are replaced by others (see Figs 11.14, 11.15). In the
decline or replant diseases this balance between the rates
of root production and root decay is altered, so that
root tip decay exceeds the rate of root tip production,
and eventually the crop dies. Species of Pythium(e.g.
P. sylvaticum) are strongly implicated in the decline
of orchard crops such as apples. Similarly, Phytophthora
cinnamomi(Oomycota) is one of the principal causes
of decline in avocado orchards and it also causes
a dieback diseaseof native eucalypt vegetation in
Australia, raising serious concerns for the future of
this unique assemblage of plant species.
One of the major lines of evidence that implicates
Pythiumand Phytophthoraspp. as the cause of decline
and replant diseases is that the acylalanine fungicides
(Chapter 17) act specifically against Oomycota, and
when these fungicides are applied to field crops ex-
perimentally (but at uneconomic levels) they can cause
spectacular improvements in plant growth. Similar
studies suggest that Pythiumspp. cause up to 10% yield
reduction in wheat crops in the Pacific Northwest of
the USA, even though the crops are apparently healthy.
These examples illustrate two important points.
First, the root tips and young tissues remain suscept-
ible to infection throughout the life of a plant. So,
even mature trees can decline if the rate of pathogen-
induced root decay exceeds the rate of root-tip pro-
duction. Second, many of these pathogens show
some degree of host-adaptation. For example, Pythium
graminicolaand P. arrhenomanesare characteristically,
but not exclusively, associated with graminaceous
hosts (the grass family), and are implicated in the pro-
gressive decline of sugar-cane crops. Experimental
studies comparing “host” with “nonhost” (nongrass)
plants strongly suggest that the host adaptation of
these two fungi is linked to their efficient zoospore
encystmenton host plants. Similarly, Phytophthora
sojaeis a host-adapted pathogen of soybean crops, and
its zoospores show strong attraction to the flavonoids
released from soybean roots (Chapter 10).
Diseases of senescence: the stalk-rot
pathogensSome fungi that damage seedlings can also grow on
senescing plant tissues, exploiting the declining host
resistance. Important examples include the stalk-rot
pathogens of maize, some of which (e.g. Macro-
phomina phaseolina) attack many crops whereas others
are quite host-specialized towards maize (Diplodia
maydis,Gibberella zeae,Colletotrichum graminicola). In any
case, their similar behavior with respect to senescence
allows them to be considered as a group.
Stalk-rot pathogens are extremely common where
maize and sorghum are grown, but they cause seriousdisease in only some sites or growing seasons, whereas
in other sites or seasons they cause little or no dam-
age. The reason is that they respond to the plant’s phys-
iology, and particularly to any stress conditions that
predispose the plants to infection. These stress factors
are many and varied. They include temporary drought
stress, poor light conditions (overcast weather), min-
eral nutrient deficiency, or insect (stem-borer) damage.
Even an exceptionally high level of grain-setting can
lead to stalk-rotting by creating a heavy demand on the
plant’s nutrient reserves. If any of these conditions
occurs at a critical stage in the crop’s growth, during
grain-filling, then the whole crop can be infected by
stalk-rot pathogens, resulting in premature death and
collapse, with heavy loss of yield (Fig. 14.4).
A unifying hypothesis to explain this type of disease
was developed early in the 1900s and was refined by
Dodd (1980). The filling of the grain in maize and
sorghum places a heavy demand on plant sugars, and
only about 80% of this sugar demand can be supplied
by “current” photosynthesis in the leaves. The other
20% is supplied from sugar storage reserves, principally
in the base of the stem. As the plant approaches matur-
ity its reserve sugars are progressively depleted, and
any stress conditions that temporarily reduce the rate
of photosynthesis at this stage cause more sugars to be
removed from the stem tissues, leading to premature
senescence. Then, the weakly parasitic stalk-rot fungi
that were already present but were held in check are
able to invade and destroy the stalk base. This is easy
to demonstrate experimentally, because stalk rot never
occurs if the developing grain is removed to eliminate
the sugar stress, but this is hardly a practical solution!
Instead, growers must use their experience to predict
the likelihood of stress conditions on their farms and
adjust the fertilizer and plant spacing accordingly. All
the fungi that cause diseases of this type are special-
ized, natural invaders of senescing plant tissues.Pathogens of fruits: the roles of pectic
enzymesRipening fruits and other sugar-rich or starch-rich
plant tissues are often rotted by fungi (Figs 14.5, 14.6).
One common example is Botryotinia fuckeliana(better
known by its asexual stage, Botrytis cinerea) which
causes gray mould of soft fruits such as strawberries,
raspberries, and grapes (see Fig. 5.20). This fungus
often initiates infection by growing on the senescing
flower remains (it is also a problem in the commercial
cut flower industry) and then invades the living tissues
of the ripening fruit. By contrast, several other fruit-rot
fungi infect through minor wounds in the fruit surface,
caused by insects or by damage during harvesting and
packing. Examples of these fungi include the “green