DAMPING-OFF IN FOREST NURSERIES. 83 



Pots with saprophytes : Emergence, 47.4±0.86 per cent ; damping-off, 9.1 per 



cent ; survival, 41.0±1.23 per cent. 

 Pots without saprophytes : Emergence, 35.7 per cent ; damping-off, 14.3 per 



cent ; survival, 29.2 per cent. 



It has been noted in the attempts to diagnose damping-off by 

 planted-plate cultures that when Rhizopus appears Pythium debary- 

 anum is not frequently obtained. It is therefore of some interest to 

 note that in this case^in the two 5-pot units which received Rhizopus 

 in addition, the parasite killed only 1.2 per cent and 3.3 per cent, 

 respectively, of the seedlings which appeared above the soil. 



At the same time pots not inoculated with parasites were sown, 16 

 other 5-pot units were inoculated with the same saprophytes as those 

 used in the Pythium inoculated pots, while 25 pots were left entirely 

 without inoculation. A certain amount of damping-off occurred in 

 these pots also as a result of accidental infection. The results were 

 as follows : 



Pots with saprophytes : Emergence, 47.8±0.8 per cent ; damping-off, 3.9 per 



cent ; survival, 43.7±0.95 per cent. 

 Pots without saprophytes : Emergence, 43.0 per cent ; damping-off, 5.2 per 



cent ; survival, 38.4 per cent. 



It is of some interest to note that in these pots also the 5-pot units 

 inoculated with Rhizopus suffered less from damping-off than the 

 average of the saprophyte-inoculated pots. 



The probable-error values given above are based on the variability 

 of the emergence and survival figures of the different 5-pot units. 

 No individual figures are available to serve as a basis for the deter- 

 mination of the variability of the pots without saprophytes. The 

 16 units which support the error determinations are, of course, not a 

 sufficient number to give an entirely reliable index of variability, 

 though these 16 units are respectively derived from the combination 

 of a total of 78 and 80 ultimate units. The distribution of the data 

 appears to be such as to justify the use of probability methods. Of 

 the 64 items which went into the germination and survival calcula- 

 tions, 34 showed a deviation equal to E s (probable error of a single 

 unit), 9 to a deviation equal to 2E S , and only one a deviation equal 

 to 3E S . 



All of the above figures are based on the results at the end of 10 

 days after average emergence in the pots. The pots were kept on the 

 benches till practically all damping-off had ceased, 36 days after 

 emergence. As additional accidental infection with saprophytes 

 certainly, and probably with parasites, occurred during this period, 

 the results at the end of the tenth day are considered to give a better 

 indication of the effect of the original inoculations. It is of some 

 interest, however, to note that during the period from the tenth to 

 the thirty-sixth day the difference between the pots to -which sapro- 



