Agriculture Reference
In-Depth Information
12.3
Particle fi lms as agents for control of
plant diseases
The concept of hydrophobic particle fi lm technology for the control of pests and
diseases was introduced by Glenn
et al.
in 1999. Hydrophobic particle fi lms are based
on the inert mineral, kaolin, which is treated with a water-repelling agent. Kaolin is a
white, non-porous, non-swelling, non-abrasive, fi ne-grained aluminosilicate mineral
(Al
4
Si
4
O
10
(OH)
8
) that easily disperses in water and is chemically inert over a wide pH
range. Kaolin particles can be made with varying degrees of hydrophobicity by coat-
ing them with waterproofi ng agents such as chrome complexes, stearic acid and organic
zirconate. By dusting fruit trees with hydrophobic kaolin particles, Glenn
et al.
(1999)
obtained control of arthropod pests and fungal and bacterial pathogens. The authors sug-
gested that disease control was achieved because plants were enveloped in a hydrophobic
particle fi lm barrier that prevented pathogen propagules or water from directly contacting
the leaf surface. Interestingly however, hydrophilic kaolin particles can also provide plant
disease control. For example, Puterka
et al.
(2000) found that hydrophilic particle fi lms
controlled fabraea leaf spot on pear, while the hydrophilic kaolin-based product Surround
WP
®
(BASF, NJ, USA) was shown to control
Zygophiala jamaicensis
and
Gloeodes
pomigena
on apple fruits and
Phoma
sp. on apple leaves, although it was not consistently
effective against the cedar apple rust pathogen
Gymnosporangium juniperi-virginianae
(Thomas
et al.,
2004). In a recent four-year study, hydrophobic kaolin particle fi lms failed
to control
Cladosporium carpophilum
or
Podosphaera leucotricha
on peach, although it
did control
Monilinia fructicola (
Lalancette
et al.,
2005). In contrast, hydrophilic kaolin
particle fi lms did not control any of the peach pathogens, leading the authors to suggest
that hydrophobicity and deposit density may be important factors for effective disease
management (Lalancette
et al.,
2005).
Whether or not kaolin particle fi lms provide disease control, other benefi cial effects of
applying such agents have been observed. For example, liquid formulations of both hydro-
phobic and hydrophilic particle fi lms were shown to double yields of pear in fi eld trials,
while delayed fruit maturation, increased fruit size, increased fruit number and increased
fruit yield were obtained following use of kaolin particle fi lms (Lalancette
et al.,
2005).
In the latter case, the authors attributed the effects to a reduction in plant stress. In fact,
reduced plant stress during extreme temperature conditions was also observed following
applications of kaolin particle fi lms to apple trees (Thomas
et al.,
2004).
12.4
It is thought that adhesion of fungal spores to the plant surface is the essential fi rst step
in the infection process (e.g. Epstein & Nicholson, 1997). In
Magnaporthe grisea
, the
causal agent of rice blast, hydration of conidia leads to the release of an adhesive muci-
lage from the spore tip that forms a viscous pad for attaching the conidium to the plant
surface (Hamer
et al.
, 1988). Once a spore has successfully adhered to the leaf surface,
germ tube formation is rapid, whereas if conidia fail to adhere to the substrate, viability is
rapidly lost (Talbot, 1995). Once formed, germ tubes must also adhere to the plant surface,
since this process is important in the perception of signals for further differentiation, for
example formation of the appressorium. The importance of spore adhesion to successful
Disrupting spore adhesion to the leaf surface
