The Rhizoctonia Damping-Off of Conifers 59 



tests. The results with the two last-named materials suggest that equal 

 amounts of chlorophenol mercury are more effective when used in solution 

 than when applied dry. 



An adequate comparison of the mercury compounds on the basis of mer- 

 cury content has not been made in these experiments. The data at hand, 

 however, are suggestive in this connection. The results of the liquid 

 treatments have indicated that mercury-equivalent solutions of mercuric 

 chloride and chlorophenol mercury are approximately equally effective. 

 In the dry form the two chlorides of mercury gave similar results when 

 applications containing equal amounts of mercury were used. Nitro- 

 phenol mercury was more effective than chlorophenol mercury of equal 

 mercury content. The results with dry applications suggest further that 

 both compounds of inorganic mercury were more toxic to seedlings, and 

 gave greater reductions in damping-off, than did either of the organic 

 compounds, where the comparison is made on the basis of mercury content. 



The evidence from the soil-treatment experiments definitely indicates 

 that the materials here used exerted not only a disinfecting action at the 

 time of their application, but a protective action as well during the period 

 of emergence and for some time thereafter. In no instances were pre- 

 cautions taken to prevent reinfection of treated beds. The nursery prac- 

 tices made reinfection from paths easily possible, yet damping-off did not 

 appear at the margins of the beds more frequently than elsewhere. The 

 evidence of chemical injury from certain concentrations also indicates that 

 the materials were present in active form some weeks after treatments 

 were made. This is well illustrated by the fall applications on nor way 

 spruce (table 13), where injury was noted on seedlings emerging during 

 early spring, five months after the treatments were made. Whether the 

 chemicals remained in an active state throughout the intervening period, 

 or whether they were brought into active form during the germination 

 processes, is not known. 



SUMMARY 



This paper brings together the results of three years of investigations 

 on the damping-off of conifers, conducted at Ithaca, New York, and at 

 Keene, New Hampshire. 



Isolations from diseased seedlings at the Ithaca nursery indicated that 

 Rhizoctonia and Pythium were the pathogenes causing the disease. At 

 the Keene nursery, Rhizoctonia was practically the only organism involved. 

 Pure cultures were obtained in more than 900 cases from approximately 

 1 300 isolations. Isolations from six other nurseries yielded a high per- 

 centage of Rhizoctonia and Fusarium, and in a few cases Alternaria. 



Rhizoctonia and Pythium were highly virulent in greenhouse inoculation 

 tests. The few tests with Fusarium and Alternaria gave negative results. 



Cross-inoculation experiments with thirteen strains of Rhizoctonia 

 were made on six species of vegetables and five species of conifers. The 



