Agriculture Reference
In-Depth Information
such as the maintenance of healthy area through compensatory adjustments in leaf growth
(see below). The nature of the 'no disease' control is also important as fungicides may
have direct growth regulatory effects on the crop in addition to those resulting from the
control or prevention of disease. For example, greening effects and anti-gibberellin activ-
ity has been reported for some triazoles (Rademacher, 2000) and yield enhancement
with the early generation strobilurins has been associated with modifi cations to auxin
and ethylene metabolism and a delay in leaf senescence (Grossman & Retzlaff, 1997;
Grossman
et al.
, 1999). Whether the fungicides are systemic or not may also be infl uen-
tial as systemic fungicides have the potential to control asymptomatic systemic infection
such as
Ramularia collo-cygni
of barley (Walters
et al.
, 2008b). Control of asymptomatic
infection by fungicides could lead to a yield increase with no associated change in visible
symptoms giving the impression that the genotype is non-tolerant if tolerance is defi ned
as yield loss per unit of visible disease.
Exclusion of pathogen challenge may have different effects from protection using
fungicides, because challenge of resistant varieties by fungal inoculum can induce
defence reactions that are metabolically expensive. Thus, resistant varieties exposed to
inoculum may appear non-tolerant as growth may be reduced with few or no visible
symptoms of disease because assimilates are diverted from growth to defence. Newton &
Thomas (1994) found that powdery mildew inoculum challenge had a greater effect than
disease on yield loss of spring barley in glasshouse experiments, but fungicide protec-
tion compared with no protection modifi ed this relationship. It is unclear how inoculum
challenge and fungicide interactions might infl uence measurements of tolerance in the
fi eld where inoculum challenge is almost invariably present. Newton & Thomas (1994)
found no correlation between fi eld and glasshouse experiments in tolerance designations
indicating that it is diffi cult to extrapolate conclusions from glasshouse experiments to
fi eld situations.
In recent years, advances have been made in the development of techniques for quanti-
fying fungal presence within plant tissues. These include enzyme-linked immunosorbent
assays (ELISA) of fungal biomass and polymerase chain reaction (PCR) methodologies
for quantifying fungal DNA (Newton & Reglinski, 1993; Foroughi-Wehr
et al.
, 1996;
Fountaine
et al.
, 2007). These techniques have enabled the presence of pathogens to be
identifi ed in otherwise symptom-less tissue (Fountaine
et al.
, 2007).
In principle, tolerance can be quantifi ed from the relationship between fungal bio-
mass (or a surrogate measure such as the quantity of DNA) and yield (Newton
et al.
,
1998). However, tolerant genotypes identifi ed from estimates of fungal biomass do not
necessarily correspond to those identifi ed from measurements of visible symptom area
(Newton
et al.
, 1998), which may refl ect genotypic variation in the relationship between
the amount of fungal biomass in the tissue, the extent of symptom development and the
disruption caused to host physiology. Consequently, when selecting a technique for quan-
tifying tolerance it is important to be aware of what is actually being measured and the
mechanisms through which the measured variable might impact on yield. Quantifi cation
of fungal presence using ELISA or PCR methodologies is more expensive than assess-
ment of visible symptoms and the improvement in ability to predict the effects of infec-
tion on yield, if any, may not justify the extra cost. For example, ELISA assessments
of powdery mildew severity accounted for less of the variation in yield between spring
barley varieties than AUDPC scores based on visible symptoms (Newton
et al.
, 1998).
