Agriculture Reference
In-Depth Information
application technologies, improved diagnostics
and seed certification systems, and improved
host resistance. This is attributable largely to
changes in
P. infestans
populations (Goodwin
et al
., 1996; Smart and Fry, 2001; Fry, 2008;
Kirk
et al
., 2009; Hu
et al
., 2012). The most im-
portant of these changes worldwide, and especially
in North America, have been the occurrence of
A
2
mating type (Deahl
et al
., 1991), the resistance
to phenylamide fungicides in the field (Shattock,
1988), and the appearance of increasingly
aggressive genotypes of
P. infestans
(Fry and
Goodwin, 1997a; Hu
et al
., 2012)
.
Before 1993, only the US-
1
genotype had
been found in the USA. The US-
1
genotype is
mating type A
1
.
P. infestans
requires two mating
types, A
1
and A
2
, to come into contact to pro-
duce a sexual spore, known as an oospore. Oo-
spores are resistant to freezing and other envir-
onmental extremes, and can survive in diseased
leaves and stems or free in soil. With the arrival
of the A
2
(e.g. US-8) and new A
1
(e.g. US-23)
mating types in North America, the potential ex-
ists for the production of oospores that can sur-
vive between seasons, though this has not yet
been proven to occur (Hu
et al
., 2012). If the
pathogen does start to produce overwintering
oospores, it may be able to overwinter more eas-
ily and may become a factor in virtually every
growing season when conditions favor the devel-
opment of late blight.
P. infestans
can survive only in living potato
tissue or, as oospores, survives as mycelia from
year to year in infected tubers placed in storage,
in piles of cull potatoes, or in infected tubers
missed during harvest that remain unfrozen
over the winter (volunteer potatoes). If infected
tubers freeze and die over winter, the disease
cycle is broken, and very often the disease does
not appear even when the weather conditions
are favorable. If the pathogen survives within
tubers, in the spring it can be transmitted from
infected tubers to potato foliage by airborne
spores
(Fig. 11.2
). Some infected tubers may rot
in the soil before emergence, and not every plant
that emerges from an infected tuber will con-
tract late blight. The fact that many reproductive
cycles are possible within a season accounts for
the rapid increase in disease once it becomes es-
tablished in a field. This is responsible for the
spread to other fields, and the spread within a
field can take place by wind and rain within a
single round of sporangia production.
P. infes-
tans
favors wet weather with moderate temper-
atures (
15-
26°C), high humidity, and frequent
rainfall. Environmental conditions during the
growing season in the many potato growing
regions of North America are sporadically but
frequently conducive to the development of epi-
demics of potato late blight, and significant fi-
nancial costs, in terms of crop protection (up to
US$700 ha
-1
) and crop losses (up to US$5000
ha
-1
), are incurred when intervention measures
to control potato late blight are unsuccessful
(Guenther
et al
., 2001).
Many interacting variables, including me-
teorological factors such as climatic change and
increasing tolerance of
P. infestans
to colder
temperatures (Kirk, 2003), represent a serious
situation for the potato industry in the USA
(Hijmans
et al
., 2000; Baker
et al
., 2004). It is
predicted that global climate change will result
in a significant increased risk of late blight epi-
demics on all continents where potatoes are
grown (Hijmans
et al
., 2000). For example, from
1950 to 2001, climatic conditions in Michigan,
USA, became steadily more conducive for the
initiation and development of potato late blight
epidemics (Baker
et al
., 2005), although this
may not be the case for all regions. Traditional
thresholds for spray initiation as predicted in
BLITECAST (Krause
et al
., 1975) were reached
earlier in the season, and conditions during
midsummer were also cooler and wetter and
resulted in more days conducive for late blight
development in Michigan (Baker
et al
., 2005).
Potato late blight prediction models have
been used to estimate environmental conditions
that are favorable for epidemic risk and fungicide
recommendations appropriate to that risk for
more than
50
years (Wallin and Schuster, 1960;
Wallin, 1962; Mackenzie, 1981). Such models
attempt to limit grower expenditures and reduce
the amount of chemical used while achieving
optimal control of potato late blight. One of the
downfalls of these models is that they use wea-
ther data that are at best real time. The incorpor-
ation of extended range forecast data into a dis-
ease risk system would render these systems
even more valuable by providing a prediction of
risk conditions up to several days in advance of
their occurrence. Extended range forecast model
output statistics (MOS) including
192-
h max-
imum and minimum daily temperatures have
