The Rhizoctonia Damping-Off of Conifers 25 



light in color and are loosely fused with the abundant aerial mycelium 

 (Plate II, 7-9). The four strains 1, 183, 184, and 193, on the contrary, 

 form abundant distinct sclerotia (Plate III, 1-9). Strains 192 and 196 are 

 intermediate in character between these four and the other four noted 

 above (155, 187, 167, and 175) (Plate III, 10-12). 



Conclusions from the cross-inoculation and cultural studies 



The most outstanding fact brought out by these experiments is that many 

 of the strains of Rhizoctonia isolated from conifers differ strikingly in 

 their virulence and in their morphological characters as expressed by 

 growth on certain artificial media. Apparently the conifer strains differ 

 as much from one another as they do from the vegetable strains. The 

 morphological differences, however, are more striking than the differences 

 in virulence noted, and are apparently not correlated with the latter. 



In view of the fact that the cultural studies were of a somewhat limited 

 nature, and since only gross morphological features have been considered 

 in the grouping of the strains, it is not proposed to designate the groups with 

 any particular name. With the possible exception of strain 156, which 

 will be studied in more detail later, the strains employed are all considered 

 to fall within the limits of Rhizoctonia solani Kiihn as the species is now 

 constituted. 



CONTROL 



The method of control most extensively tested was that of soil treat- 

 ment with chemicals. A limited amount of information was obtained also 

 on the effect of adding muck to the soil, and on the relation of fall seeding 

 to the occurrence of the disease. 



Application of muck 



Because of the low content of organic matter in the soil at the Keene 

 nursery, it has been the practice there to apply large quantities of muck 

 every four or five years to the areas used for seedbeds. General observa- 

 tions by those in charge had suggested the possibility that such applications 

 had been effective in the prevention of damping-off in certain parts of the 

 nursery. Opportunity was afforded to make direct observations on seed- 

 beds of norway spruce sown during the fall of 1926 and the spring of 1927 

 in a section of the nursery in which muck had never been used. The muck 

 was added in November to several 100-foot beds as a layer several inches 

 thick, after which it was harrowed into the soil. A part of this area was 

 seeded in the fall, and the remainder during the following spring. On 

 the fall-sown beds both with and without muck, approximately 30 per 

 cent of the seedlings that emerged were destroyed by damping-off. More 

 detailed observations were made on the spring-sown beds, where the 



