Agriculture Reference
In-Depth Information
In one of the relatively few reports of the use of induced resistance under field
epidemic conditions, Calonnec
et al.
(1996) showed that resistance in wheat induced
by inducer races of
Puccinia striiformis
, applied two days prior to infection, reduced
tiller disease severity due to virulent
P. striiformis
by 44-57%. However, the
durability of this resistance needs to be established. Although it is possible that
resistance elicitors act through the induction of horizontal resistance mechanisms, it
is possible that the widespread use of elicitors could lead to some erosion of their
effectiveness; however this is unlikely to lead to a sudden loss of effectiveness or to
the production of a new pathogen 'race' (Lyon and Newton, 1997). Variety-elicitor
interactions in the field have been demonstrated. Such differences in varietal
response could be due to the uptake of the elicitor, the pathogen recognition mech-
anism, the control of resistance induction, the resistance mechanism available, or to
the delivery of the resistance response (Lyon and Newton, 1997). The variability in
varietal response does suggest that reaction to resistance elicitors could be selected
for in a plant breeding programme.
5.6 NON-HOST IMMUNITY
At present it is not known whether there are any differences between the mecha-
nisms or basis of resistance expressed by non-host plants and by resistant cultivars
of the host (Heath, 1981). Immunity is definable only as resistance, of whatever
kind, which renders most plants resistant to most pathogens and most pathogens
avirulent to most plants (Browning, 1974). The fact that host specificity exists
among parasites, and in most cases has not changed over recorded history, suggests
that this form of resistance is not easily overcome (Heath, 1981). It is, therefore, not
only absolute but also of great durability. For example, Bhat and Subbarao (2001)
demonstrated that the immunity shown by broccoli (
Brassica oleracia
var.
botrytis
)
against
Verticillium
dahliae
is stable both with respect to host age and continued
exposure of
V. dahliae
isolates to brassica hosts. These authors proposed that the
mechanism of immunity was active since neither autoclaved broccoli leaves nor a
broccoli extract had any effect on the growth of the pathogen in soil, or its ability to
produce microsclerotia.
In theory, therefore, the exploitation of non-host immunity could provide an
effective and lasting control of crop diseases (Peacocke, 1995). Non-host immunity
may provide a significant contribution to resistance breeding as our knowledge of the
mechanisms involved increases. There is only limited knowledge of the possibly
complex additive and complementary nature of this form of resistance and of the
possible influence of environmental criteria for its expression (Peacocke, 1995).
5.7 TOLERANCE
Although tolerance has been expressed in a number of ways (Posnette, 1969;
Robinson, 1969; Schafer, 1971), by definition tolerant plants are susceptible to
pathogen attack but show lower levels of yield loss than those expressed by other
susceptible lines supporting the same levels of infestation (Browning, 1974;
