189et al. 2011a, 2015c), can act as the initial inoculum that can kill seedlings in nurser-
ies and infest the soil on farms. A survey done by Davison et al. ( 2006 ) recorded the
presence of Pythium and Phytophthora species and some nematodes in consign-
ments of potting media. Also, Pythium aphanidermatum and Fusarium oxysporum
have been isolated from commercial peat moss while farmyard manures have been
reported to have high levels of contamination with pathogenic fungi (Al-Sadi et al.
2011a).
Soil previously infested with fungi is often regarded as the most important source
of fungal propagules as most incidences of soil-borne diseases occur after sowing/
transplanting into infested soil (Stanghellini and Phillips 1975 ; Al-Sa’di et al.
2008b). Disease severity can vary between and within the same planting row, which
is usually attributed to the distribution and different densities of fungal propagules
in the soil (Stanghellini and Phillips 1975 ; Al-Sadi et al. 2012 ). Wind-driven dust in
dry areas of the world (Al-Sa’di et al. 2008b, 2011a), insects (Gardiner et al. 1990 )
and irrigation water (Pottorff and Panter 1997 ) are also potential sources of fungal
inoculum.
Survival of fungal pathogens in soil is affected by several factors such as the type
of spores produced and the characteristics of the soil habitat. Mycelia survive for
relatively short periods, usually a few days (Martin and Hancock 1986 ). Spherical
sporangia produced by some Pythium species usually survive longer than species
producing inflated filamentous or lobulate sporangia (Martin and Loper 1999 ).
Survival of sporangia can be from a few days (Stanghellini and Burr 1973 ) to a few
weeks (Peethambaran and Singh 1977 ) or several months (Stanghellini and Hancock
1971b). Long-term survival is achieved via the thick-walled resting spores.
Depending on the species and the soil characteristics, spore survival can range from
months (DeVay et al. 1982 ) to years (Stanghellini and Nigh 1972 ). Some spores do
not germinate immediately or simultaneously but enter a dormancy period (Hancock
1981 ) which may be advantageous to escape the prospect of germinating all at once
in unfavorable conditions. Upon germination, plant infection results in the coloniza-
tion of the host, disease development and the production of new spores to complete
the disease cycle.
Several airborne fungal pathogens usually survive in infected buds or leaves in
the form of mycelium or resting spores. Others can thrive in decaying plant tissues
for several years. Disease severity depends on several factors, including the aggres-
siveness of the isolate and susceptibility of the host (Al-Sadi 2016 ). The spores of
some fungal pathogens, especially rust-inducing fungi, can spread long distances
via the wind, which is common in dry places. Short distance spread is usually by
rain splashing. Puccinia’s aeciospores absorb water, which exerts a force on the
fruiting body's wall which ruptures to enable spread around the area. Insects are
attracted to sugary and odorous material produced by pycniospores and receptive
hyphae of rust fungi. Many airborne fungal pathogens usually cause serious epi-
demics because they can spread more rapidly and some produce high numbers of
spores (e.g. rust and powdery mildew diseases).
The exchange of genetic factors between or within fungal populations, either by
interbreeding or migration, can impact on the genetic diversity of a fungal pathogen,
Epidemiology and Management of Fungal Diseases in Dry Environments