Three Rust Diseases of the Apple 11 



The necrotic lesions involve less of the fruit surface, the fungus appar- 

 ently being arrested at a comparatively early stage in the development 

 of the fruit (Plate II, 9). Thus an older lesion sometimes resembles 

 bitter pit in superficial appearance (Plate II, 7). Not infrequently the 

 necrotic tissue cracks to a considerable depth, sometimes exposing the 

 seed cavities. In the interior of the fruit the necrosis is more extensive 

 and tends somewhat to follow the larger vascular elements (Plate II, 

 10). Aecia have not been found on the extreme necrotic type of lesion, 

 and pycnia are not abundant, In the hyperplastic lesions, pycnia and 

 aecia may be present in considerable abundance. 



The most definite and striking diagnostic characters are found in the 

 fruiting structures of the fungus. The pycnia, aecia, aeciospores, and 

 peridial cells of G. germinal e are distinctly larger than those of G. juni- 

 peri-virginianae or G. globosum. The 'aeciospores in mass are of a 

 bright orange color, in contrast with the dull brown color of the spore 

 mass in G. juniperi-virginianae . The peridium of G. germinate dehisces 

 by irregular shredding and does not become revolute. The peridial 

 cells are wet with difficulty and adhere in a pseudoparenchymatous 

 plate when mounted in water. In face view these are distinctly verru- 

 cose (Plate I, 5), in contrast with those of G. globosum which in face 

 view are comparatively smooth in the center and rugose at the margins 

 (Plate I, 4 A). 



ETIOLOGY 



Kern (1912) has published detailed descriptions of the three fungi 

 under consideration, in a monograph of the genus Gymnosporangium. 

 Our findings agree in most of the essential details with those of Kern. 

 It seems desirable, however, to present in this paper certain characters 

 in somewhat greater detail. 



In figure 1 are shown the approximate frequency distributions of the 

 three species in regard to length of aeciospores, based on measurements 

 of 200 or more spores in each species. The aeciospores of G. globosum 

 and G. juniperi-virginianae are similar in color and shape, and cannot 

 be distinguished by casual examination. However, if 50 or more spores 

 are measured and the mean length is computed, the two species are 

 readily separated, since the mean length of G. globosum is roughly 

 21.60±0.17/m while that of G. juniperi-virginianae is 27.45±0.01 /n. The 

 aeciospores of G. germinate have a mean length of 31.00±0.22 fi, and in 

 addition are lighter in color and more smoothly rounded than the spores 

 of the other two species. 



The aeciospores of the three species are more variable in width. The 

 mean width of G. globosum aeciospores is 18.32 ±0.41 /x, while that of 

 G. juniperi-virginianae aeciospores is 22.67 ±0.51 fj. and that of G. ger- 



