4 DISEASES OF CULTIVATED PLANTS 



parasites, the spores of these fungi cannot effect an entrance 

 through an unbroken surface, but only through some wound. 

 Many kinds of wounds to which trees are liable, as those 

 caused by hailstones, frost, branches broken by wind, etc., 

 are beyond our control ; on the other hand, the injuries caused 

 by bad pruning, by workmen in planting trees, neglecting to 

 protect cut surfaces at once by a coating of gas-tar, and 

 many other examples, may be cited as primary causes of 

 disease, over which we have the means of perfect control, 

 which are but rarely applied. 



An epidemic, say, caused by a fungus, is usually the 

 outcome of the spores of that particular fungus having been 

 previously present in the vicinity, hence, honestly speaking, 

 I consider sheer negligence as the most frequent primary 

 cause of disease, as when diseased plants are left lying 

 about instead of being promptly burned or buried. More 

 will be said on this subject under the heading, Cleanliness. 



On the other hand, numerous insects, mites, eelworms, fungi, 

 and even flowering plants are undoubtedly primary causes of 

 disease. There is no denying the fact that many different 

 kinds of insects, during some period of their development, 

 attack and destroy perfectly healthy plants. The same is true 

 of the minute organism called Plasmodiophora, which is the 

 primary cause of the disease known as ' anbury ' or ' finger-and- 

 toe,' in the roots of turnips, cabbages, and various other 

 cruciferous plants. Aphides or ' green-fly ' of various kinds, 

 Thrips, ' eel-worms,' and * scale-insects,' of which the ' woolly 

 aphis' or American blight is a well-known example, also 

 belong to the same category. 



Among fungi, as primary causes of disease, may be 

 enumerated the whole of the great family popularly known as 

 rusts wheat rust, hollyhock rust, etc. ; the * smuts ' and 

 4 bunts ' of cereals. Finally, flowering plants furnish such 

 destructive parasites as the 'dodders,' which attack flax, 

 clover, hemp, etc. The broom-rapes and mistletoe also 

 kill or injure other plants. 



In some instances the presence of one parasite renders the 

 host more susceptible to the attack of a second parasite. I 

 have proved by means of repeated experiments that the very 

 destructive disease known as larch canker, caused by the fungus 

 Dasyscypha calycina (Fekl.), is enabled to establish itself at 

 new points on the tree, where the surface has been wounded 

 by the larch aphis (Chermes laricis). In like manner apple 



