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stalk rot complex was lower in Bt-maize hybrids than in non-Bt-maize (Gatch and
Munkvold, 2001). Also the
Fusarium
ear rot incidence was reduced (Munkvold
et al.
, 1997). The type of interaction between insect and pathogen remains unclear.
Besides the formation of entry wounds for the fungi the corn borer larvae carry
spores of
Fusarium
species to the maize plant surface (Gatch and Munkvold, 2001).
Disease incidence was positively correlated with damage caused by the European
corn borer.
4.3.2 Entry by active penetration
Fungi that are able to penetrate the plant surface directly do so by enzymes that can
digest components of both the cuticle and the cell wall or by mechanical force. A
combination of both mechanisms is most likely but the relative contribution of both
are unclear. Recently, two species of the fungus-like
Oomycota
group were compared
regarding the penetration process: the phytopathogen
Pythium
graminicola
, a
widespread pathogen of
Graminaceae
, and
P. insidiosum
, a mammalian pathogen
(MacDonald
et al.
, 2002). Hyphal apices of both species exerted minor pressures (0.19
MPa for
P. graminicola
and 0.14 MPa for
P
.
insidiosum
). Measurements of the
mechanical resistance of the epidermis of the grass roots performed by
micropenetration with glass microprobes showed values of 1-12 MPa. That of
mammalian skin reached 10-47 MPa (Ravishankar
et al.
, 2001). These results show
clearly that the tissue strength exeeds the pressures exerted by hyphae of these
pathogens. Thus, the force of the hyphae of these species is not sufficient to
penetrate the tissue surfaces of their hosts without the concerted activity of secreted
tissue degrading enzymes (MacDonald
et al.
, 2002).
(a) Formation of appressoria
In many fungi, wall penetration, whether by enzyme action or by pressure, is
initiated by the differentiation of appressoria (for review, see Deising
et al.
, 2000).
Appressoria attach themselves firmly to their substrate and increase the area of
contact between the fungus and the host. Depending on the species, appressoria are
positioned over stomata or they develop in random distribution over the leaf surface.
From a pore in the appressorial base a penetration hypha (or infection peg) grows
directly through the cuticle and cell wall into the host tissue or, in the case of
dikaryotic stages of rust fungi, through the stomatal aperture into the substomal
chamber.
The production of appressoria is under both environmental and genetic control.
For several plant pathogenic fungi, such as
Magnaporthe grisea
(Xu and Hamer,
1996),
Colletotrichum lagenarium
(Lev
et al.
, 1999),
Cochliobolus heterostrophus
(Takano
et al.
, 2000) and
Pyrenophora teres
(Ruiz-Roldan
et al.
, 2001) production
was found to be under the control of an homologous mitogen-activated protein
(MAP) kinase gene. In each case these genes are not necessary for vegetative growth
but for successful infection of the host plant. As found by fusion with the green
