DAMPING-OFF IN FOREST NURSERIES. 29 



been treated with sulphuric acid followed later by lime. The other 

 experiments included in the graphs were on autoclaved sandy loams 

 in pots in the greenhouse. In these graphs are included all of the 

 results in which the same groups of strains were used repeatedly in 

 different experiments. In figure 1, the values plotted for experi- 

 ments 36, 49, and 51 are for the number of seedlings which appeared 

 above ground, the heavy inoculations and favorable conditions for 

 damping-off in these experiments being such that even weak strains 

 caused heavy losses and the survivals therefore do not give differ- 

 ential results. Comparison of the survival data in the other experi- 

 ments in figure 1 with the emergence data for the same strains in 

 that figure and in figure 10 indicates that the strains best able to 

 reduce survival are also the ones best able to reduce emergence. 



While the data presented in the graphs are not entirely consistent, 

 it is very evident from them that strains 147, 213, and in a lesser 

 degree 206 Avere regularly more virulent than most of the strains in 

 tests conducted several years apart on different species of Pinus. 

 It is also evident that certain strains of 186 and 189 which appear 

 in figure 2 are quite regularly of low or doubtful virulence. Strains 50, 

 183, 192, 211, 212, 230, and 233, whose virulence is apparently inter- 

 mediate, show a greater variability. In experiments 36, 45, 47, 49, 

 and 51, in which conditions especially favor parasitism, they may 

 cause practically as serious loss as the regularly virulent strains, the 

 best differential results being shown in experiments in which the 

 disease is less favored. The apparent variation in the relative viru- 

 lence of such strains in different experiments may, of course, mean that 

 their virulence is differently affected by different conditions. It 

 seems rather more probable that the variation in relative activity is 

 to be classed as accidental variation, necessarily great with small 

 units which are subject to numerous uncontrollable variables. It 

 seems entirely possible, however, that part of the observed differences 

 in relative activity may be due to differences, not in virulence, but in 

 the ability of the different strains to maintain themselves saprophyt- 

 ically in different soils during the period between inoculation and 

 the commencement of germination. For example, strains 230 and 

 233 came from a nursery in southwestern Kansas in which the soil- 

 acidity exponent, as determined by Dr. L. J. Gillespie, of the United 

 States Bureau of Plant Industry, is 8.4. It seems entirely possible 

 that these strains, rather strongly parasitic in some of the experi- 

 ments, including an experiment on the soil from which they were 

 taken, might prove less able than strains from some other habitats 

 to maintain themselves on some of the eastern soils used in the green- 

 house tests. The source of strains 230 and 233 was furthermore a 

 locality where high soil temperatures are to be expected. The fact 



