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began to spread across most potato-growing regions
of the world, presumably as a result of international
trade, and this has led to an increased diversity of the
P. infestanspopulation, which was already diverse as
evidenced by the existence of many different isozyme
profiles.
In field conditions, potato blight is first seen as
black, spreading lesions on the foliage, and in cool
humid conditions these lesions produce masses of
sporangiophores that emerge from the leaf stomata.
Potato tubers become infected later in the growing
season, perhaps by the spread of motile zoospores
(Chapter 10), then the tubers start to rot and are
destroyed by secondary bacterial invaders. The main way
in which P. infestansis disseminated is by the produc-
tion of detachable sporangia which can be wind-borne
or splashed onto foliage (see Fig. 5.17). These spor-
angia can germinate in two ways, depending on the
climatic conditions, and this can be mimicked in
laboratory conditions. At temperatures of around
20°C or higher, the detached sporangia germinate by
forming a hyphal outgrowth (Fig. 14.19), whereas at
temperatures of 12°C or lower the sporangia undergo
cytoplasmic cleavage to produce motile zoospores.
The production of zoospores at low temperatures
could be especially significant in early-season spread of
the disease, whereas “direct” germination of sporangia
could be more important later in the season – for
example, in the infection of tubers. This dual strategy,

also shared by P. erythrosepticawhich causes pink rot

of potatoes, is part of the formidable arsenal of P.

infestans.

However, the major weapon of this pathogen is,

undoubtedly, its ability to overcome the “major” race-

specific resistance genes that have been bred into

potato cultivars over the years. P. infestans is a

hemibiotrophic plant pathogen: in its early stages of

colonizing a leaf it feeds by means of haustoria which

tap into the host cells and draw nutrients from them,

but at a later stage it switches to a necrotrophic mode

and causes generalized tissue breakdown. Every new

major resistance gene introduced into potatoes was

soon overcome by a mutation in the pathogen, lead-

ing to a new “race” of P. infestansthat could not be

controlled by the existing single major genes. Now a

new breeding strategy is in place, to try to exploit gen-

eral resistance (also termed horizontal, rate-limiting or

multigenic resistance) coupled with the use of fungi-

cides and cultural practices.

Phytophthora ramorum: sudden oak death

A new species of Phytophthora was discovered in

Germany and the Netherlands from 1993 onwards, and

was described formally in 2001 as P. ramorum. It

causes damage to the branches of Rhododendronand,

less often, Viburnumbushes, sometimes leading to

300 CHAPTER 14

(a)

Fig. 14.19Phytophthora infestans. (a) A sporan-

giophore, with attached sporangia, emerging

from a stoma on the underside of a potato leaf.

(b) Two detached sporangia, producing germ-

tube outgrowths from near the apical papilla.

Note the characteristic broken “stalk” at the

base of the sporangia.

(b)