1916 1 PARASITIC RHIZOCTONIAS IN AMERICA 359 



When carnation cuttings were infected, the strains used, with but 

 two exceptions, whether from carnation or from other hosts, were 

 able to cause more or less loss, the mortality of the cuttings ranging 

 in either instance from to 100 percent. Again, the same strains 

 varied in virulence from one year to another, in most cases decreas- 

 ing in virulence with age. When cuttings other than carnation were 

 used, the results were the same. 



When young rooted carnation plants were inoculated, the percent- 

 age of loss was much less than with cuttings. Here, however, the car- 

 nation strains seemed to be slightly more virulent than those from 

 other sources, altho there was still a great difference in the strains 

 from carnation themselves. Only one of the strains from other sources 

 was unable to attack young rooted carnation plants. 



On old carnation plants in the greenhouse which were inoculated 

 by contact, even the carnation strains did not cause a high percentage 

 of infection. However, when plants growing under these same condi- 

 tions were slightly wounded and then inoculated, the percentage of 

 loss was very high in nearly all the strains studied. When conditions 

 (temperature and moisture) were favorable to the fungus, most of the 

 strains studied were able to infect carnation plants as readily as the 

 carnation strains themselves. 



In the majority of cases all strains were able to cause damping-off 

 of various seedlings. There was a great difference in the virulence of 

 strains when inoculated on the same host from which they had been 

 isolated and when inoculated on other hosts. Only occasionally was 

 there any indication of marked specialization, and in no case was such 

 indication corroborated in succeeding experiments. 



In older plants, a marked difference in susceptibility was found in 

 the different species. As a rule, the root crops were highly susceptible 

 to attacks of Rhizoctonia. Among these, beet appeared to be the most 

 susceptible. Tomato and eggplant showed a very marked resistance 

 to Rhizoctonia, and this was true to some extent of the potato also, 

 altho under certain conditions it was quite susceptible. This varia- 

 bility of resistance held true for most of the vegetable and field crops 

 other than root crops. Under ordinary conditions, the majority of 

 floricultural plants were not subject to attacks of Rhizoctonia, altho 

 the mycelium of this fungus was known to be present in the soil or 

 even on the plant itself. 



From the fact that all the strains studied showed the ability to 

 attack the same species of plant and produce the same characteristic 

 symptoms, it seems clear that they can be included under one form, 

 R. Solani. These experiments show further that the virulence of 

 R. Solani is very variable, as is also the degree of resistance of the vari- 

 ous host plants, both depending on a number of varying factors. 



