DAMPING-OFF IN FOREST NURSERIES. 51 



strains, they are not much more representative than the smaller num- 

 ber of Corticium strains experimented with. All of the strains from 

 soil direct and 11 of the strains from pine were taken at approxi- 

 mately the same time from the same nursery in Michigan by Mr. 

 Glenn G. Hahn; despite the fact that these were the most recently 

 isolated of the strains used, nearly all of them proved weak in the 

 inoculations. Of the IT strains which proved the weakest (out of 

 35) , all but 3 were from this Michigan nursery. The 18 strains from 

 other sources (5 from Calif ornia/2 from Minnesota, 2 from Kansas, 

 1 from Wisconsin, 2 from an unknown locality, and 6 from Washing- 

 ton, D. C, representing two coniferous and three dicotyledonous host 

 genera), as shown by solid circles in figure 13, for the most part were 

 rather closely grouped within the more virulent portion of the range. 

 The coefficient of variability in the survivals allowed by the 35 

 Pythium debaryanum strains is 39+3.6 per cent, while for the 

 smaller number of original strains of Corticium vagum in experi- 

 ments 71 and 72 it is 63 ±9.7 per cent. It is evident from figure 13 

 that if there had not been a disproportionately larger number of 

 strains from the Michigan nursery the variability of the P. de- 

 baryanum strains would have been much less than 39 per cent. 

 The number of strains was, of course, altogether insufficient for either 

 fungus to represent adequately a population as immense as the total 

 number of strains of either of these omnipresent species. The above 

 data, however, contain the only available information of which the 

 writer is aware on variation in the virulence of different strains of 

 P. debaryanum. '■ 



The evidence as a whole, both from the results shown in figure 13 

 and the experience with 6 other strains which were not used in the 

 experiments on which figure 13 was based, lead the writer to believe 

 that most strains of Pythium debaryanum taken from lesions in 

 plants are ordinarily likely to prove rather virulent parasites on pine 

 seedlings. It' further appears that the variation in virulence between 

 the different strains of P. debaryanum on pine seedlings is less than 

 the variation in strains of Corticium vagum. 



PYTHIUM INOCULATIONS ON UNHEATED SOIL. 



Inoculations with Pythium debaryanum were made in western 

 Kansas on a fine sand containing little humus after treating the soil 

 with acid followed by lime. Commercial sulphuric acid was applied at 

 the rate of 14.8 c. c. per square foot of bed, followed two days later 

 by 25.5 grams of air-slaked lime raked into the soil (0.16 liter of 

 acid and 0.274 kg. of lime per square meter). The acid was diluted 

 before applying with 256 volumes of water. The seeds were sown 

 in drills, and inoculum was placed in the drills at the time of sowing. 

 Each unit involved approximately 11 linear inches of drill, and all 



