Agriculture Reference
In-Depth Information
Chapter 12
Control of plant disease by disguising
the leaf surface
Dale Walters
Scottish Agricultural College, Edinburgh, UK
12.1
Parasitism is essentially a nutritional relationship and if pathogens are to gain access to
the nutrients available in the apoplast and within plant cells, they must fi rst get through the
outer layers of the plant. On leaf surfaces, the fi rst layer the pathogen will encounter is the
cuticle, the insoluble polymeric compounds of which constitute the main physical obstacle
to penetration by fungal pathogens (Royle, 1975). Plant surfaces may also be a determi-
nant in the recognition and attachment of fungal spores. For example, evidence provided
by Wright
et al
. (2002) suggests that chemical interactions on the barley leaf surface
are required for adhesion of powdery mildew conidia, with consequences for subsequent
development of the powdery mildew germling. Interestingly, for rust fungi, germinated
uredospores 'sense' the leaf surface and several steps are involved in germ tube tropism
on such surfaces: adhesion, directional growth, appressorium formation over stomatal
openings, directional emergence of infection pegs and adherence of haustorial mother
cells (Figure 12.1) (Wynn & Staples, 1981). This germ tube tropism is greatly infl uenced
by surface features of the leaf and any alteration of the leaf surface can alter topography
and infl uence the tropism of uredospore germ tubes. Thus, removal of epicuticular waxes
may induce tropism-related mistakes and result in reduced infection frequencies (Wynn,
1981). More recent work (Collins
et al
., 2001) provided evidence for the involvement of
both topographical and chemical signals associated with the wheat stomatal complex, in
the induction of appressoria by the stem rust fungus
Puccinia
graminis
f. sp.
tritici
.
The importance of leaf surface features in the early development and establishment of
foliar pathogens suggests that interference with leaf topography will disrupt pathogen devel-
opment and lead to reductions in infection. Research on a variety of agents which coat the
leaf surface has shown that interference with leaf topography can indeed lead to reductions in
pathogen infection. The following sections of this chapter deal with the various approaches
that have been used to coat or disguise leaf surfaces in order to control foliar pathogens.
Introduction
12.2
Controlling disease using fi lm-forming polymers
Film-forming polymers are widely used in agriculture and horticulture as antitranspirants
and as spray adjuvants. Their main use as adjuvants is as fi lming agents to reduce weathering
