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barriers to penetration or fungitoxic chemicals, and

they can either be preformed or formed in response

to infection. In the following sections we discuss

the characteristic features of these host-specialized

pathogens.

The role of inoculum: a comparison of

Botrytis fabaeandB. cinerea

Botrytis cinereaand B. fabaeare closely related fungi,
but B. cinereainvades the compromised tissues of
many plants, whereas B. fabaeis a host-specialized
pathogen of broad bean (Vicia faba). Both fungi pro-
duce ovoid conidia, but those ofB. fabaeare larger
(20 –25μm long) than those of B. cinerea (10 –15μm),
representing a roughly sixfold difference in spore
volume. As we saw in Chapter 10, larger spores have
a higher efficiency of impaction onto leaves, especially
onto wide leaves such as those of broad bean plants.
Large spores also have more endogenous food reserves
to support infection.
The spores of both B. fabaeand B. cinereagerminate
when placed in water drops on the surface of broad
bean leaves, but B. cinereaoften fails to penetrate the
leaf cuticle whereasB. fabaepenetrates and causes a
rapid host response – the death of the penetrated cells
and their immediate neighbors, giving a large necrotic
spot (Fig. 14.8). This reaction to infection is termed the
hypersensitive response, and it is associated with an
accumulation of fungitoxic metabolites in the dead cells,
often sufficient to prevent further infection.
The difference in initial penetration by theBotrytis
species is largely explained by their nutrient reserves.
B. cinerea will penetrate and cause a necrotic spot if
sugars are added to the inoculum drop, or sometimes
even if the normal leaf surface microorganisms are killed
by antibiotics so that B. cinereacan use the leaf-surface

exudates. Conversely, aged spores of B. fabae, taken from

old parts of agar colonies, can fail to penetrate because

their endogenous reserves have been depleted, but they

penetrate if supplied with exogenous sugars. Thus, B.

cinereahas evolved a strategy as a general invader of

senescent tissues: it produces many more spores from

a given amount of resource but these spores depend

on nutrients released from compromised host tissues.

B. fabaeproduces fewer spores from the same amount

of resource, but each of them has a significant chance

of initiating infection of the healthy host.

Such differences in the infectivity of pathogens can

be quantified, to predict the likelihood of infection in

different conditions. Fig. 14.9 shows this forB. cinerea

and B. fabaewhen water drops containing different

numbers of spores were placed on broad bean leaves.

Infection was scored as the presence of absence of a

necrotic spot beneath each water drop. The results are

plotted as the logarithm of spore dose (spore number)

against the percentage response (infection, expressed

as a probit value, where the probit of 5=50%). This

type of plot converts a skewed sigmoid curve into a

straight line, and the estimated doseof spores that gives

50% probability of infection (the ED 50 value) can be

read from the graphs. The same type of plot is used

widely in plant and animal experiments to obtain

LD 50 values for lethaltoxicity of chemicals, etc. The host-

specialized fungusB. fabaewas found to have an ED 50

of 4 in water drops, corresponding to a 16% chance

of any single spore being able to initiate infection. By

contrast, B. cinereahad an ED 50 of about 500, corres-

ponding to a 0.14% chance of a single spore initiating

infection.

Experiments of this type provide information relev-

ant to field conditions. For example, several different

Colletotrichumspp. produce leaf spots on rubber leaves

in Malaysia. By inoculating spores in droplets it was

possible to predict: (i) the species most likely to infect

from low spore numbers and (ii) the age at which the

leaves are most infectible, before the development of

a thick leaf cuticle prevents any further infection.

The roles of tissue-degrading enzymes

Many necrotrophic fungi produce tissue-degrading

enzymes as a primary mechanism of pathogenesis,

like the pectic enzymes discussed earlier. But the grass

family, Gramineae, seems to be unusual because grasses

have different wall matrix components compared with

many other plants. Instead of pectin in the middle

lamella they have mixed polymers of β-1,4-xylan and

α-1,3-arabinose, with lesser amounts of noncellulosic

β-1,4- and β-1,3-linked glucans. Cooper et al. (1988)

investigated the enzymes produced by three stem-base

pathogens of cereals – Rhizoctonia cerealiswhich causes

288 CHAPTER 14

Fig. 14.8Broad bean leaflets inoculated with water

drops containing spores of different Botrytisspp. Only B.

fabaeis a host-specialized pathogen of broad bean plants;

the other Botrytisspp. have different hosts, but B. cinerea

is a weak pathogen. (Courtesy of J. Mansfield.)