376 BULLETIN No. 189 [June, 



or more before the plant showed any signs of being diseased. In fact, 

 a certain amount of mycelium is always present in the carnation soil 

 in the greenhouse, but it is only when the temperature is high that 

 the fungus is able to attack the plants. This explains why stem rot 

 of carnations is more severe during the summer months than in the 

 winter. (See Experiment 6, page 349.) In the field similar conditions 

 are necessary to result in infection of a plant. 



Investigations to determine how much vigor the mycelium must 

 attain before the fungus can attack a plant are now in progress, as 

 is also a histological and enzymatic study. 



SUMMARY 



1. At the present time there are recognized in America two 

 species of truly parasitic Rhizoctonias : The common form, Rliizoctonia 

 Solani Kiihn (C6rticium vagum B. & C.), widely distributed and oc- 

 curring on a great number of hosts; and R. Crocorum (Pers.) DC., 

 with a limited distribution on alfalfa and potato tubers. A third 

 Rhizoctonia, Corticium ocliraleucum (Noack) Burt, is found on the 

 leaves of pomaceous fruit trees, while a fourth species isolated from 

 damped-off onion seedlings is of questionable parasitism. 



2. The plants thus far listed as more or less subject to attacks of 

 RJiizoctonia Solani Kiihn in the United States number about 165 

 species. All the more important families of dicotyledons are included 

 in this list, as well as a number of monocotyledons, several gymno- 

 sperms, and Equisetum. Most of the floricultural plants, vegetable 

 and field crops, herbaceous plants, and many weeds are susceptible to 

 attacks of this fungus. 



3. The symptoms produced by RJiizoctonia Solani Kiihn in nat- 

 ural infection are largely similar when appearing on the same type 

 of host. The damping-off of seedlings and cuttings of various plants 

 is identical, as is the rotting of a number of root crops. In most her- 

 baceous plants a stem rot is produced, the symptoms of which are also 

 identical on the various hosts. On very resistant plants lesions only 

 are formed ; these are apparently the same on the different hosts. 



4. From these inoculation experiments with a large number of 

 different types of plants, we must conclude that all the strains studied, 

 which were obtained from a wide range of hosts of diverse geographi- 

 cal origin, can attack the same species of plant and produce the same 

 characteristic symptoms. No marked specialization was noted in any of 

 the strains. Thus all the strains studied can be included under one 

 form, Rliizoctonia Solani Kiihn. The inoculation experiments show 

 further that the virulence of R. Solani is very variable, as is also the 

 degree of resistance of the various host plants, both depending on a 

 number of varying factors. 



