80 PREVENTIVE AND COMBATIVE MEASURES. 



(4) The massing of numbers of the same species of plant 

 together is dangerous, because it presents a favourable oppor- 

 tunity for the rapid spread of epidemic diseases. On this 

 account the smaller fields of small holdings tend to prevent 

 any epidemic from assuming serious proportions. Still better 

 is a system where, as in Northern Italy, a few rows of vines 

 alternate with narrow strips of Indian corn with gourds or 

 melons on the ground below, and strips of grass or millet 

 intervene here and there. 



Wherever similar plants must be cultivated in close neigh- 

 bourhood over extensive areas, as in vine cultivation, any 

 epidemic, which may obtain a hold, soon produces disastrous 

 effects. Our cultivated forest plants, when occupying extensive 

 areas, are particularly open to attacks of certain fungus-diseases. 

 Thus Pines from Hysterium pinastri, Caeoma pinitorquum, and 

 Peridermiuvi pini ; pole-forests of pure spruce from Hysterium 

 macrosporum, all plantations of conifers from Trametes radi- 

 ciperda and Agaricus melleus, the latter especially if preceded 

 by beech forest, the stools and dead roots of which offer the 

 Agaricus an opportunity for easy and abundant development. 



The prevention of many epidemic diseases is one of the 

 advantages claimed by Gayer ^ in favour of natural regeneration 

 and mixed plantations. On exposed areas the prevailing strong 

 winds facilitate distribution of many fungus-spores, while, at 

 the same time, they introduce the supplemental hosts of 

 of heteroecious fungi (e.g. aspen, ragwort, cowberry, etc.), which 

 would be excluded from a closed permanent mixed forest natur- 

 ally regenerated. Of course, we do not maintain that, under 

 these conditions, diseases are entirely absent, because it is just 

 on naturally-sown beech seedlings in closed forests that Phytoph- 

 thora finds a habitat. Similarly Trichosphaeria on silver fir, 

 and other parasites, are in closed forest provided with that 

 degree of atmospheric moisture which favours them. In fact, 

 several parasitic fungi exhibit adaptations to such conditions. 

 Diseases, speaking broadly, are less dangerous in mixed forest ; 

 they never attain the same distribution, and they are more 

 easily restricted where trees of different dispositions are grown 

 together. Thus, the forests of Bavaria consist, in the lower 

 elevations, of mixed beech, silver fir, and spruce ; higher up 

 ' Gayer, Der Waldbau. 



