528 Journal of Agricultural Research voi. xv, no. 10 



field tests in the Middle West heated soil plots not specially protected 

 against reinfection have in a number of cases suffered even more heavily 

 from damping-off than plots not treated, despite the demonstrated killing 

 of the parasites originally present in the soil. 



Heavy inoculation with material consisting largely of substances like 

 corn-meal mush, rice, and various nutrient agars, as has been customary, 

 undoubtedly results also in soil conditions decidedly different from those 

 which occur in nature. It is believed that the presence of abundant rich 

 food material for the parasites decidedly increases the ability of some of 

 them to attack the seedlings. Heavier seeding than that used in practical 

 seed-bed work has been a part of the experimental procedure in most of 

 the writers' work if not in that of others, and also appears to create con- 

 ditions abnormally favorable to the parasites. It is felt that all of the 

 work that has been done, including that presented in the following dis- 

 cussion, must be supplemented by further experiments on unsterilized 

 soil, with unheated water, with a density of seed sowing corresponding to 

 that used in regular seed-bed practice, and with inoculum consisting of 

 spore suspensions where practicable, or with small quantities of mycelium 

 in other cases, but in no case with any addition of a nutrient medium. 

 Inoculation in outdoor seed beds, as well as in pots in greenhouses, will be 

 desirable. Conclusive results in such experiments w411 not always be 

 easy to get. Because of the inevitable damping-off in the controls of 

 experiments so conducted, positive results will have to consist of a heavier 

 damping-off loss than occurs in the controls, a thing which can be demon- 

 strated only by averaging the results obtained from a large number of 

 pots. Until such work is done the relative importance of the various 

 parasites active under field conditions can only be guessed at, no matter 

 how thoroughly parasitism has been demonstrated in experiments of the 

 conventional type. 



CORRELATION OF INOCULATION RESULTS 



The method of determining the results of inoculations has been to 

 count and remove all damped-off seedlings, making the examinations 

 every two or three days in the case of species like jack pine, which have 

 small seed. Because of the extreme brittleness of the stems of young 

 coniferous seedlings a certain number were accidentally broken in 

 almost every experiment. These were removed as found, and recorded 

 separately. The experiments were ordinarily closed from 10 to 20 days 

 after fairly complete germination had been attained, in order to minimize 

 the effect of accidental contamination on the results; and the surviving 

 seedlings were counted. By adding the number of seedlings lost by 

 damping-off and breakage to those surviving, the total number germi- 

 nating was secured. By the number germinating is meant not the total 

 germination but rather the number which developed far enough to break 

 through the soil cover, this criterion being the only one it was practicable 



