Agriculture Reference
In-Depth Information
cells, granulation of cytoplasm, increased branching and collapsed empty cells (Sutherland
& Walters, 2001). The mechanism(s) by which Bond and Vapor Gard alter hyphal
morphology is not known. However, Ethokem is a cationic surfactant and as such may
be active at the cell membrane. Wade
et al
. (1993) reported that cationic surfactants such
as ethoxylated tallow amines increase plasma membrane permeability in plants. The pos-
sibility that Ethokem may exert its effects on hyphal morphology by altering the perme-
ability of fungal membranes is worthy of investigation.
From the above, it is unclear whether the fi lm-forming polymers act either as a physical
barrier or to disguise the leaf surface. In fact, two mechanisms have been proposed to
account for the effects of fi lm-forming polymers in reducing fungal infection on leaf sur-
faces (Gale & Poljakoff-Mayber, 1962; Ziv & Frederiksen, 1983; Osswald
et al
., 1984).
First, it has been suggested that leaf surfaces coated with polymers are hydrophobic,
leading to low water potential at infection sites (Gale & Hagan, 1966). Second, various
workers have suggested that coated leaf surfaces may be impenetrable due to the thick-
ness, hardness, or resistance to enzymic attack of the fi lm-forming polymer (e.g. Ziv &
Frederiksen, 1983). Working on the effects of fi lm-forming polymers on development of
the rust
Puccinia recondita
f. sp.
tritici
on wheat, Zekaria-Oren & Eyal (1991) found that
polymers applied prior to inoculation had a greater effect on rust infection than compounds
applied post-inoculation. They found that increasing the concentration of the fi lm-form-
ing polymers led to a progressive reduction in infection intensity, suggesting that effi cacy
was related to thickness and uniformity of the coat on the leaf surface. Using fl uorescence
microscopy and scanning electron microscopy, these workers found that surfaces coated
with fi lm-forming polymers interfered with fungal penetration of the leaf (Zekaria-Oren &
Eyal, 1991). They observed that both the orientation of the germinating uredospore
towards the stomata and the formation of appressoria were altered on coated surfaces.
The leaf surface is known to provide various stimuli that orient the germinating uredo-
spore towards a stoma (Wynn, 1981; Wynn & Staples, 1981) and induce appressorium
formation (Collins
et al
., 2001). Zekaria-Oren & Eyal (1991) suggested that the sig-
nifi cant reduction in appressorium formation and the distribution of appressoria on the
coated leaf surface was associated with disruption of mechanisms responsible for ori-
entation of the germinating uredospores towards stomata and triggering appressorium
formation. In their work on the effects of fi lm-forming polymers on powdery mildew
infection of barley, Sutherland & Walters (2002) suggested that the polymers might have
disrupted the 'fi rst touch' phenomenon described by Neilsen
et al
. (2000). The 'fi rst
touch' of conidia of
Blumeria graminis
on the barley leaf surface is associated with
conidial uptake of anionic, low-molecular-weight materials before germination. Neilsen
et al
. (2000) suggest that this could be a mechanism for recognition of the host and deter-
mination of the direction of growth of the emerging germ tube toward the leaf surface.
There is an urgent need for work on the mechanisms by which fi lm-forming polymers
reduce fungal infection on leaf surfaces. Since the fungal pathogen on a leaf surface
will capitalise on any break or weakness in the fi lm produced by the polymer, research
on polymers with increased stretching or 'self-healing' properties would also be use-
ful. Interestingly, polymer composites with 'self-healing' properties have been reported
(White
et al
., 2001; Brown
et al
., 2004), although such polymers have been designed for
military and medical purposes and are likely to be too costly and impractical for crop
protection purposes.
