Agriculture Reference
In-Depth Information
Scandinavia (Zadoks, 1967). Further evidence of long-distance dispersal of rust is
given by the population structure of
P. triticina
in Western Europe (Park
et al.,
1996, 2001). The isolates of the four most common pathotypes generally have the
same RAPD phenotypes, supporting the hypothesis of a clonal migration over
considerable distances.
The wind dispersal of urediniospores of
P. striiformis
f.sp.
tritici
from Great Britain to Denmark, and later to Germany and France allowed an
explanation of the pathotype distribution in western Europe after the outbreak of the
Yr17
virulent pathotype (Brown and Hovmøller, 2002; Hovmøller
et al
., 2002).
Another path for
P. graminis
f.sp.
tritici
and
P. triticina
is from the South Indian
Hills to the Central Indian Plains (Nagarajan and Singh, 1990). Urediniospores
survive throughout the year in south India (in the Nilgiri Hills) and are dispersed to
central and northern India by autumn tropical cyclones (following a recurrent pattern
of synoptic situations). Unlike in North America, there is no feedback of inoculum
to the south.
The possible South African origin of several strains of
P. graminis
f.sp.
tritici
found in Australia in 1968 was speculated because of the similarity in both
pathogenicity and isozyme patterns between isolates found in the two continents
(Watson and de Sousa, 1982). Stripe rust development in Australia since 1979
illustrates rapidity and long-distance dispersal of the pathogen.
A continental
Puccinia
pathway has also been observed from western China to
the main wheat belt, southern Gansu and northern Sichuan being a source for the
dispersal of diverse
P. striiformis
f.sp.
tritici
pathotypes (Shan
et al.,
1998; Brown
and Hovmøller, 2002).
This long-distance dispersal has prompted the suggestion to organize the
introduction of resistance genes in crops against the main wind direction, for
example, Limpert (1987) for barley powdery mildew in Europe, and Bahadur
et al.
(1994) for stem and leaf rusts in India. However, long-distance transport may
include diverse populations. Using virulence polymorphism and RAPD markers,
Kolmer
et al
. (1995) hypothesized that the two major groups of
P. triticina
isolates
currently found in eastern and western Canada originated from different
introductions. No relationship between pathogenic and molecular variation was
found for
P. striiformis
f.sp.
tritici
in the US Pacific northwest by Chen
et al.
(1993).
15.2.7 Consequences of climatic factors on polycyclic epidemics
Environmental factors can affect different phases of the life cycle differently so that
it is not clear what conditions will lead to severe epidemics. Moreover, the progress
of an epidemic is heavily influenced by the susceptibility of the crop plant, which is
also affected by environmental conditions, for instance, the expression of adult plant
resistance against powdery mildew in barley plants is more pronounced in dry and
hot weather (Aust and von Hoyningen-Huene, 1986).
For epidemics of mildew, development is optimal between 15 and 22°C and it is
retarded at temperatures above 25°C (Mathre, 1982). Epidemics can reach high
disease levels if the initial disease level is high and the temperature is relatively
warm, for instance, more than 10°C from the beginning of the three-leaf stage until
