Diseases and pest insects in natural ecosystems

If we aim to control pests and diseases on farms and in commercial forests, by means other than chemical pesticides, one approach is to ask whether we can learn by studying more natural ecosystems. Fungi and insects do not wipe out all the other species in those systems, so in some sense they must be under control. Maybe this can give us ideas on how to control them in our more artificial systems. This section describes some examples of insects and diseases that attack wild plants and animals, and considers whether there are any useful messages for pest control.

Chestnut blight

Phytophthora cinnamotni in Australia

The first message is that epidemics can occur in near-natural ecosystems, even when they are quite species rich. An example is chestnut blight in North America, which is caused by the fungus Cryphonectria parasitica (Van Alien 1982; Buck 1988; Newhouse 1990; Anagnostakis 1995). This spreads in the bark of stems, and once it has formed a girdle right round a stem all the branches above that point die. The fungus probably reached the United States in young chestnut trees imported from Asia. It was first noticed in 1904 in New York, and within the next 50 years had spread through the whole natural range of the native chestnut (Castanea dentata) in the eastern USA. Once a major tree of mixed deciduous forests in the Appalachians, the chestnut now occurs there only as regrowth coppice shoots from surviving stumps. These shoots rarely live long enough to set seed, so whether the species will survive indefinitely in the area is uncertain. The fungus also reached Mediterranean Europe, where it killed many of the native Castanea sativa. There, however, after about 15 years less damaging 'hypovirulent' strains of the fungus began to appear, which caused some symptoms but did not kill the tree. The character for hypovirulence is carried by a virus, which can transfer to virulent strains, reducing their virulence. This suggests an opportunity for biological control, which will be discussed later.

Chestnut blight provides an example of a disease caused by a parasite which can attack plants only within a single genus {Castanea). In contrast, some other plant pathogens have a wide host range: one example is Phytophthora cinnamomi. This fungus infects the roots of many woody plant species, causing root rot, which can be followed by die-back of branches and ultimately the death of the whole plant. The fungus is confined to soil and roots and has no airborne spores, so it normally spreads slowly, in water or with eroding soil. Yet it occurs in every continent

Table 8.2 Records from a permanent plot in Eucalyptus woodland in the Brisbane Ranges, Victoria, Australia, in 1975 and 1985. Phytophthora cinnamomi was present from 1975 onwards. From Weste (1986)

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