July IS, 1920 Presoak Method of Seed Treatment 389 



The^ method of treatment discussed has, therefore, a two-fold advan- 

 tage. On the one hand, wheat-seed injury due to the use of formalin and 

 copper sulphate is eliminated or reduced to a minimum. On the other 

 hand, the blackchaff organisms on the seed coats are rendered particu- 

 larly sensitive to the action of the disinfectant by being previously 

 brought into a vegetative condition. 



The same physiological principles discussed above should hold true 

 for the general problem of seed treatment for various seed-borne patho- 

 gens. The consistency of the results obtained by this method with nine 

 varieties of wheat and with other cereals, using formalin and copper 

 sulphate, indicates the possibility of the use of the presoak method with 

 other kinds of seeds as a means of minimizing or preventing seed disin- 

 fectant injury. Similarly, other pathogenic organisms, bacteria, or 

 fungus spores, may be stimulated by the presoak method into increased 

 susceptibility to the disinfectant. 



The presoak method of seed treatment with chemical disinfectants 

 may be formulated for general purposes as consisting of two parts: 

 First, the presoak period, in which seeds are soaked in water for 10 min- 

 utes, drained, and kept covered and moist for a definite period of time, 

 which is 6 hours for wheat, barley, and oats and 10 to 18 hours for maize — 

 in no case sufficient to begin seed germination; second, the disinfectant- 

 treatment period immediately following, in which the disinfectant is 

 applied exactly as now practiced. The relative time of the presoak and 

 subsequent treatment for other diseases, probably varying with each 

 kind of seed and pathogen, is dependent on the following factors: 



(i) Susceptibility of the kind of seed used to the disinfectant. 



(2) Susceptibility of the pathogen to the disinfectant. 



(3) Rate ®f absorption of water by the seeds. 



(4) Time at which seed germination begins. 



(5) Time at which vegetative activity of the pathogen begins. 



A proper balance of these factors must be obtained, such that the 

 optimum seed germination and the optimum germicidal efficiency are 

 secured, as reported for the blackchaff disease of wheat. 



The length of the presoak period should not exceed half or two-thirds 

 of the period necessary for seed germination to begin, since germination 

 before treatment with the disinfectant would result in extreme sensi- 

 tiveness to injury. On the other hand, the pathogen, especially if bac- 

 terial in nature, usually has a much shorter germination period, which 

 should come within the limit of the time of presoak and thus render it 

 susceptible long before the seed has begun to germinate. The period 

 necessary for the absorption of about 30 per cent by weight of water 

 appears to be sufficient, and in the case of the cereals so far tried seems 

 to counteract disinfectant injury. In wheat, oats, and barley this is five 

 to six hours. The length of time necessarj' for other kinds of seeds to 

 absorb about 30 per cent of water is suggested as the presoak period 

 when not conflicting with the other factors involved. 

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