Introduction to Fungi, Third Edition

470 LOCULOASCOMYCETES
Fig17.10 Lewiainfectoria. (a) Asci, one intact, the
other stretched by expansion of the endotunica.
(b) Germinating ascospore. (c) Alternaria
infectoria conidial state developed in culture
from a germinating ascospore. (d) A conidium
from an intercalary position in a spore chain
showing a scar at each end of the conidium.
(e) A conidium from the end of a spore chain
with only one basal scar.
spore, reducing its settling velocity (McCartney
et al., 1993).
Some species of Alternaria are of considerable
economic significance (Chelkowski & Visconti,
1992; Rotem, 1994). Alternaria alternata (¼ A.
tenuis) is the name given to a widespread and
cosmopolitan opportunistic saprotroph, reported
on all kinds of senescent plant material.
Unfortunately, because of difficulties in identification,
it is probable that the name encompasses
several distinct taxa (Roberts et al., 2000).
Alternaria spp. are associated with diseases of
crops, often showing a degree of host specificity
as indicated in Table 17.2. Most of these diseases
are seed-borne; seeds become infected from the
flowering stage onwards (Rotem, 1994). The hostspecific
pathogens may have evolved from nonspecific
saprotrophic forms of ‘A. alternata’ by the
selection and multiplication of mutants capable
of secreting host-specific toxins (Scheffer, 1992;
Rotem, 1994; Thomma, 2003). Many of these
host-specific toxins have been characterized
chemically (Otani & Kohmoto, 1992). Numerous
mycotoxins are also produced by Alternaria spp.
(Montemurro & Visconti, 1992), and some have
severe and fatal consequences if they accumulate