Agriculture Reference
In-Depth Information
traits on tolerance, and for quantifying the potential costs of these traits in the absence of
disease. Figure 7.4 shows the results of a sensitivity analysis in which the effect of dis-
ease on above ground dry matter production of spring barley was estimated after varying
several leaf and canopy traits. The traits varied were canopy size (GAI), extinction coef-
fi cient (
k
), the virtual lesion size (
; Bastiaans, 1991), and changes in photosynthetic rate
and size of the fl ag leaf in response to disease on the lower leaves. The analysis demon-
strates that for spring barley, changes in canopy morphology had a relatively small effect
on tolerance compared to compensatory adjustments in leaf growth and photosynthetic
rate or virtual lesion size. This type of approach can be used to target research at those
traits likely to have the greatest impact on tolerance.
β
7.8
Although pathogen tolerance has the potential to contribute to disease management,
for most crop species we are not yet in a position to be able to include tolerance as an
objective in plant breeding programmes. Further research effort is needed before this can
be achieved. The necessary steps involved are outlined below.
Strategy for improving tolerance
(a)
Tolerance traits must be identifi ed for individual pathosystems. A number of candidate
traits have been highlighted above, but traits appropriate for one pathosystem may not
be suitable for another because of differences, for example, in host phenology, timing
of epidemic development, the nature of the infection and the type of damage induced.
The extent of intra-specifi c variation in the traits identifi ed must be determined.
(b)
For some traits, for example the light extinction coeffi cient, there is evidence of
variation within some species that could be exploited. However, for most traits little
is known about the extent of any variation, and also, importantly, how the trait of
interest infl uences tolerance in the fi eld. A major shortcoming of much of the research
on tolerance to date is that few fi eld studies have quantifi ed both tolerance and the
expression of putative tolerance traits simultaneously. If insuffi cient variation is found
within the existing germplasm, it may be necessary to look to wild relatives of crops
for a source of tolerance (Sabri
et al.
1997; Akhkha
et al.
, 2003). However, incor-
porating genes from wild relatives into the adapted background of elite germplasm
requires a considerable breeding effort (Ellis
et al.
, 2000).
Determine the heritability of the trait in fi eld crops. Ideally the expression of tolerance
(c)
traits should be relatively insensitive to environmental conditions, so that tolerance
can be used as a predictable and robust component of disease management. An
improved mechanistic understanding of tolerance and the underlying traits that confer
it will provide greater predictability should expression prove to be sensitive to some
environmental conditions (Tiffi n, 2000). Tolerance could then still be of practical
value in particular crop situations.
Screening and selection of tolerance. Depending on the nature of the tolerance trait,
(d)
specifi c high-throughput screening or marker-assisted selection techniques may need
to be developed to enable the trait to be selected within a breeding programme.
The fi nal stage of the process will be to incorporate disease tolerance into an
(e)
integrated disease management programme. Tolerance is unlikely to give suffi cient
protection of yield on its own, but in situations where resistance is incomplete, or
