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impact the distribution and diversity of these pathogens. Some fungal species com-
plete their life cycle in the soil, with no aerial dispersal of propagules. These fungi
usually have limited genetic diversity and show sub-structuring (Al-Sadi et al.
2012 ). Others complete most of their life cycles on the aerial parts of plants and
therefore show higher levels of gene flow and genetic diversity. Their dissemination
from location to location is usually a result of human activity including irrigation,
tillage practices, movement of equipment in fields and, in many cases, movement of
infested soil, potting media, plant residues, or colonized plant material (Al-Sa’di
et al. 2008b, 2011a, 2015c, Kazeeroni and Al-Sadi 2016 ). However, some fungal
spores (e.g. zoospores of Oomycetes) can move by themselves when water is avail-
able. Infection of plants can lower the quality of propagated plant material, produce
poor plant stands, stunt growth, cause wilting and death of affected seedlings and
plants, and reduce yield.
Dry regions of the world are usually characterized by relatively higher tempera-
tures and lower precipitation rates. Despite these challenging factors, farmers in
these regions grow a variety of crops, including field crops, vegetables and fruit.
When the conditions are not suitable for certain high-value crops, farmers turn to
protected agriculture in the form of greenhouses. Disease epidemics caused by fun-
gal pathogens are usually affected by environmental conditions, the type of patho-
gens present, the type of crops grown, and the type of cultural practices employed
by farmers. This in turn will affect the cultural, chemical or biological management
strategies used for these pathogens.
This chapter focuses on some of the pathogens and diseases threatening crops
under dry environments, with specific emphasis on wheat (Triticum aestivum L.)
diseases. It presents the latest findings in the areas of population structure and
dynamics of pathogens in dry environments, the factors affecting disease epidemics,
plant–pathogen interactions, case studies of some diseases, and disease
management.
2 Epidemiological Aspects of Plant Diseases in Dry
Environments
2.1 Population Structure and Dynamics of Fungal Pathogens
in Dry Environments
The source and quantity of fungal inoculum represent important determinants of the
severity of plant diseases. Fungal propagules can come into contact with seeds,
seedlings, and plants from numerous sources including potting mixtures, cultivated
soil, irrigation water, wind-driven dust, and insects.
Potting mixtures are frequently used for seed germination before transplanting to
farms or greenhouses. Infestation of potting mixtures with fungal propagules, as
reported by several workers (Cartwright et al. 1995 ; Davison et al. 2006 ; Al-Sadi
A.M. Al-Sadi