DAMPING-OFF IN FOREST NURSERIES. 69 



most easily controlled by soil disinfection (see the bottom line in 

 the last four columns of Table IX). Its poor adaptation for aerial 

 dissemination would lead one to expect to find it seldom in beds 

 treated with efficient disinfectants. The entire absence of Corticium 

 in heated soil therefore seems somewhat significant. The rather 

 high Corticium yield in the formaldehyde plats is of some interest 

 in view of the reported inefficiency of formaldehyde in destroying 

 Corticium vagum on potato tubers (48, 50). As will be noted from 

 the data given, more than one suspected parasite was often found in 

 what appeared to be a single focus. This was probably in some 

 cases due to independent foci being nearly concentric; it also in 

 some cases undoubtedly means that one of the organisms found was 

 only secondary. In the beet-seedling cultures by Busse and his 

 associates, individual seedlings yielded two or more potential para- 

 sites in 100 of their nearly 1,300 examinations. It not infrequently 

 happened in the work on pine seedlings that no fungus recognized 

 as a likely parasite could be isolated. This was especially true 

 in plate cultures when Rhizopus or Trichoderma happened to be 

 abundant, as both are very fast growing and often suppress para- 

 sites. This is an additional reason for the development of some 

 method as a dilution plate of lesion fragments for diagnosing damp- 

 ing-off. 



Even an accurate and complete census of the organisms present in 

 the different foci could not be directly interpreted in terms of rela- 

 tive importance. None of the parasites so far used in inoculation 

 have been vigorously parasitic under all conditions. Of both Corti- 

 cium vagum and Pythium debaryanum some strains, microscopically 

 indistinguishable from the others, are very weak as parasites. Only 

 part of the Fusarium species are parasitic on pine, and data showing 

 which are and which are not parasitic are known for only a very 

 few. There is therefore no fungus which can be said positively to 

 be the cause of any particular damping-off " patch " simply because 

 it was found in some of the dead seedlings in the patch. In an occa- 

 sional exceptional case, such as the large Corticium patch in figures 

 7 and 8, there is such a vigorous growth of the fungus that its pre- 

 dominance is undoubted, but such cases are rather rare. A census 

 throws light on the importance of the different fungi, but can be 

 interpreted only in the light of inoculation results. 



For Pythium and Corticium the inoculation data do not permit 

 any simple comparison between the two, for the reason that neither 

 is uniform. Each has strains of high virulence and strains having 

 practically no effect on pines. In the inoculations in autoclaved soil at 

 sowing time the strongest strains of Corticium vagum have on the 

 whole caused more damage than any of the Pythium strains, but, on 

 the other hand, there has seemed to be a higher proportion of very 



