Dec.23, i9js Parasitism of Cronartium ribicola 625 



infection is shown in Plate 48, B, a. McCubbin (29) reports that out of 

 1,007 pine infections examined 925 originated at the bases of leaf fasci- 

 cles, 14 originated in wounds, and 68 were indeterminate. These con- 

 clusions controvert the scarcely tenable theory of Marchal (55) that in- 

 fection generally takes place through wounds. Observations on a large 

 number of infections at Kittery Point, Maine, indicate that most infec- 

 tions can be classed as either nodal or intemodal. However, it should be 

 borne in mind that infection probably takes place very often in the bud 

 while the nodal and internodal cells are scarcely diflferentiated. There- 

 fore the point at which infection appears to originate after the tissues in 

 the bud have elongated may be either nodal or intemodal, depending 

 entirely on the chance growth of the sporidial germ tube. In discussing 

 Peridermium pint Pers., parasitic on Pinus sylvestris I^., Hartig (19) 

 states that infections with this rust also originate either at the node or 

 the base of the leaf fascicle. 



The swelling and etiolation of the bark noted above are the two 

 most prominent indications in early stages after infection of the presence 

 of the parasite. The swelling is confined to the bark alone, the wood 

 actually becoming constricted in old infections, and is due to the fact 

 that the mycelium of the parasite forces apart the phloem and cortex 

 cells of the host. No evidence of any increased division has been ob- 

 served in the bark cells, and there seems to be no stimulation toward 

 gall formation, such as occurs in other cases — ^for example, on Pinus 

 virginiana Mill, under the attack of Peridermium cerebrum Peck. In 

 the case of the nodal infection in Plate 48, A, it will be noted that there 

 is little or no swelling; but, as stated above, both swelling and etiola- 

 tion may occur at the same time. The irregular edge of the etiolated 

 area marks fairly definitely the advancing tips of the invading hyphae, 

 which generally extend a little beyond the line. 



Other less common external indications of the parasite are bunched 

 needles on seedlings or transplants and occasionally on older trees, and 

 short adventitious branches which spring from the infected nodes. 

 It frequently happens that certain secondary fungus parasites become 

 established on the area already attacked by the rust, and suppress 

 the latter so completely that spore production is partly or completely 

 inhibited. Under these conditions the bark dies and shrinks more 

 rapidly than under the attack of the rust alone, and the stem then 

 appears to be constricted, more or less irregularly, at the infected part. 

 All of these external indications are valuable aids to early recognition 

 of Cronartium, ribicola. 



The period of incubation, from the time of infection to spore pro- 

 duction, varies from one to several years, depending possibly on whether 

 the infection takes place in early summer, and therefore under favorable 

 conditions for growth before winter, or in early fall, when growth has 

 unquestionably slowed down. The growth of the mycelium in the bark 



