32 



FU^S^GI AXD FUNGICIDES 



Tast number of whicli may deyelop from the mycelium 

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

 idly. Toward the end of summer the mycelium devel- 

 ops small round black masses just yisible 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 

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

 gus oyer wdnter. 



This pow^dery mildew may develop on either surface 

 of the leaf, and is especially likely to a23pear on the 

 leayes and stems of young shoots. It is also usually 

 much more abundant on young trees than old ones, and 

 is especially destructiye to nursery stock ; so much so, 

 in fact, that wdiere no treatment with fungicides takes 

 j)lace it often prevents the successful budding of a large 

 j)roportion of the young trees. Leayes severely attacked 

 by the fungus drop off prematurely, and so preyent the 

 proper grow^th of the tree. The disease is usually most 

 preyalent during seasons of dry w^eather. 



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

 that this disease is sufficiently destructive to require 

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

 moniacal solution of copper carbonate, making the first 

 application w^hen the leaves are about half-grown, and 

 repeating four or five times at intervals of twelye days. 

 Mr. B. T. Gallow^ay has shown that nursery stock can 

 be sj^rayed with this solution fiyc 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. TVaite, may be found in the 1888 

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

 while notices of remedial experiments aj^pesiY in bulletins 

 of the Diyision of Vegetable Pathology. 



