Agriculture Reference
In-Depth Information
In spite of the lack of consensus in the literature about the relative importance of disease
versus pathogen tolerance to overall tolerance, we consider that useful progress can be
made in identifying traits that may contribute to tolerance provided that terminology is
defi ned clearly from the outset. In fact there is a danger in setting criteria for identifying
cases of tolerance that are too stringent because it can make quantifying tolerance more
diffi cult, especially in large fi eld experiments, thereby hindering rather than facilitating
progress. A further consideration is that for an improved understanding of tolerance to be
exploited commercially, it must be defi ned and measured in terms that relate to disease
management practice. At present, management decisions for most foliar pathogens are
based on assessments of visible infection or disease symptoms. Understanding and
predicting the extent of yield loss for a given severity of visible infection will aid deci-
sions regarding the need for fungicide in a particular crop or variety. Thus, for the purpose
of this chapter we defi ne tolerance as less than expected yield loss in response to a given
severity of visible disease symptoms or pathogen-induced loss of green area.
7.3
Yield formation in many crop species has been analysed in terms of the quantity of
photosynthetically active radiation (PAR) incident upon the crop (Q), the fraction
of the radiation intercepted by green tissue (I), the effi ciency with which the energy
from intercepted PAR is converted into dry matter (radiation use effi ciency, RUE) and
the partitioning of dry matter into the harvested parts (harvest index, HI). This can be
summarised as follows:
Yield formation
Y = Q
*
I
*
RUE
*
HI
(7.1)
Equation (7.1) has provided the conceptual framework for modelling the effects of
pathogens and pests on crop growth and yield (Johnson, 1987; Waggoner, 1990; Rossing
et al.
, 1992; Gaunt, 1995; Paveley
et al.
, 2001; Bancal
et al.
, 2007) and is a useful basis
for identifying potential tolerance traits. Pathogen infection and subsequent disease can
reduce yield through effects on radiation interception, RUE and dry mater partitioning to
yield bearing structures, as illustrated in Figure 7.1. A more quantitative treatment of the
relationships linking symptom area and yield is given by Gaunt (1995) and Paveley
et al.
(2001). It follows, that tolerant genotypes will be those that possess traits that enable them
to maintain high levels of PAR interception, RUE, and the formation of and partitioning
of dry matter to yield bearing structures in spite of pathogen infection.
The type and extent of damage infl icted by foliar pathogens depends on the species in
question and its mode of nutrition (Walters
et al.
, 2008a). Necrotrophic fungi kill tissue
in advance of colonisation by fungal hyphae. Tissue death results in a loss of green area
and some shrinkage of the leaf surface. Necrotic regions within leaves may continue
to intercept light, but without contributing to photosynthesis. Typically, necrotrophs and
hemibiotrophs have relatively limited effects on host photosynthetic metabolism. Thus,
yield loss of potatoes following infection by
Phytophthora infestans
, the cause of late
blight, has been attributed to the reduction in light interception by green tissue, with
no change being found in RUE (van Oijen, 1990, 1991; Rossing
et al.
, 1992). We must
sound a word of caution at this point. Care must be taken when interpreting the impact of
