INFECTION STRATEGIES OF PLANT PARASITIC FUNGI - The Epidemiology of Plant Diseases - page 119

host cuticle. Apparently, adhesion of the pads is improved by these enzymes. The
spores have reduced ability to attach to the leaf surface when these enzymes are
inactivated (Deising et al. , 1992).
In contrast to propagules of most fungi, conidia of barley powdery mildew
( Blumeria graminis, syn. Erysiphe graminis ) begin the process of adhesion in the
absence of free water in a wide range of high relative humidities. Carver et al.
(1995) showed that the extracellular material released by germlings of B. graminis
f.sp. avenae sticks the fungus firmly to the leaf surface. The conidia of B. graminis
f.sp. hordei produce an esterase containing liquid in response to a non-specific
contact stimulus with the barley leaf surface or to a moistened cellophane surface
(Nicholson et al. , 1988). Furthermore, this exudate contains cutinase activity
(Pascholati et al. , 1992) that alters the cuticle surface and may help the fungus to
penetrate the leaf surface more efficiently.
4.2.2 Host surface perception
During the early phase of the infection process before invading the plant tissue, the
developing fungus greatly depends on favourable environmental conditions such as
surface moisture, relative humidity, temperature and light. In addition, nutrient
availability is an important stimulus especially for necrotrophic pathogens. The
precise mechanism by which germination occurs is an interplay of several factors.
Spore germination, growth direction of germ tubes and subsequent induction of
appressorium formation all have been shown to be triggered by chemical or physical
features of the substratum. To place the appressorium in the optimal site for
penetration, a recognition event is required which includes topographical, chemical
or environmental signals (for review, see Staples and Hoch, 1997).
The germ tubes of several pathogenic fungi have been described as growing
along the anticlinal cell walls of their host plants. In vitro studies of germinating
spores of Cochliobolus sativus indicate that both chemical and topographic signals
given by anticlinal cell wall junctures over epidermal cells are involved in the
appressorium induction (Clay et al. , 1994).
Appressorium formation by urediospore germ tubes of the bean rust Uromyces
appendiculatus is induced by physical differences in the topography of the leaf
surface, such as stomatal lips of guard cells, or by defined ridges of 0.5 µm height
formed on an artificial surface (Hoch et al. , 1987; Kwon and Hoch, 1991). In
addition, it has been shown that many rust fungi exhibit species-specific responses
on membranes with defined topographies (Allen et al. , 1991).
Surface contact on host leaves or artificial substrates was found to be essential
for the formation of appressoria by germ tubes of Magnaporthe grisea (Xiao et al. ,
1994). Furthermore, a high surface hydrophobicity and light favoured the formation
of appressoria but these factors were not essential (Jelitto et al. , 1994). Gilbert et al.
(1996) reported evidence for the regulation of appressorium formation of M. grisea
by chemical signals: they found that plant cutin monomers of host plants and non-
host plants induced infection structure formation. From Colletotrichum trifolii , the
causal agent of alfalfa anthracnose, a lipid-induced protein kinase (LIPK) required