SOIL SUEVEY 



By means of a local soil survey, it was found that Rhizoctonia So- 

 lani (Corticium vagum) is abundant in sod and cultivated land with 

 any crop, where it may live either on dead organic matter in the soil or 

 on weeds and other plants. 



PARASITISM OF RHIZOCTONIA 



That Khizoctonia Solani (C. vagum) is an active parasite under 

 certain conditions would never be (questioned by anyone who had seen 

 a severe attack of carnation stem rot in the field or greenhouse. In the 

 cutting bench this fungus causes damping-off of cuttings in an in- 

 credibly short time, and of seedlings almost as quickly. At times 

 Rhizoctonia causes considerable loss in potato fields. In fact, it may 

 become epidemic and cause serious injury to most of the field, vege- 

 table, and floricultural crops. 



The epidemics are apparently due to a combination of factors, such 

 as the presence of a virulent strain of the fungus, a susceptible variety 

 of plant, and optimum conditions of temperature and moisture for in- 

 fection and development. Under ordinary conditions most of the 

 strains appear to be weak parasites. 



The apparently universal presence of Rhizoctonia in the soil, 

 where, under ordinary conditions, it can live indefinitely on dead or- 

 ganic matter, makes it a dangerous fungus. The fact that it shows no 

 marked specialization and can attack a large variety of weeds assists in 

 the harboring of the fungus and in keeping up its virulence. The 

 sclerotia and mycelium of Rhizectonia can live under adverse condi- 

 tions for several years. 



In all but one of the experiments inoculation was brought about 

 without wounding the plants in any way, in many cases the fungus be- 

 ing simply mixed with the soil in which the plants were growing. The 

 results furnish convincing proof of the parasitism of the fungus. The 

 conditions under which all strains manifested their greatest para- 

 sitism were primarily a high temperature (above 88 F.) and a mois- 

 ture content of the soil either too low or too high for the best develop- 

 ment of the plant. 



Repeated observations in the greenhouse and field have shown that 

 a certain amount of the mycelium must be present before the fungus 

 is able to attack and kill the plant. A small amount of mycelium has 

 always been observed around a carnation plant in the bench a week 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 car- 

 nations is more severe during the summer months than in the winter. 

 In the field similar conditions are necessary to result in the infection 

 of a plant. 



