Agriculture Reference
In-Depth Information
(a)
(b)
Figure 12.1
(a) Scanning electron micrograph of germ tubes growing from uredospores (u) of
Puccinia
graminis
on an inert replica of a wheat leaf. Growth of the germ tubes (seen as dark, narrow lines) is ori-
entated at right angles to the pattern of ridges and grooves of the leaf surface cells. Lines of stomata (s)
occur at intervals across the leaf surface replica. (b) Scanning electron micrograph of germ tube tips of
Puccinia graminis
growing perpendicular to precisely spaced ridges and grooves of a polystyrene replica
of a microfabricated silicon wafer. The germ tubes are about 4 µm in diameter; the lower region of their tips
is fl attened against the replica, presumably enabling them to sense the topography. Courtesy of Professor
Nick Read, University of Edinburgh.
and extend pesticide effi cacy, and as stickers or spreaders to improve distribution and
adherence of agrochemicals (Backman, 1978). Film-forming polymers used as anti-
transpirants include waxes, silicons and a variety of plastic polymers. When used in this
role, they form a fi lm over the stomata, increasing resistance to water vapour loss (Gale
& Hagan, 1966) and are used particularly on seedlings and transplants to decrease water
stress and wilting and to improve water use effi ciency in arid conditions (Quarles, 1991).
The fi rst report of disease control provided by an antitranspirant was made nearly
50 years ago. During fi eld trials of the effect of an antitranspirant on the water balance of
sugar beet, Gale & Poljakoff-Mayber (1962) found that the incidence of powdery mildew
was reduced on treated plots. A number of studies on fi lm-forming polymers have found
several to be effective in controlling foliar pathogens of a variety of plants, including
cereals (Ziv & Frederiksen, 1983, 1987; Walters, 1992), vegetables (Osswald
et al
., 1984;
Han, 1990) and fruit (Han, 1990). Elad
et al
. (1989) examined the effects of four fi lm-
forming polymers (Vapor Gard
®
[1-p-menthene], Wilt Pruf
®
[
-pinene], Safe Pack
®
[a wax emulsion] and Colfi x
®
[40% polyvinyl]) and a number of commercial fungicides,
including fenarimol, on control of powdery mildew on cucumber. In pot experiments
they found that Vapor Gard and Wilt Pruf reduced powdery mildew on cucumber by up
to 82% and 55%, respectively, while under cover, Safe Pack and the fungicide fenarimol
reduced infection by up to 67% and 96%, respectively (Elad
et al
., 1989). Cucumber
powdery mildew was controlled most effectively by a mixture of Safe Pack and fenari-
mol. According to Elad
et al
. (1989), fi lm-forming polymers can only form an effective
barrier on the leaf surface if applied at concentrations greater than 3%. Since powdery
mildew infection of cucumber was reduced by concentrations of polymers between 0.5%
and 3%, they suggested that the polymers were exerting a direct effect on the pathogen.
In fact, their studies showed that fi lm-forming polymers reduced germination of powdery
mildew conidia when used at concentrations as low as 0.5% (Elad
et al
., 1989). Similar
results were obtained with the grey mould pathogen
Botrytis cinerea
. Here, the fi lm-
β
