DAMPING-OFF IN FOREST NURSERIES. 87 



(3) The most serious losses in conifers are ordinarily from the 

 root-rot type of damping-off, occurring soon after the seedlings 

 appear above ground and while the hypocotyls are still soft. Losses 

 due to the killing of dormant or sprouting seed by parasites before 

 the seedlings appear above the soil are also frequently serious, some- 

 times necessarily more so than the later types, as in extreme cases 

 more than half of the seed or young seedlings are destroyed in this 

 way. Damping-off due to infections of parts above the soil surface 

 is serious only under extremely moist atmospheric conditions. The 

 late type of damping-off, in which the roots are rotted after the 

 stem becomes too rigid to be easily decayed, is ordinarily less im- 

 portant than the early types. Seedlings more than 2 months old are 

 ordinarily able to recover from infections by the damping-off fungi. 

 Even after the first month, seedlings with part of their root system 

 killed often recover. 



(4) It is possible that damping-off has a certain value as a selec- 

 tive agent by eliminating weak individuals in the seed-bed stage and 

 allowing only the best trees to go into forest plantations. This 

 value, however, is believed to be slight. Disinfectant treatments of 

 seed beds, even when controlling early parasitic losses very well, 

 allow a considerable percentage of disease during the last part of 

 the damping-off period, often as much as occurs at the same stage of 

 development in untreated beds. As it is only this late damping-off 

 in which differences in individual resistance of the seedlings seem to 

 be of importance in determining whether or not they succumb, it is 

 believed that whatever selective value the disease may have will 

 appear in a larger proportion of the damping-off in the treated than 

 in the untreated beds. 



(5) Of the different conifers, reports are available as to the sus- 

 ceptibility of 63 speeies. Species which are especially susceptible 

 at some nurseries may prove more resistant than the average at 

 others. Pinus resinosa, which is especially subject to loss at some 

 nurseries, is believed to be so because its growth at these nurseries is 

 slow and its period of susceptibility is therefore especially long. 

 In its early stages it does not seem especially susceptible. Repre- 

 sentatives of all the commonly grown genera of the Abietoideee have 

 been reported by one observer or another as decidedly susceptible. 

 The reports on junipers and other members of the Cupressoidese, on 

 the other hand, have indicated a considerable amount of group re- 

 sistance to damping-off. 



(6) The best control method appears to be the disinfectant treat- 

 ment of the seed-bed soil before or immediately after seed is sown. 

 Sulphuric acid has been found very useful for conifers, as they are 

 apparently especially tolerant of acid treatment. No method has yet 

 been worked out to a point at which all of its details are entirely 



