Agriculture Reference
In-Depth Information
for appressorium formation was induced specifically by plant cutin as well as cutin
monomers, but not by structurally-related synthetic analogues (Dickman
et al.
,
2003).
4.3 ENTERING THE PLANT TISSUE
Fungal pathogens may penetrate through wounds, through natural openings such as
stomata and lenticels, through stigmas, or by breaching the plant surface and
entering by active penetration.
4.3.1 Entry through wounds or natural openings
Entering a host plant requires the recognition of possible openings. Post-harvest
diseases caused by
Botrytis
or
Monilinia
are often the result of infections through
wounds that result from handling injuries during or after harvest. In addition,
B. cinerea
(syn.
Botryotinia fuckeliana
), often invades senescent or damaged plant
tissue. Although this and other wound-infecting fungi are able to penetrate the
cuticle and cell wall directly, some factors of the wound, such as humidity or
nutrients, may stimulate spore germination. However, conidia from
B. cinerea
do
not strictly need a specific signal in order to germinate, as they usually germinate
under humid conditions. Germination is strongly stimulated by the presence of low
concentrations of glucose and organic phosphate (Rijkenberg
et al.
, 1980). It is
assumed that nutrients released from wounds of various host plants lead to greater
susceptibility to infection (Harrison, 1988). Apparently the fungus senses conditions
that are well suited for growth, resulting in the stimulation of germination.
Furthermore, some pathogens show preferential growth toward stomata. For
example, zoospores of pathogenic oomycetes enter their host plants through both
stomata and lenticels. Penetration through these natural openings requires that the
fungus can locate them. It is assumed that fungal zoospores have receptors able to
detect signals that influence the direction of motility; an excellent review of the
molecular basis of recognition between
Phytophthora
pathogens and their hosts was
published by Tyler in 2002.
Studies of
Phialophora malorum
, the causal agent of side rot (a post-harvest
disease of pears), demonstrated that infection depends on the relationship between
wound size and hydrostatic pressure in immersion tanks. Infection of wounds less
than 1 mm in diameter generally depended on immersion depth, whereas with
wound diameters more than 1 mm infection took place at all immersion depths.
Furthermore, it was shown that wound exudates stimulate spore germination (Sugar
and Spotts, 1993).
Wounds caused by physical damage from insects, hail and wind stress plants and
encourage numerous pathogens by creating entry points. For example, in maize
injuries due to the feeding of the European corn borer (
Ostrinia nubilalis
) encourage
the development of both
Fusarium
ear rot and maize stalk rot (Christensen and
Schneider, 1950). Recently, it was shown that the reduced
Ostrinia
injury in
Bt-maize hybrids lead to a shift in species composition. The species diversity of the
