ANNUAL REPORT, 1937 33 



with Pythium and Rhizoctonia, basal ends of some cuttings were dipped (dry) 

 in zinc oxide powder. This proved to be very injurious to cuttings of all species 

 with which it was used. 



Copper carbonate, 5.5 gm. per sq. ft., mixed with this medium before the 

 insertion of cuttings, was no less harmful, the only species not much injured by it 

 being two barberries, Berheris Julianae Schneid. and B. stenophylla Lindl. 



Acetic acid does not remain in the soil long enough to protect against damping- 

 off the seeds and seedlings of species which germinate slowly. Copper oxalate, 

 6 gm. per sq. ft., applied to soil before seeding, gave much better results with 

 Sciadopitys verticillata Sieb. & Zucc, seeds of which germinated 3 months after 

 seeding. 



Chemical Soil Surface Treatments in Hotbeds for Controlling Damping-Off 

 of Early Forcing Vegetables. (\V. L. Doran with E. F. Guba.) A soil disinfectant 

 may be applied to the soil (1) before seeding, (2) after seeding, but before germina- 

 tion, or (3) after the emergence of seedlings. The first of these methods has been 

 most common in the past, but there is a continued demand for more convenient 

 methods and these may include treatments to be applied later. 



The writers have found no treatment effective against damping-ofT which at the 

 same time is safe to use after the emergence of seedlings, which means with living 

 plants. Vinegar, in amount too small to protect against damping-off, was injurious 

 to seedlings of most species even when variously diluted. 



Such a treatment could not improve germination anyway; and more attention 

 has therefore been given to the application of a soil disinfectant immediately after 

 seeding and before germination. 



The important point is not so much when as how a chemical is applied to soil, 

 there being often no more than ten minutes' difference between the times of 

 application of treatments before and after seeding. It should be further under- 

 stood in this connection that a chemical applied to soil after seeding cannot, 

 because of the earlier presence of the seed, be worked into or mixed with the soil 

 and that it is, therefore, more concentrated near the soil surface and, consequently, 

 near the seeds. 



That being the case, the possibility of injury to the seeds is greater and applica- 

 tions which were safe enough if applied to soil immediately before seeding were 

 injurious in some cases if applied immediately after seeding. Thus as little as 

 4 cc. formic acid (90 percent) diluted to 250 cc. (per sq. ft.) was harmful if applied 

 to soil immediately after seeding, although 7 cc. applied before seeding was not 

 injurious to the same species. Formaldehyde 7 cc. and acetic acid (80 percent) 

 8 cc. per sq. ft., or either one of these diluted with 5 times those volumes of water 

 and applied to soil immediately after seeding, was injurious to species to which 

 they were quite harmless if mixed with soil before seeding. 



Treatments applied after seeding were generally least injurious to species the 

 seeds of which germinate relatively slowly, as do those of pepper, and more 

 injurious to seeds, such as cabbage, which germinate in fewer days or before 

 enough of the volatile chemical has escaped from the soil. 



When formaldehyde is diluted 1:300 and applied to soil at the rate of 0.75 

 qt. per sq. ft., there is only about 2.3 cc. formaldehyde per sq. ft.; but even this 

 light application, when made immediately after seeding, prevented most early 

 damping-off. It did not injure tomato, eggplant, pepper or lettuce, but it did 

 injure cress, the only crucifer with which it was used. It was not proved that such 

 a light application would be for long effective in preventing fungi from growing 

 up from the soil below, but such methods are convenient, if sufficiently effective, 

 and they are being further investigated. 



Vinegar 175 cc. (per sq. ft.) diluted with an equal volume of water and applied 



