Pathogenic Ascomycetes

Pathogenic Ascomycetes
Microsphueru ulni is an ascomycete that infects the leaves of the common
lilac. It produces conidiospores at the ends of hyphae by abstriction. Ascus
production occurs in a completely enclosed fruiting body called a cleistocarp.
The formationo f asci is not associated with a fusiono f gametic nuclei. When
an infected lilac leaf is examined, the conspicuous element is the whitish
mycelium with its powdery profusion of conidiospores. It is a powdery
mildew. Careful examination with ale ns discloses small, black dotst;h ese are
the cleistocarps. In figure 19-7 a cleistocarp having anchoring filaments has
ruptured, and asci-bearing ascospores are being extruded.
Endothecia purasiticu is another ascomycetous plant pathogen. It has
caused the near extinction of chestnut trees in the United States. The fungus
was inadvertently brought to this country on stock brought from the Orient
near the year 1900. From the initial point of infection in Long Island, New
York, it spread throughout the country and destroyed nearly all the chestnut
trees. Although a few resistant trees remaina nd a comeback is possible, iits
not yet much evident.
Cerutostomelh ulmi causes Dutch Elm disease, so-called because it was
first describedi n the Netherlands. This fungus infecttsh e American Elm tree.
The infection began in this country in 1930; again, the source was imported
stock. It has spread widely and caused vast destruction among American Elm
trees. The destiny of the American Elm appears to be the same as that of the
American Chestnut.