Agriculture Reference
In-Depth Information
Centre estimated that annual costs of late blight were approximately US $3.2 billion
(Raman
et al
., 2000).
It is logical that the pathogen and the disease have received much attention.
There are numerous studies, reviews, book chapters and topics devoted to the
biology of the pathogen and disease. Some classic studies such as those by Crosier
(1934) and Van der Zaag (1956) are still used. Of the many reviews, those by
Andrivon (1995, 1996) and Harrison (1992) provide a wealth of references. Some
topics have been published as proceedings of conferences, but others have been
published solely to investigate and illustrate the breadth and depth of knowledge on
the disease (Ingram and Williams, 1991; Lucas
et al
., 1991; Dowley
et al
., 1995;
Erwin and Ribeiro, 1996). With such a rich literature on late blight, this chapter will
explore the implications of insights gained from recent studies on the population
biology of this very familiar pathogen.
17.2 POPULATION BIOLOGY OF
P. INFESTANS
Pathogen population biology is a more inclusive term than epidemiology because it
encompasses both ecological and genetic aspects of populations (Milgroom and
Peever, 2003). In the last 15 years, there has been an explosion of information about
the biology of populations of
P
.
infestans
. Studies were carried out in temperate
regions as well as in tropical and subtropical areas of the world. While this
knowledge will improve late blight management everywhere, the benefit is expected
to be particularly important in the tropics and subtropics.
The life history and evolutionary position of an organism can be very helpful in
interpreting population phenomena. Our understanding of the basic biology of
P. infestans
has changed dramatically during the latter part of the 20
th
century. The
organism was first described as
Botrytis infestans
by Jean Montagne in the mid 19
th
century. Since then our understanding of the phylogenetic relationships of
P.
infestans
has been changing. Oomycetes are now generally regarded as existing in a
kingdom separate from the true fungi, plants, animals, and prokaryotes. Some
suggest that they belong in the kingdom Protoctista but others believe they should be
in the kingdom Chromista (discussed in Erwin and Ribeiro, 1996). Regardless of
kingdom, evidence is mounting that oomycetes and some algae belong in the same
kingdom (Sogin and Silberman, 1998; van West
et al
., 2003). These organisms share
ribosomal ITS similarity, have biflagellate (tinsel and whiplash) zoospores, have
similar sexual structures (antheridia and oogonia), are diploid and have cell walls of
glucans.
Phytophthora infestans
is heterothallic, with two mating types (A1 or A2),
and bisexual. When the organism reproduces sexually, a thick-walled spore -
oospore - is formed. However,
P. infestans
can exist quite successfully as an asexual
organism; as such it is a near obligate parasite in nature. Because the life history
may be essentially asexual in one geographic region, but sexual and asexual in
another, the epidemiology of the pathogen can be dramatically different in different
regions. For example, some sources of inoculum are important in some regions and
completely unimportant in other regions.
