Agriculture Reference
In-Depth Information
Incident
radiation
Fungal
biomass
Visible disease
symptoms
Disruption to
host physiology
Intercepted
radiation
RUE
Biomass
Partitioning
Yield
Figure 7.1
Schematic representation of mechanistic links between fungal infection and reduction in
crop yield. The broken arrow refers to possible disruption to host physiology prior to formation of, or in
the absence of, visible symptoms of infection.
pathogens on radiation interception and RUE, because estimates will depend on whether
radiation interception has been assessed on the basis of total leaf area (diseased plus
non-diseased) or just green (healthy) area. Thus in contrast to the above work where
P. infestans
was reported to reduce radiation interception and not RUE when assessed
on the basis of green tissue,
Mycosphaerella pinodes
of pea reduced RUE but not radia-
tion interception when interception was assessed on the basis of total leaf area (Béasse
et al.
, 2000).
Infection by necrotrophs can result in altered partitioning of carbohydrates and inor-
ganic nutrients, but effects tend to be less pronounced than those observed with biotrophic
pathogens (Whipps & Lewis, 1981). Thus export of photosynthates from infected leaves
can be reduced, and in some cases, accumulation of nutrients occurs at the infection sites
where rates of water loss are high (Ayres & Jones, 1975; Whipps & Lewis, 1981).
In general, host-pathogen interactions with biotrophic fungi are more complex than
those with necrotrophs, as they derive their resources for growth and sporulation from
living host cells (Walters
et al.
, 2008a). Pathogens, such as rusts and mildews, reduce light
interception by masking the leaf surface with pustules and by accelerating leaf senescence
(Spitters
et al.
, 1990). Infection may cause a temporary stimulation in the rate of photo-
synthesis, but then the rate usually declines and chlorophyll is lost (Scholes, 1992). Rates
of respiration are also typically increased. Together these effects on photosynthesis and
respiration may reduce the RUE of infected leaves (RUE defi ned as dry matter production
per unit of PAR intercepted by green tissue), since net carbon fi xation can be reduced in
non-symptom expressing (i.e. green) parts of infected leaves (Kral
et al.
, 1993; Scholes &
Rolfe, 1995). RUE might also be infl uenced by effects of pathogens on stomatal regulation
and the alteration of normal water relations caused by rupture of the epidermis during
sporulation (Ayres, 1981b; Prats
et al.
, 2006). Biotrophic fungi signifi cantly alter the par-
titioning of carbohydrates and inorganic nutrients within the host plant since the growing
