Dec. 23. i9i8 Parasitism of Cronariium ribicola 651 



parasitic fungi, or saprophytic fungi, and insects, which hasten the 

 decomposition of the weakened susceptible tissue. These secondary 

 fungi may also gain entrance through the pycnial spots. Rathay (47) 

 has shown that the pycnia of many rusts are visited by insects, appar- 

 ently attracted by the sweet drops. Both pycnia and secia in C. ribicola 

 may be eaten out by various insects, and unquestionably these insects 

 bear on their bodies or legs viable spores of fungi capable of growing 

 in the bark tissue. In many instances these fungi are so rapid in their 

 growth that they overrun the infected area and completely suppress the 

 rust, so that it never fonns aecia. In such cases, the bark shrinks and 

 the infected stem is actually constricted, and the girdle thus formed is 

 often more quickly effective than in the cases where rust works alone. 



In leaves of Ribes spp. the mycelium sometimes causes the death of 

 isolated infected spots; but in other cases the hyphae penetrate to all 

 parts of the tissue without causing death of the cells, and without pro- 

 ducing hypertrophy. The large spaces among the mesophyll cells and 

 the fact that the hyphae rarely form solid mycelial masses in leaves of 

 Ribes spp. probably help to explain the lack of hypertrophy and destruc- 

 tive effect. When defoliation occurs during the course of severe epi- 

 demics of the rust, there is, of course, a consequent poor crop of berries. 

 The variation in the effect of the parasite on the different species of 

 Ribes is a subject which must have separate treatment and therefore 

 can not be considered fully at this time. 



It will probably be found on further investigation that a close agree- 

 ment exists among caulicolous Peridermia with respect to the structure 

 of the pycnium. The external appearance of the pycnium, or of the 

 thin layer of tissue overlying it, will, however, be found to vary according 

 to the outer bark texture of the host. In the case of Pinus parviflora 

 the exterior appearance of the pycnium is almost identical with the 

 pycnium of P. strobus. The value of the pycnial spots as diagnostic 

 characters has been briefly outlined in an earlier short note (9), and re- 

 ferred to above in connection with the discussion of the form.ation of the 

 pycnium and pycniospores. Hartig (19, Taf. 4, fig. 7, b) was one of the 

 first observers to call attention to the pycnial spots on the bark of P. 

 strobus, although he at that time thought the fungus on this pine was 

 identical with Peridermium pini. Wolff (64) copied the same figure in 

 his paper on the latter fungus. Kirchner and Boltshauser (22) in Plate 

 15 of their atlas show what are evidently three pycnial spots, but they 

 do not definitely refer to them in their description of the figures. The 

 ability to recognize the pycnial spots of Crotiartium ribicola is almost 

 absolutely essential in the field study of control methods. 



The description of the formation of the aecium given above emphasizes 



the remarkable agreement which exists in the fundamental processes 



involved in the production of seciospores in the rusts. The figures of 



Hartig (jp), Wolff {64), and Sappin-Trouffy {31, fig. 63) on the aecium of 



88096°— 19 3 



