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90 CHAPTER 5

survive desiccation or exposure to UV irradiation. So,

the spores often germinate immediately and the germ-

tube produces a melanized appressorium which persists

on the host surface until the onset of host senescence.

Some of the fungi that depend on melanized

infection structures can be controlled by antifungal

antibiotics that specifically block the pathway of

melanin biosynthesis. Both Magnaporthe grisea and

Rhizoctonia oryzae(which causes sheath blight of rice)

are examples of this, although the level of disease con-

trol in practice has been disappointing because these

pathogens can easily mutate and become resistant to

the antibiotics (Chapter 17).

The morphogenetic triggers for

differentiation of infection structures

Several physical and chemical factors have been

reported to influence the development of appressoria

and other infection structures, but the main require-

ment is contact with a surface of sufficient hardness.

In vivothis could be a leaf cuticle or an insect cuticle.

In vitroit can be simulated by various artificial mem-

branes. Thus, contact-sensingseems to be one of the

key morphogenetic triggers.

Contact-sensing can be either topographical or

nontopographical. In nontopographical contact-

sensing the fungus merely responds to the presence

of a hard surface, and this is true of the air-borne

conidia of Blumeria (Erysiphe) graminis(powdery mildew

of cereals). These spores have minute warts on their

surface, and within a few minutes of landing on a leaf

or of being placed on a glass surface, the warts in con-

tact with the surface secrete an adhesive containing

wall-degrading enzymes. This is a localized response

because the warts on the rest of the spore surface do

not secrete the adhesive. By contrast, topographical sens-

ing is more specific, because the fungus responds to

ridges or grooves of particular heights (or depths) or

spacing on the host surface, and recognition of this

surface topography is used to locate the preferred

infection site. Examples of this are found in the germ-

tubes produced from the uredospores of several cereal

rust fungi (Puccinia graminis, P. recondita, etc.). As

shown in Figs 5.5 and 5.6, the germ-tubes initially grow

at random on cereal leaves, but when they encounter

the first groove on the leaf surface (the junction of

two leaf epidermal cells) they orientate perpendicular

to this groove and then grow across the leaf surface.

This is thought to maximize the chances of locating

a stomatal pore, because the stomata are arranged

in staggered rows along the leaf. These fungi show

precisely the same behavior on inert grooved surfaces

such as leaf replicas made of polystyrene, confirming

that the response is to topographical signals and not

to chemical stimuli.

The “nose-down” orientation of the germ-tube tips

shown in Fig. 5.6 indicates that the Spitzenkörper is

Fig. 5.3(a – c) Bananas infected by Colletotrichum musae, photographed over a 10-day period after the fruit ripened.

Small brown flecks on the fruit surface (a) result from the activation of single-celled, melanized appressoria that remained

dormant for several months. The lesions progressively expand and coalesce, leading to softening and over-ripening of

the fruit tissues.

(a) (b) (c)

Fig. 5.4Dense clusters of flask-shaped phialides (P) and
spores (S) of Colletotrichum musaescraped from the sur-
face of a brown lesion on a ripe banana.