Origin and evolution of conifer rusts in the light of continental drift

Many genera and species of present-day aecial rusts on seed plants are derived directly or indirectly from a limited group of these pathogens on northern conifers, whereas conifer rusts are believed to have descended from their progenitors on ferns. It is worth noting that only the Pinaceae, Taxodiaceae and Cupressaceae serve as alternate hosts to an extensive rust flora on ferns and angiosperms which are distributed predominantly in the northern hemisphere. Numerous conifers in the southern hemisphere do not bear rusts, except a few endemic species on Araucariaceae in Malaysia and south central Chile.Such a disjunctive distribution of conifers and their specialized rusts is attributed now to extensive continental drift. Evidence indicates that the breakup of the primary land mass Pangaea into northern Laurasia and southern Gondwanaland, with successive splitting and drifting of these supercontinents, has also scattered ancient land faunas and floras.Plant Science Research Division, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland. Plant Introduction Investigation Paper No. 29.