Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 11
point to its role in resistance development (Kuc, 1976). It was clarified
that the rapidity and magnitude of phytoalexin accumulation were
important in disease resistance, and that potato tubers could be
protected against disease caused by a compatible race of P. infestans by
inoculation with an incompatible race. Rishitin has also been found to be
induced in potato tuber discs 24 h after inoculation with Fusarium
sambucinum, which causes dry rot in stored potatoes (Ray and
Hammerschmidt, 1998). Other sesquiterpenoids that have been found in
potatoes may also play a role in tuber disease resistance; they include
rishitinol (Katsui et al., 1972), lubimin (Katsui et al., 1974; Stoessl et al.,
1974), oxylubimin (Katsui et al., 1974), solavetivone (Coxon et al., 1974),
and others. The terpenoid phytuberin was found to be constitutively
present in tuber tissues at low levels, but it was further induced after
inoculation with F. sambucinum. The phytoalexins, phytuberol and
lubimin appeared in potato discs by 48 h after inoculation, while
solavetivone was produced in very low quantities. At least eight additional
terpenoid compounds were induced in potato tubers in response to
inoculation with pathogenic strains of F. sambucinum, and they appeared
48-70 h after inoculation. Successful infection was necessary for terpenoid
accumulation. No inducement of terpenoid accumulation occurred
following inoculation of tuber discs with non-pathogenic strains of
F. sambucinum (Ray and Hammerschmidt, 1998).
Rishitin and solavetivone were recently isolated from potato tuber
slices inoculated with Erwinia carotovora. These sesquiterpenoids
suppressed mycelial growth of the potato pathogen P. infestans on a
defined medium (Engstrom et al., 1999). A similar effect, however, was
recorded for the naturally occurring plant sesquiterpenoids abscisic acid,
cedrol and farnesol, although these compounds are found in healthy
plant tissue and are not associated with post-infection responses. At
concentrations of 50 or 100 ^g ml^ rishitin and cedrol showed the
strongest inhibitory effects on P. infestans, while solavetivone, abscisic
acid and farnesol were somewhat less inhibitory. P. infestans has
frequently been used for testing the activity of potato phytoalexins, the
criteria for inhibition being the rates of spore germination, germ-tube
elongation, mycelial growth and the inhibition of fungal glucanases
(Ishizaka et al., 1969; Hohl et al., 1980). The effects of these
sesquiterpenoids - phytoalexins as well as non-phytoalexins - were found
to be much lower than the effect of the fungicide metalaxyl. In general,
phytoalexins are not considered to be as potent as antibiotic compounds
(Kuc 1995; Smith 1996).
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