DAMPING-OFF IN FOREST NURSERIES. 79 



within reasonable limits, are expected to increase the speed with 

 which the disease works, but these should also hasten the develop- 

 ment of the host to a point at which infections are unable to cause 

 death. It is the total amount of damage in the beds rather than the 

 damage per unit of time which is of practical importance. For a 

 number of reasons, then, the method followed in obtaining the data 

 for these graphs can not give information of maximum value. While 

 data of the sort mentioned are of undoubted interest and would be 

 of still more value if the records had been commenced when the first 

 seedlings appeared instead of a few days later, the relation of any 

 specific factor to the total extent of the disease can be better deter- 

 mined by comparing plats in series in which the factors are as far 

 as possible controlled and varied one at a time. To vary soil moisture 

 and soil temperature independently will prove somewhat difficult, 

 but it can be done with the proper facilities. Some work with en- 

 vironmental factors should be done under conditions of artificial in- 

 oculation in the greenhouse, in which the different damping-off 

 parasites can be experimented with separately, as it is obvious that 

 the factors which favor the activity of one may not be favorable for 



another. 



CHEMICAL FACTORS. 



Chemical factors are presumably also important, as the soil is 

 in most cases the culture medium for both the parasite and the host. 

 The much greater activity of Pythium debaryanum in autoclaved 

 soil than in untreated soil may be due to the larger quantity of 

 soluble organic matter commonly present in autoclaved soil. Pythium 

 debaryanum has been found more sensitive to unfavorable substrata 

 in artificial culture than Gorticium vagum and is apparently more 

 dependent on soil organic matter in the nurseries than is G. vagum. 

 For example, in the normal humus-containing surface sand in the 

 beds -at Cass Lake, Minn., both Pythium and Corticium occurred 

 frequently in the damped-off seedlings, while in beds a few feet 

 distant, from which enough of the surface soil had been removed to 

 leave no humus, nearly all the damping-off foci contained abundant 

 Corticium, and no Pythium could be found. With both fungi and, 

 in addition, with two species of Fusarium (68) heavy inoculation has 

 been more successful in experiments at the time of sowing than 

 light inoculation. This has been thought possibly due in part to the 

 larger amount of nutrient substratum added in the heavy inocula- 

 tions, allowing better saprophytic development of the fungus in the 

 soil. In each of the two experiments with Pythium reported in 

 Table XI, a 5-pot unit was treated with corn-meal infusion and 

 another with prune infusion at the time of inoculation. In both 

 experiments germination was lower, damping-off after germination 

 higher, and the survival less than half as great in the pots with 



