242 The Spraying of Plants. 



the Section of Vegetable Pathology at Washington, and the 

 following are the conclusions reached : * 



" 1. The disease can be effectually prevented by the applica- 

 tion of the ammoniacal solution of carbonate of copper. 



" 2. In the nursery the total cost of the treatment need not 

 exceed twelve cents per 1000 trees. 



" 3. The first application should be made when the leaves are 

 about one-third grown, and should be followed by at least five 

 others at intervals of ten or twelve days." 



Rust (Rcestelia pirata, Thax., and Gymnosporangium macropm, 

 Link.). Description. The fungus which causes the rust of 

 apples is one of the most peculiar in which the horticulturist 

 is interested. Unlike many fungi, this one lives upon two host- 

 plants during its course of development. These host-plants are 

 the apple, and the cedar or juniper. There are probably several 

 species of rusts which attack cultivated apples, 2 but the histories 

 of all are essentially the same. The most common one is now 

 supposed to be Roestalia pirata, Thax. 3 



The effects of this fungus upon the apple are first noticeable 

 during the latter part of May, or in early June. The leaves are 

 then dotted with bright yellow spots, the so-called rust; the 

 fruit is also attacked about the same time. Such fruit becomes 

 worthless, as the growth is increased at the diseased point, and 

 the swollen part produces spores, which ruins the apples. Spores 

 are also produced upon the under side of the leaves. They ap- 

 pear and ripen during midsummer. They will not germinate 

 and grow upon either the leaves or fruit of the apple, but they 

 will develop the fungus upon the cedar. There the mycelium 

 enters the tissues, and as growth advances, enlargements appear 

 upon the branches of the tree. Such swellings, or " cedar-apples," 

 as they are called, are from half an inch to almost two inches in 

 diameter; they become full grown early in spring. During 

 April and May, horn-shaped masses an inch or more in length 

 are produced by the cedar-apple. They are of a bright yellow 

 color and can readily be seen among the green branches of the 

 cedars. Upon these soft, yellow bodies the spores are borne; 

 these spores will not grow upon cedars, but only upon the leaves 



1 Ann. Rep. V. S. Com. Agrie. 1889, 415. 



2 Byron D. Halsted, Ann. Rep. U. S. Com. Agrie. 1888, 3T6. 



3 Scribner, " Fungous Diseases of the Grape and other Plants," 1890, 84. 



