Agriculture Reference
In-Depth Information
from that reported in Japan and Korea; there is also circumstantial evidence of
sexual reproduction in Russia (Elansky
et al
., 2001).
Migrations into the United States and Canada have occurred during the 1980s
and the early 1990s. These were first detected in the early 1990s (Deahl
et al
., 1991;
Goodwin
et al
., 1994b). The first migrations were detected in production regions
along the Pacific Coast of Canada and the United States. Subsequently, immigrant
strains were detected along the East Coast and then throughout the United States and
Canada (Fry and Goodwin, 1997b). The immigrant strains in the United States
brought characteristics that were previously not present: A2 as well as A1 mating
types; enhanced pathogenicity to either potatoes or tomato; and metalaxyl resistance
(Fry and Goodwin, 1997a).
The 1990s migrations into the United States and Canada were detected because
of a dramatic increase in the severity of late blight (Johnson
et al
., 1997; Daayf and
Platt, 2000). In locations where late blight became more severe, exotic strains were
detected. Often the previous indigenous strain was not detected. In 1994 and 1995,
there were locally devastating epidemics that had severe economic repercussions on
individual growers throughout the United States (Fry and Goodwin, 1997b; Johnson
et al
., 1997). Because the migrations were observed as they happened, it was
possible to compare immigrant strains directly with the previous indigenous strain.
In most of these comparisons, the immigrant strains appeared to have enhanced
pathogenicity to tomatoes, to potato foliage and tubers (Legard
et al
., 1995; Kato
et al
., 1997; Lambert and Currier, 1997; Miller
et al
., 1998). Regarding tuber
infection, it now appears that there is significant diversity among genotypes of
P. infestans
for pathogenicity on tubers (Lambert and Currier, 1997).
Currently (early 2005) the populations of
P
.
infestans
worldwide vary from
clonal to recombinant (Table 17.1). In countries where it is clonal, there are a single
or few lineages which can be more adapted to a particular host species. Host
specificity in
P
.
infestans
is not absolute, i.e. isolates adapted to potato can infect
and cause lesions on tomato and the opposite can also occur. There could be
differences regarding isolate aggressiveness when inoculated in different hosts (see
section 17.3.4). Interestingly, the old lineage US-1 is still present in several places.
Comparison of exotic strains to previous strains in relation to their reactions
to abiotic influences suggests that the diverse genotypes may respond somewhat
differently to abiotic factors than did previous very simple populations. In an
investigation of the germination of sporangia, recent isolates of the US-1 clonal
lineage (a representative of the population present in the United States perhaps since
the mid 19
th th
century (Crosier, 1934). However, germination of sporangia of the US-8 clonal
lineage responded significantly differently. Whereas sporangia of US-1 germinated
rather well at 20°C sporangia of US-8 did not (Mizubuti and Fry, 1998). Similar
responses were obtained for a new lineage (BR-1) associated with potato in Brazil
(Maziero, 2001). This observation suggests that plant pathologists throughout the
world need to ascertain the responses of new local populations of
P. infestans
to
important abiotic factors such as temperature, moisture, and solar radiation.
century) responded the same as reported by Crosier in the early 20
