of seed balls. Because of the rather limited number of pots used 

 in each test, differences that are less than four times the probable error 

 are not considered significant. 



As shown by the data in table 5, four of the seven strains from conifers 

 (160, 77, 156, and 13) were highly virulent on conifers. In one case only 

 (strain 77 on austrian pine) the difference is not significant — due, however, 

 to contamination in several of the check pots. Strain 192 did not appear 

 to be pathogenic to conifers. Strains 187 and 167, particularly the latter, 

 were of relatively low virulence, as shown by the low average for all species 

 tested and by the fact that in two and three cases, respectively, the differ- 

 ences are not significant. 



Of the six vegetable strains tested on conifers, strains 183 and 196 were 

 highly virulent, strains 1 and 184 were mildly virulent, strain 195 was of 

 very low virulence, and strain 193 was non-pathogenic. 



There is little evidence from the data in table 5 to indicate that a given 

 strain is more virulent than others on the species from which it was origi- 



