Fungal Diseases of Trees

4.2.2.1 Leaf Blight of Rubber

Although the latex-producing rubber tree, Hevea brasiliensis, is a native of Brazil, today almost all rubber plantations are in Thailand, Malaya, and Indonesia. Brazil lost its rubber monopoly of the world due to ravages of the dreaded leaf blight, Dothidella ulei (Ascomycotina). Attempts to revive rubber cultivation in Brazil were abandoned because of the reappearance of leaf blight.

4.2.2.2 Chestnut Blight

At one time, the majestic American chestnut, Castanea dentata, was one of the most important timber trees in the eastern parts of the United States. The wood was easy to work and the nuts served as food. The fungus Endothia parasitica (Ascomycotina) was introduced into the United States from Asia and was first observed in 1904 as causing cankers on trees growing in New York City. Within ten years it spread from Maine to North Carolina and westward to Iowa and Nebraska, killing and eliminating chestnut trees throughout (http://www.apsnet.org/online/feature/chestnut/top.html). The fungus is carried from tree to tree as spores and plant quarantine legislation was enacted to reduce the chances of such a catastrophe's happening again. A biological control has promise: hypovirulent strains of the blight fungus were imported from Italy, cultures of which caused a regression when inoculated into cankers. The hypovirulent strains have an infectious viral double strand RNA that is transmitted through vegetative hyphal fusions to lethal strains (Monteiro-Vitorello et al., 1995).

4.2.2.3 Dutch Elm Disease

Another disease of concern has been the Dutch elm disease (first seen in the Netherlands, hence the name) and has spread through continental Europe and into the United States. It is caused by the fungus Ophiostoma ulmi (http://helios.bto.ed.ac.uk/bto/microbes/dutch-elm.htm) and is spread by beetles as they carry fungal spores on their bodies from infected trees to the bark of healthy trees where they feed and breed. The fungus spreads rapidly in the xylem vessels as infected trees first show wilting, curling and yellowing of leaves in small branches in the upper portions. Another mode of transmission of the fungus is through the root system via natural root grafts between trees growing close together, where the fungus spreads through water conducting vessels (xylem cells). The tree forms gums within these vessels in response to the presence of the fungus, obstructing water movement and causing the tree to wilt, its leaves to turn yellow and drop off and the eventual death of the tree. Among the fungal diseases of trees, the Dutch elm disease caused by Ceratocystis ulmi (Ascomycotina) is an important disease of American and European species of the elm trees. A large number of oak trees prized as shade trees in the midwestern United States have been killed by the oak-wilt fungus, Ceratocystis fagacearum (Ascomycotina), which clogs vascular tissues, preventing water movement to the crown and results in the wilting of leaves and finally in the death of the tree.

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