3-2 



INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



rust perish at GO" C. fliO° F.), those of l)rown rust at 45''-50'' C. (113°- 

 122° F.). Herzber<f has compared the resistance of spores of ditt'erent 

 ages to the action of heat, and found the following temperatures as 

 those at which they succuaibed : — 



TABLE IV. — Shoiving the Temijeratiires at tvliich the Spores of different Species 

 of Ustilago are Killed. 



It follows that the young spores are more resistant than the old 

 ones, and that the temperature required to destroy the vitality of a 

 spore is not the same for spores of different species. 



Use. — The sensitiveness of parasites to hot water and the com- 

 parative resistance of plants have permitted the use of different treat- 

 ments, especially preventive, to free plants from certain diseases. 



Immersion or Steeping. — This consists in dipping seeds or 

 plants in hot water to free them from disease germs adhering to their 

 surface. 



Hot Water Steeping of Seed-Corn to Kill Disease Si)ores. — Steeping 

 seed-corn in hot water is practised before sowing to destroy the 

 dormant spores of smut and bunt, which adhere to their surface 

 and help to propagate these diseases. Brefeld, observing that cold 

 water was injurious to the development of the spores of bunt, 

 tried washing seed with cold water, and so obtained an appreci- 

 able but incomplete result. On the other hand, hot v*ater has 

 been recognized as deadly to the spores of these fungi, and treating 

 the seed by hot water would wholly suppress cryptogamic diseases if 

 the seed formed the sole factor of their propagation. To get satis- 

 factory results steeping the seeds in hot water should be done in a 

 strictly scientific manner. It is only efificacious if the temperature of 

 the bath has been rigorously maintained at a certain degree. A greater 

 heat than that required to kill the spores should be avoided, for it will 

 appreciably diminish the germinative capacity of the seed, and may 

 even destroy it. If there be a difference of sensitiveness between the 

 seeds and the spores as regards heat, it is so small that a difference 

 of a few degrees may be fatal. A low temperature should also be 

 avoided, as it favours the disease instead of preventing it. Warm 

 moisture helps, in fact, a premature development of the spores. The 

 promycelium and the sporidia formed then attack the young plant as 

 soon as hatched. It is a known fact that bunt as well as smut 

 are more injurious to the plant the younger they attack it. When 

 well done, steeping in hot water imparts to the seed in many cases a. 



' To bring to Fahrenheit, multiply by 9, divide by 5, and add 82. 



