Introduction to Fungi, Third Edition

112 STRAMINIPILA: OOMYCOTA
firmly to the leaf. Penetration of the cell wall is
probably achieved by a combination of mechanical
and enzymatic action and can occur within
2 h. Within the leaf tissue, an intercellular
mycelium develops and haustoria are formed
where hyphae contact host cell walls (Fig. 5.21).
The resulting lesion acquires a dark green watersoaked
appearance associated with tissue disintegration
(Plate 2e). Such lesions are visible
within 3 5 days of infection under suitable conditions
of temperature and humidity. Around
the margin of the advancing lesion on the lower
surface of the leaf, a zone of sporulation is found
in which sporangiophores emerge through the
stomata (Fig. 5.20a). Sporulation is most prolific
during periods of high humidity and commonly
occurs at night following the deposition of dew.
In potato crops, as the leaf canopy closes over
between the rows to cover the soil, a humid
microclimate is established which may result
in extensive sporulation. As the foliage dries
during the morning, the sporangiophore undergoes
hygroscopic twisting which results in the
flicking-off of sporangia. Thus the concentration
of sporangia in the air usually shows a
characteristic diurnal fluctuation, with a peak
around 10 a.m. Although sporangia can survive
drying if they are rehydrated slowly (Minogue &
Fry, 1981), in practice the long-range spread of
inoculum is probably by sporangia in contact
with water drops (Warren & Colhoun, 1975).
The destructive action of P. infestans is directly
associated with the killing of photosynthetically
active foliage. When about 75% of the leaf tissue
has been destroyed, further increase in the
weight of the crop ceases (Cox & Large, 1960).
Thus, the earlier the onset of the epidemic, the
more serious the consequences. To a certain
extent, the crop reduction may be offset by the
fact that epidemics are more common in rainy
cool seasons which are conducive to higher crop
yields.
Phytophthora infestans can also cause severe
post-harvest crop losses because tubers can be
infected by sporangia falling onto them, either
during growth or lifting. Such infected tubers
may rot in storage, and the diseased tissue is
susceptible to secondary bacterial and fungal
infections.
Chemical control
By spraying with suitable fungicides, epidemic
spread of the disease can be delayed. This results
in a prolongation of photosynthetic activity of
the potato foliage and hence an increase in yield.
Fungicides developed against the Eumycota are
often ineffective against Oomycota such as
Phytophthora because the latter differ in fundamental
biochemical principles, including many
of the molecular targets of fungicides active
against Eumycota (Bruin & Edgington, 1983;
Griffith et al., 1992). In 1991, about 20% of the
total amount of money spent on chemicals for
controlling plant diseases worldwide was used
for the control of Oomycota (Schwinn & Staub,
1995).
The first of all fungicides was Bordeaux
mixture, an inorganic formulation containing
copper sulphate and calcium oxide which was
found to be effective against downy mildew
of vines caused by Plasmopara viticola, another
member of the Oomycota (see p. 119; Large, 1940;
Erwin & Ribeiro, 1996). Oomycota in general are
extremely sensitive to copper ions, and Bordeaux
mixture is still widely used (Agrios, 2005).
The dithiocarbamates such as zineb or
maneb (Fig. 5.27a) were among the first organic
fungicides to be developed. They act against
a wide range of fungi, including Oomycota,
because of their non-selective mode of action.
The molecule is sufficiently apolar to diffuse
across the fungal plasma membrane; once inside,
it is metabolized, and the released isothiocyanate
radical (Fig. 5.27b) reacts with the sulphydryl
groups of amino acids (Agrios, 2005).
The most important agrochemicals against
Oomycota are the phenylamides such as metalaxyl
(Fig. 5.27c) which are systemic fungicides,
i.e. they can enter the plant and are translocated
throughout it. Metalaxyl appears to inhibit the
transcription of ribosomal RNA in Oomycota
but not Eumycota (Davidse et al., 1983). This is
an inhibition of a specific biochemical target,
and the immense genetic variability of P. infestans
enabled it to develop resistance against metalaxyl
in the early 1980s shortly after this was
released for agricultural use (Davidse et al.,
1991). Resistance is now widespread and has
serious implications for future control of