A comparison of the morphology and anatomy of normal Abies balsamea and that infected by Melampsorella cerastii (Pers.) Schroet.

Melampsorella cerastii (Pers.) Schroet. is an organism which causes witches` brooms (Hexenbesen) in our firs (Abies spp.) and spruces (Picea spp.). It belongs to the group of higher fungi (Basidiomycetes) known as the rusts (Uredinales). The alternate hosts are various species of the Caryophyllaceae e.g. Cerastium and Stellaria (Alsine). Pycnia and aecia are produced on the coniferous host, while the uredinia and telia are borne on the caryophyllaceous host. The mycelium is perennial in both hosts, frequently living as long as the host does, and thus rendering the parasite more permanent even if the required alternate hosts are rare. In fact, it appears that the aecidial host is not necessary for its continued existence (Wilson 1934). Infection takes place in the buds of the spruces and firs and causes hyperplasia and hypertrophy of the tissues. Apical dominance is lost, or, at least, reduced and numerous lateral branches develop resulting in the formation of conspicuous witches’ brooms. These brooms may eventually grow to large size: Pady (1941) shows one on Picea which is six feet long and two or three feet in diameter. Those on Abies, however, are much smaller, more compact and usually more nearly spherical. Typical brooms are shown in Plates 1(a) and 1(d), the first in its fourth year and the latter in its sixth.[...