32 FUNGI AND FUNGICJM 



vast number of which in;iy develop from the mycelium 

 of a single leaf the fungus is able to spread very rap- 

 idly. Toward the end of summer the mycelium devel- 

 ops small round black masses just visible to the naked 

 eye, which are the cases containing the winter spores. 

 These cases are technically called perithecia, and when 

 magnified resemble Fig. 15, d. They remain on the 

 leaves when the latter fall in autumn, and tide the fun- 

 gus over winter. 



This powdery mildew may develop on either surface 

 of the leaf, and is especially likely to appear on the 

 ^leaves and stems of young shoots. It is also usually 

 much more abundant on young trees than old ones, and 

 is especially destructive to nursery stock ; so much so, 

 in fact, that where no treatment with fungicides takes 

 place it often prevents the successful budding of a large 

 proportion of the young trees. Leaves severely attacked 

 by the fungus drop off prematurely, and so prevent the 

 proper growth of the tree. The disease is usually most 

 prevalent during seasons of dry weather. 



Treatment. As a rule, it is only in the nursery 

 that this disease is sufficiently destructive to require 

 remedial treatment. To prevent it, spray with the am- 

 moniacal solution of copper carbonate, making the first 

 application when the leaves are about half-grown, and 

 repeating four or five times at intervals of twelve days. 

 Mr. B. T. Galloway has shown that nursery stock can 

 be sprayed with this solution five times, at a cost of 

 eight cents per thousand trees, estimating the copper 

 carbonate at forty cents per pound, and the ammonia 

 (26) at eight cents per pound. 



Literature. An excellent general account of this 

 disease, by Mr. M. B. Waite, may be found in the 1888 

 Keport of the Department of Agriculture (pp. 352-357) ; 

 while notices of remedial experiments appear in bulletins 

 of the Division of Vegetable Pathology. 



