Introduction to Fungi, Third Edition

PYTHIALES
113
Fig 5.27 Fungicides against P. infestans. (a) The dithiocarbamate maneb which is active against Oomycota and Eumycota. (b) The
isothiocyanate radical released by metabolism of dithiocarbamates by fungal hyphae. (c) The phenylamide metalaxyl which is active
only against Oomycota. (d) Aluminium ethyl phosphonate (fosetyl-Al). (e) Cyazofamid, a new fungicide specific against Oomycota.
(f) Famoxadone, a new fungicide active against Oomycota and Eumycota.
Phytophthora spp. (Erwin & Ribeiro, 1996).
Phenylamides are now protected by being used
in a cocktail, e.g. with the less-specific dithiocarbamates,
and tailor-made application regimes
are recommended for each year and each region
(Staub, 1991).
The phosphonates are a different type of
fungicide against Phytophthora spp. Fosetyl Al
(aluminium ethyl phosphonate; Fig. 5.27d) is
readily taken up by plants in which it is broken
down to release phosphorous acid (¼ phosphonate),
which seems to be the active principle
(Griffith et al., 1992). Fosetyl Al as well as phosphorous
acid can move downwards through the
phloem and upwards in the xylem, showing
similar transport characteristics as sucrose
(Ouimette & Coffey, 1990; Erwin & Ribeiro,
1996). The mode of action of phosphonates is
not known but is likely to be complex, with a
stimulatory effect also on the host plant immune
system (Molina et al., 1998). Although active
only against potato tuber blight but not foliar
blight caused by P. infestans (L. R. Cooke & Little,
2002), phosphonates are effective against a wide
range of root-infecting Phytophthora spp. and
even show good curative properties (Erwin &
Ribeiro, 1996).
A useful introduction to current fungicides
and their modes of action has been provided
by Uesugi (1998). Because of the enormous
economic significance of P. infestans and other
Oomycota, new fungicide candidates are continually
being developed and introduced into the
market. Two recent examples are cyazofamid
(Fig. 5.27e) and famoxadone (Fig. 5.27f). Both
inhibit mitochondrial respiration. However,
whilst the former is specific against Oomycota
(Sternberg et al., 2001), famoxadone inhibits
both Oomycota and Eumycota (Mitani et al.,
2002). Its molecular target is different from
that of cyazofamid but probably the same as
that of the strobilurins (see Figs. 13.15e,f),
as indicated by the development of crossresistance
in fungal pathogens against famoxadone
and strobilurins.
Disease forecast
To avoid unnecessary spraying and to ensure
that timely spray applications are made, it has
proven possible to provide forecasts of the
incidence of potato blight epidemics for certain
countries. Beaumont (1947) analysed the incidence
of blight epidemics in south Devon
(England) and established that a ‘temperature
humidity rule’ controls the relationship between
blight epidemics and weather. After a certain
date (which varies with the locality) and assuming
that inoculum on volunteer plants is always