if 



53 



already noted. The principal causes of the disease of plants may b« 

 divided into two classes, namely, those due to attacks of living orgaor 

 isms ; as, for example, insects and fungi ; and those brought aboat by 

 conditions in which life takes no part, as unfavourable conditions of soil 

 and climate, peculiar atmospheric influences, and physiological changes 

 in the plant itself which cannot be assigned to'any known cause but which, 

 nevertheless, eventually bring about death or a sickly condition render- 

 ing it worthless to man. 



As we are concerned only with the parasitic enemies of plants, we 

 may dismiss all the others with che simple statement that the injuries 

 they occasion are comparatively insignificant. This brings us now to 

 the point of further dividing these enemies into vegetable parasites and 

 animal parasites. Under the former are classed the fungi, while to the 

 latter group may be referred the insects. It is of the fungi particularly 

 that we propose to speak. Of course there are diseases caused by vege- 

 table and animal parasites which cannot properly be referred to either, 

 fungi or insects, but they do not concern us here. 



In these days we see, hear, and read a good deal about fungi ; but 

 despite all this, very few people have a fair conception of what they are 

 and in what manner they can cause the destruction of entire crops. To 

 make these matters clear it will be necessary to devote a few words to 

 the place in nature occupied by these plants, for such they are, after 

 which it will be proper to say something about how they live, cause dis- 

 ease and death among the higher plants, and finally die themselves. To 

 most persons fungi are the common mushrooms and toadstools which 

 j^row in pastures, on the trunks of dead trees, and other similar places. 

 While these are true fungi they form a comparatively small part of the 

 great group of plants, numbering over 30,000 species. By far the 

 majority of fungi are so small that powerful microscopes are necessary 

 to see and study them, and for this reason, perhaps, more than any 

 other, many erroneous ideas prevail about them and the effects they 

 produce. 



To get a better idea as to what a fungus is, especially a fungus be- 

 longing to the disease producing group, let us follow the life history of 

 one which is perhaps familiar to all, occuring as it does on the grape 

 which is grown everywhere in this country. The parasite under con- 

 sideration is commonly known among grape-growers as **mildew," but, 

 to use a descriptive term as well as one which will enable us to distin- 

 guish it from other members of this group, we usually speak of it as th« 

 *'downy mildew of the jjrape." 



The ''downy miMew" may be found in almost any vineyard any 

 time during the months of August and September. An examination of 

 the leaves at this time reveals, here and there on the upper sides, pale 

 yellow spots of various sizes and more or less circular shape. Opposite 

 these spots, on the under side, may be seen a whitish, frost-like, downy 

 growth which has led to the adoption of the name already mentioned. 

 In many cases where the malady is severe the entire leaf will appear 

 yellow and sometimes red, shriveled, and dry as if scorched by fire. 

 This is all the eye distinguishes, so there is no wonder that so many 

 <3ouflicting opinions exist regarding the cause of the trou- 

 ble. The microscope aided by the trained eye, however, reveals the 



