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and is affected by several factors; most importantly, the ability of a pathogen to
move from one area to the other. This occurs faster for airborne fungal pathogens
such as rusts, which can travel distances over 12,000 km (Watson and de Sousa
1983 ) and slower for soil-borne pathogens (Al-Sadi et al. 2012 ). Limited migration
and gene flow may result in a phenomenon known as subdivision or sub-structuring
(Loveless and Hamrick 1984 ), where isolated fungal populations, despite little
intra-population diversity, show higher levels of genetic differentiation between
populations compared to many foliar pathogens with wind-dispersed spores. This
has been documented in seven populations of Pythium irregulare obtained from dif-
ferent geographical locations rather than within each population (Harvey et al.
2000 ).
Genetic drift, defined as random fluctuations in allele frequencies, is another fac-
tor affecting the population structure of pathogens, especially in small populations
as there is a greater loss of allelic diversity (McDonald and McDermott 1993 ).
Random genetic drift—due to rapid asexual cycles, large fluctuations in population
size and man-induced changes to the host—is considered an important factor in
shaping populations of plant pathogens (Drenth and Goodwin 1999 ).
Environmental conditions, host range and management practices can deploy
selection pressure on populations of fungal species. The effect of environmental
conditions becomes apparent with the occurrence of different fungal species in
some geographical areas but not others (Van der Plaats-Niterink 1981 ). Fungal spe-
cies exhibiting a wide host range are expected to show higher levels of genetic
diversity (Harvey et al. 2001 ). In addition, the deployment of specific host-resistant
genes by man is likely to eliminate pathogen populations without the virulent genes
(Drenth and Goodwin 1999 ). Management strategies, especially the application of
fungicides, can lower the level of genetic diversity within fungal populations and
selects for fungicide-resistant isolates (Al-Sadi et al. 2012 ). One example is an
increase in metalaxyl-resistant genotypes of Phytophthora infestans after the intro-
duction of metalaxyl fungicides (Davidse et al. 1989 ).
2.2 Factors Affecting the Development of Epidemics
Some physical and biological factors reportedly affect the development of plant
disease epidemics and include environmental factors, inoculum level, mode of
spread, mode of reproduction, cultivar resistance, uniformity, and crop age.
Temperature affects the composition of fungal species infecting plants under
given climatic conditions. Temperature and precipitation influenced the distribution
of Pythium species infecting wheat in eastern Washington State (Paulitz and Adams
2003 ). Pythium aphanidermatum usually causes severe diseases during summer
when temperatures are above 23 °C while P. dissotocum is more aggressive during
winter when the temperatures are between 17 and 22 °C (Bates and Stanghellini
1984 ). The development of Fusarium-induced wilt diseases is higher when tem-
peratures increase, and is mainly attributed to the stress imposed on plants and
A.M. Al-Sadi