Agriculture Reference
In-Depth Information
Buddenhagen and de Ponti, 1983). Nelson (1973) suggested that tolerance negates
infection by desensitization, whereas hypersensitivity negates infection through the
localization of infection sites. Recently, more refined definitions of tolerance have
been proposed. For ecologists, plant fitness, measured for example by flowering
frequency and plant survival under epidemic conditions, have been used as
indicators of tolerance. In natural plant communities, Roy
et al
. (2000) cited
examples of small fitness consequences of infection despite levels of disease
incidence ranging as high as 100%, suggesting low resistance and either high host
tolerance or low pathogen virulence. Evidence for tolerance would be indicated if
host genotypes that were exposed to the same pathogen isolate, and showed identical
amounts of infection (and thus showing similar resistances), had different fitness
consequences of infection (Roy
et al
., 2000) (Fig. 5.1). Thus when disease pressure
is high tolerance is likely to have physiological costs due to shifting allocation away
from growth to re-growth (Simms and Triplett, 1994).
For agronomists and plant pathologists, an examination of the physiological
manifestations of tolerance and the consequent impact on disease progress are
usually more desirable. Thus, comparisons of citrus rootstocks have demonstrated
that infection of young roots occurs very rapidly and at the same rate on susceptible
and tolerant hosts, but that root rot damage, due to
Phytophthora
nicotianae
and
fungal reproduction are subsequently limited on tolerant hosts (Graham, 1995). A
further refinement, for quantitative epidemiological studies was made by Dan
et al
.
(2001) reporting the use of PCR diagnostics to identify tolerance in potato clones to
Verticillium dahliae
, where a clone was designated as tolerant if the amount of
fungus present in the host tissue was equal to or greater than the collective average
amount for all clones in the symptom category above.
Figure 5.1. Norms of reaction diagram illustrating differences among three host genotypes in
their ability to tolerate one pathogen strain. Each line represents an idealized regression line,
the slope of which measures tolerance (adapted from Roy
et al
., 2000).
