The Rhizoctonia Damping-Off of Conifers 11 



the season, and partly upon counts. They represent percentages of 

 damping-off after emergence, on the basis of plants emerged, and are 

 therefore low. 



It is apparent that the disease has been most prevalent on norway spruce, 

 white pine, and red pine — the three major species here grown — and on 

 Colorado spruce. Mugho pine, white spruce, and european larch suffered 

 serious losses in 1926. The disease has been found also on scotch and 

 austrian pine, as well as a trace on oriental arborvitae. Much of the 

 variation between the same species in different years, and between different 

 species in the same year, can be accounted for by the location of the seed- 

 beds. 



From these data one can obtain some idea of the losses that may result 

 from this disease. There is in the first place a reduction in the quantity 

 of salable stock. The tendency of most nurserymen in this situation is to 

 sow large numbers of surplus seedbeds in order to maintain the production. 

 This involves the added cost of seed, the price of which varies from $1 

 to $3 per pound for norway spruce and white pine, to $15 per pound for 

 red pine; the increased cost of the seeding operations; and, most important 

 of all, the greater expense of weeding and caring for the seedbeds during 

 the two years between sowing and transplanting. There must then be 

 considered the irregularity of production, which becomes particularly 

 important in those cases in which long-term contracts are made for annually 

 supplying large quantities of nursery stock. The combination of these 

 factors serves to make such epiphytotics a problem of major importance. 



ETIOLOGY 



Isolation of organisms from diseased seedlings 



Wherever possible, seedlings were collected for purposes of culturing 

 the pathogenes soon after the seedlings exhibited symptoms of the disease. 

 When the field work was first begun, the method of handling these was as 

 follows. Short pieces of the stem were removed in such a manner as to 

 include a part of the healthy tissue and an adjacent bit of the diseased 

 tissue. These were immersed in a 1-1000 solution of mercuric chloride 

 for one minute, rinsed in distilled water, and transferred to petri dishes of 

 potato-dextrose agar. After several days, transfers were made from the 

 margin of the advancing mycelium to other dishes of the same medium. 

 The organisms were then transferred to agar slants. Since Alternaria, 

 Fusarium, Pythium, Rhizoctonia, and other common fungi grew well on 

 this agar, it was adopted as the standard medium throughout the course of 

 these studies. Common saprophytes, such as species of Penicillium and 

 Mucor, were discarded as soon as they were detected. 



The method just described was somewhat modified in later work at the 

 Keene nursery, the chief variation consisting in washing the sections of 

 diseased stems with sterile water from a wash bottle and plating them 



