1916] PARASITIC RHIZOCTONIAS IN AMERICA 375 



The results of these experiments admit of no question as to the 

 presence of the fungus Khizoctonia Solani in the soil in the vicinity 

 of Uj-bana. 



PARASITISM OF RHIZOCTONIA SOLANI KUHN 



That R. Solani is an active parasite under certain conditions would 

 never be questioned by anyone who had seen a severe attack of car- 

 nation stem rot in the field or greenhouse. In the cutting bench 

 this fungus causes damping-off of cuttings in an incredibly short time, 

 while seedlings damp oft' almost as fast. At times Rhizoctonia causes 

 considerable loss in potato fields. In fact, it may become epidemic 

 and cause serious injury to most of the field, vegetable, and floricul- 

 tural 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 infection and development. Under ordinary conditions most of 

 the strains appear to be weak parasites. 



The apparently universal presence of Rhizoctonia in the soil, 

 where it can live indefinitely on dead organic matter under ordinary 

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

 marked specialization and can attack a large variety of weeds assists 

 in harboring the fungus and in keeping up its virulence. The sclero- 

 tia and mycelium can live under adverse conditions for several years. 

 Transfers from soil cultures started in December, 1911, kept in the 

 laboratory, and allowed to dry out, yielded pure cultures as late as 

 July, 1914. Soil cultures left in the field during the entire winter 

 yielded the fungus in the spring. 



In all but one of the experiments inoculation was brought about 

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

 being 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 soil 

 moisture content either too low or too high for the best development of 

 the plant. When carnation plants growing in soil inoculated with 

 Rhizoctonia were given a heavy watering and the soil was then allowed 

 to dry out, they were killed more rapidly than plants growing under 

 the same conditions but in continually over-watered soil. Plants 

 watered normally died off slowly and the percentage of loss was very 

 much less. 



Repeated observations in greenhouse and field have shown 

 certain amount of the mycelium must be present before the fungus 

 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 



