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Natural and introduced pathogens are present in exotic plantations. Exotic pathogens commonly enter new areas via germ plasm. Root pathogens, which tend to require soil and plant roots for their spread, have traveled less frequently between continents because of long-standing and strict quarantines on the movement of soil. However, many species of stem, leaf, and shoots pathogens have been introduced into new areas with seeds and plant debris associated with seed.

Root pathogens. Root diseases in exotic plantations are caused predominantly by native pathogens. Nevertheless, there are some intriguing examples of root diseases of exotic plantation trees caused by pathogens that are in all likelihood also introduced. One of these is Rhizina root disease, caused by Rhizina undulata (Figure 1), which can be extremely damaging in southern Africa (Gibson 1979, Germishuizen 1984). This pathogen is …