70 PREVENTIVE AND COMBATIVE MEASURES. 



and causing disease, than on its capacity to kill spores which 

 light on the leaf, or to prevent their development to a dangerous 

 extent. On this account crops liable to attack should be dusted 

 or sprayed in early spring, and at intervals thereafter as long 

 as there is any risk of disease. Used in this way, fungicides 

 soon repay themselves in increased yield of healthy produce ; 

 on the one hand, they hinder the development of the fungus, 

 on the other hand, they act like antiseptic wound-treatment in 

 preventing infection. What part the copper compounds play 

 is as yet not completely established ; Eumm ^ considers that 

 they are not actually absorbed by the plant, but only give rise 

 to some electrical effect. 



The advantage to be gained from the use of fungicides may 

 be greatly increased if all diseased plants or portions of them 

 be removed before the remedy is applied. Precautions must 

 also be taken against reappearance of the disease. In the case 

 of infected forcing boxes, frames, or glass-houses, disinfection 

 by some of the above fungicides is certainly advisable. Leaves 

 on other plant-remains containing resting-spores of the fungus 

 should be burnt, and soil containing diseased material should 

 be watered with a fungicidal solution which will kill the fungus 

 while it does no harm to the leaves or roots of plants. Finally 

 a rotation of crops of as long a duration as possible will do 

 much to keep epidemic fungoid diseases in check. 



(3) Frequently the ravages of a parasite can be considerably 

 reduced, although not completely stopped, by destroying its 

 reproductive organs. Methods of this kind are particularly 

 useful in the case of the Polyporeae which inhabit the wood of 

 many fruit-trees. The excision of the sporophores must be 

 carried out once or twice a year, because the mycelium remains 

 alive inside the stems and continually gives off new sporophores 

 on the surface. The diseased tree lives on and produces fruit 

 for many years, maybe till the wood of its stem becomes so 

 much decayed that death ensues. Fungi of this family are 

 even more destructive on those trees which are cultivated not 

 for their fruit alone but also for timber, e.g. olive, sweet chest- 

 nut, and hazel. 



(4) By the removal and destruction of dead plants or portions 

 of plants containing reproductive or hibernating organs of para- 



' C. Rumm, Berichte d. deiUsch. hotan. Oes. 1893. 



