The Rhizoctonia Damping-Off of Conifers 55 



and a failure of the plant to continue normal elongation of the stem and 

 production of leaves. Only with the copper-carbonate treatments was 

 there evidence of that extreme type of injury expressed by an almost entire 

 prevention of root development and by the failure of the cotyledons to 

 grow out of the seed coats. Injury sufficient to produce killing of dormant 

 seed was not encountered in any of these experiments. When injury 

 resulted from a single treatment made at the time of seeding, the rate of 

 application was in most cases higher than that necessary for effective con- 

 trol of the disease. 



With the applications made after the seedlings had emerged, chemical 

 injury frequently resulted, in some cases even from treatments that were 

 ineffective in preventing the spread of the disease. This injury was mani- 

 fested by both checking of root development and direct killing of stems 

 and cotyledons. 



A marked variation was found among the different species of conifers 

 in susceptibility to chemical injury. Norway spruce was much more 

 susceptible to such injury than were red pine and white pine. Limited 

 tests with several other species indicated that austrian pine and scotch 

 pine reacted similarly to red pine and white pine. Colorado spruce and 

 engelmann spruce, however, like norway spruce, were more liable to such 

 injury. White spruce and european larch were even more susceptible to 

 chemical injury than were the three species of spruce noted above. 



Weed control 



Many investigators have reported a reduction in the number of weeds 

 occurring in coniferous seedbeds where chemicals were applied for the 

 control of damping-off. Similar results were noted throughout these 

 investigations, the weeds being much fewer in most of the treated plats. 

 The information obtained was largely observational in character, since in 

 most of the plats weeding operations were carried out as a part of the regular 

 nursery practice. In one experiment with Colorado and engelmann spruce, 

 however, the weeds were not removed. The condition existing in a few 

 of the plats three months after seeding is illustrated in figure 6. 



There were some indications that aluminum sulfate and sulfuric acid 

 were in general more effective in weed control than the mercury materials, 

 and that liquid treatments were more effective than the applications of 

 dry chemicals. 



Relative cud of different treatments 



A consideration of soil treatments for controlling damping-off of conifers 

 necessarily involves the question of the expense of making such treatments. 



