i6o DISEASE I\ I'LAXTS. 



things he must avoid in deahng with certain 

 plants, or otherwise the)' will not succeed ; in 

 other words they will succumb to diseased con- 

 ditions and die. It is parti)- owing to the 

 want of systematisation of this knowledge, and 

 its extension in other directions, that such 

 extraordinary blunders are made in ignorant 

 practice, and trees for instance are planted in low- 

 lying frost beds which would succeed in slightly 

 higher situations, or seeds subject to damping- 

 off are sown in beds rife with the spores of 

 Peronospora or Pythim)i, and so forth. 



Many diseases, however, are not preventible in 

 the present state of our knowledge, or prev'ailing 

 conditions are such that the risk must be run of 

 endemic diseases gradually becoming epidemic, 

 and thus the natural desire for some means of 

 checking the ravages of some pest or another 

 has led to innumerable trials to minimise the 

 effects by prophylactic measures. The procedure 

 almost invariably followed where parasites are 

 concerned, consists in either dusting the plants 

 with some chemical in the form of a powder, or 

 spraying it with a liquid, or occasionall)' in envel- 

 oping the plant in some gas, in each case poisonous 

 to the insect- or fungus-pest. The principal rules 

 to be observed are : (i) the poison employed must 

 be sufficiently strong or concentrated to kill the 

 parasite, but not sufficiently powerful to injure the 

 host ; (2) it must be applied at the right period, 

 as suggested by a knowledge of the life-history 

 of the fungus or insect in question. 



