Agriculture Reference
In-Depth Information
In this chapter examples of fungi and fungi-like parasites with widely differing
pathogenic lifestyles are introduced and phases of their infection processes are
described.
4.2 THE PRE-PENETRATION PHASE
4.2.1 Attachment of fungal spores
The initial step of establishing infection is the adhesion of fungal propagules to the
plant surface. A binding process is essential to resist displacement by wind or water.
Although adhesion to plant surfaces is common among all fungal species, there are
differences in the factors that induce the attachment process and in the composition
of adhesive material. Reviews concerning these earliest events of fungal infection
are reviewed by Nicholsen (1996) and Tucker and Talbot (2001).
Of special importance is the attachment of zoospores of soilborne oomycetes
living in water-saturated soil to make sure that the pathogen is not washed away by
water before invasion of the host tissue can occur. Zoospores of
Phytophthora
cinnamoni
that adhere to a surface during the first 3-4 min of encystment remain
strongly attached. It was found that, although the adhesive material was still present,
cell adhesiveness had declined rapidly 5 min later (Gubler
et al.,
1989).
Considering the differences in surface composition and properties (e.g.
hydrophobicity) of aerial plant organs and roots, it seems obvious that fungi use
different mechanisms to bind to the host surface. One important factor is the amount
of water available during infection. In some cases, the hydration of fungal
propagules leads to a rapid release of mucilage that is involved in a passive, non-
specific adhesion to a variety of substrates. For example, the spores of the rice blast
fungus
Magnaporthe grisea
release a carbohydrate-containing adhesive material
from the tip region of the germ tube as a result of hydration expansion (Howard
et al.
, 1991). About 20 min after hydration, conidia of
Cochliobolus heterostrophus
secrete a material at the spore tips that serves as non-specific adhesive (Braun and
Howard, 1994).
Ungerminated conidia of
Colletotrichum graminicola
, the causal agent of
anthracnose (leaf blight of corn), begin to adhere within minutes of their contact
with the leaf surface. Adhesion is shown to be essential to the success of disease
establishment (Mercure
et al.
, 1994). The spores produce a water-soluble glyco-
protein-rich material with properties to protect spores against unfavourable
conditions such as dry periods (Mercure
et al.
, 1995).
Weak adhesion of germlings of
Botrytis cinerea
to hydrophobic substrates
occurs immediately upon hydration. In a second stage, viable conidia strongly attach
to either hydrophobic or hydrophilic substrates by means of secretion of a sheath of
material (Doss
et al.
, 1995).
Contact and adhesion to the substratum has been found as a prerequisite for the
germination of several phytopathogenic fungi, for example
Bipolaris sorokiniana
(Apoga
et al.
, 2001). The uredospores of the rust fungus
Uromyces fabae
form an
adhesion pad and release a cutinase and two specific esterases after contacting the
