580 ANNUAL REPORT OF THE Off. Doc. 



APPLE BUST 



By PROF. H. R. FULTON, State College, Pa. 



The fact that certain of the rust fungi attack two entirely dif- 

 ferent host plants during their complete life cycle, is strange enough 

 to challenge perhaps immediate belief. One of the most notorious 

 examples of this is the stem-rust of small grains, which has a second 

 stage on the leaves of the European barberry, a rather uncommon 

 plant with us. In New England, where it early became established, 

 growers of wheat noted what was also noted in the Old World, that 

 grain rusted most when growing near barberry bushes; and so the 

 practical patriots of Massachusetts, some years before the colonies 

 became a nation, passed a law requiring the destruction of barberry 

 as a protection for their grain crops against rust. Unfortunately, 

 this rust has the ability to bridge over from one grain crop to the 

 next without the aid of barberry, largely through infection of the 

 fall-sown grains and certain wild grasses. Actual proof of the 

 relation of the stem rust to grain crops on the one hand and to bar- 

 berry on the other was first made by a German botanist in 1864. 



Two years later a Danish botanist followed up the clue furnished 

 by horticulturists' traditions, that apple and other pomaceous plants 

 contract orange rust from cedars; and proved experimentally that 

 certain European species of rust do pass from cedars to apple and 

 related plants, and vice versa. 



For certain American forms of cedar rusts, Drs. Farlow, Thaxter 

 and others have established the relationships to pomaceous plants, 

 and we now know with the certainty that comes from rigid experi- 

 mental proof that at least four distinct species of rust fungi, all 

 belonging to the genus Gymnosporangium, and all passing a part 

 of their lives on junipers, attack apple in the other stage of their 

 existence causing the sometimes troublesome orange or cedar rust. 



The three forms commonly occurring on red cedar in Pennsylvania 

 are readily distinguishable. The most common (Gi/mnosporangium 

 macropus) forms on cedar twigs slowly during twenty months the 

 peculiar smotish, brown swellings as large as a marble known com- 

 monly as "cedar apples." In spring and early summer these send 

 out in moist weather, bright orange, elongated, gelantinous horns; 

 and then perish. The second species (G. glohosum) also forms "cedar 

 apples," but the galls are rough and scaly, and the long gelatinous 

 horns are replaced by short protrusions of rusty red masses. These 

 galls persist for several seasons. The third species (G. clavipes) 

 attacks larger twigs and causes very slight swelling, but the bark 

 is roughened, and through cracks there protrude reddish masses 

 of spores; it is also perennial, and may attack the low, straggling 

 common juniper as well as red cedar. At maturity, always in the 



