174 



The white parasitical fungi, that are either the 

 cause or injurious consequence of the peach-leaf mil- 

 dew, are Oidium erysiphoides, Sporotrichum macro- 

 sphorum, Torula hotryoides, and Erysiphe pannosa. 

 We have little doubt that these fungi never attack 

 plants that are in good health, for we entertain the 

 opinion that it is only the sap of diseased plants — sap 

 m a state of decomposition — that is suited to be the 

 food of the fungi. Prevention, therefore, is prefera- 

 ble to curative applications, and we have no doubt 

 that if the peach tree is kept in due vigour by having 

 the soil well drained, and prevented from excesses of 

 either moisture or of dryness — and if its leaves are 

 similarl}^ protected from being exposed to sudden at- 

 mospheric changes — they will never be visited by 

 mildew. 



We are justified in this conclusion, because with 

 this disease our peach trees, in the whole course of 

 our practice, (which has extended over thirty years,) 

 have seldom or never been troubled. Mildew of 

 all kinds generally proceeds from an impeded root 

 action, and we have generally found that stagnation 

 suddenly caused, whether by excessive heat or drought, 

 is liable to produce it, more especially if succeeded 

 by much solar light. We have little doubt that in 

 such cases the elaboration (by overtaking or being in 

 advance of the absorbing power) produces more highly 

 concentrated juices, which are adapted as pabulum 



