191increased activity of the invading pathogens (Al-Mawali et al. 2013 ). Low air mois-
ture can further increase wilt diseases due to increased transpiration from plants in
which water absorption and/or translocation are affected by vascular fungal patho-
gens. Germination of the dryland root rot pathogen of wheat (Bipolaris sorokini-
ana) is optimum at moderate to high temperatures (Fig. 1 ).
Moisture is an important determinant of the severity of plant disease, as higher
soil moistures usually favor more disease (Hendrix and Campbell 1973 ). High soil
moisture increases the mobility of spores, especially zoospores, saprophytic growth
and spore germination (Martin and Loper 1999 ). However, several plant diseases
are favored by drought conditions such as dry rots and charcoal rots caused by
Macrophomina species (Lodha et al. 2014 ; Sharma et al. 2015 ). Increased soil mois-
ture increases the amount of CO 2 within the soil which may offer some fungal spe-
cies the opportunity to cause more disease while microbial competition decreases as
many other microorganisms are sensitive to higher CO 2 concentrations (Gardner
and Hendrix 1973 ).
The conduciveness of soil to pathogen development can have a significant effect
on disease; even if the inoculum level varies in different soils, there may not be a
strong positive correlation to disease levels due to differences in soil conduciveness
(Stasz and Harman 1980 ). Several workers including Kaiser and Hannan ( 1983 )
found higher levels of disease from soil with a low inoculum level than soil with a
high inoculum level. Also, the type and amount of propagules and the age of seed-
lings can affect disease development (Al-Sadi et al. 2011b). A study by Al-Sadi
et al. (2010a) showed that increased soil and water salinity stimulates the develop-
ment of damping-off epidemics.
Disease epidemics are also affected by the inoculum level and the mode of repro-
duction (Al-Sadi 2016 ). Some pathogens only have one or few disease cycles in a
00347595 95
8131(^00)
20
40
60
80
100
120
51015202530354045
Temperature ( C)
Germination
%
°
Fig. 1 The effect of temperature on germination percentage of conidia spores from 11 B. sorokini-
ana isolates obtained from wheat and barley (Al-Saadi 2002 )
Epidemiology and Management of Fungal Diseases in Dry Environments