DISEASES OF SHADE AND FOREST TREES. 99 



RUSTS. 



The rusts are among the most highly developed of the 

 parasitic fungi and attack foliage and fruit. Some of them 

 are verv destructive while others are compara- 



. , .' . . ^ f f. , , Importance. 



lively msignmcant. Many ot them have very 

 complicated life histories and require two host plants to com- 

 plete their life cycle and in many cases to perpetuate them- 

 selves. 



One of the most conspicuous of the rusts is the Gymno- 

 sporangium macropus* Lk., which attacks the red cedar and 

 the apple. On the red cedar they cause the formation of the 

 large reddish brown bodies known as "cedar Cedar 

 apples." During the period of early spring apples. 

 rains the mature cedar apples produce long yellowish or 

 "range colored horns within which are produced great masses 

 of fungus spores. These spores will not attack the cedar 

 but are carried to neighboring apple trees where they attack 

 the leaves, and sometimes the young twigs and fruit, causing 

 yellowish or reddish orange spots. On the under side of 

 each spot are produced a number of small cup-like cavities 

 within which are borne the spores. These spores are in turn 

 carried to the cedars where they attack the young shoots and 

 eventually cause the formation of the next year's crop of 

 'Vedar apples." 



Treatment. 



This fungus may be held in check by removing the cedar 

 apples early in the spring before the maturing of the orange 

 ''lured horns, and by the proper spraying of the apple 

 orchards. 



A moms other interesting diseases of this kind 



Other plants 



are the rust, or leaf cast, of the Jersey or scrub affected by 

 pine, the rust of the Scotch and pitch pines 

 which has an alternating stage on the sweet fern, the leaf 

 rust of the hemlock, rusts of the willows, poplars and ashes. 



* There are several species of Gymnosporangium attacking cedars and with 

 various species of the Pomaceae as their alternating hosts. 



