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affected by soil moisture, soil temperature, cultural practices, pathogen population
in the soil, and time of infection. Disease severity increases when the plant is under
stress. Differences in soil surface residue can affect soil moisture and temperature,
root growth and nutrient uptake as well as microbial activity, which can modify the
conditions for root infections. The severity of the disease in wheat is related to the
soil population of B. sorokiniana at the time of planting, which is highest in the top
10 cm. This is mainly due to sporulation of the fungus on culm bases, crown roots,
stubble, straw, sub-crown internodes and seminal roots near the soil surface (Duczek
1990 ). Drought and warm temperatures make wheat more susceptible to common
root rot disease.
Common root rot can affect the yield and quality of wheat as a result of the
effects of the disease on seedling emergence and growth, leaf area and growth, num-
ber of tillers per plant, kernel number and weight, shoot and root weight, and size of
internodes (Zhang et al. 1999 ; Tunali et al. 2008 ). B. sorokiniana can also cause root
rot disease in rye, oats and other crops (Ovsyankina 2005 ; Poole et al. 2015 ).
4.3 Spot Blotch
Spot blotch, caused by Bipolaris sorokiniana, is a common disease in wheat and
barley in dry environments (Acharya et al. 2013 ; Agostinetto et al. 2015 ; Singh
et al. 2015 ; Zhu et al. 2015 ). It is one of the most important foliar diseases limiting
wheat production in warmer non-traditional growing areas. The symptoms of spot
blotch disease on wheat appear with distinct, elongated, brown–black lesions that
rarely exceed 1 cm in diameter (Agostinetto et al. 2015 ). Lesions have a distinct
margin which varies in size. Enlargement of the spots may form blotches that cover
large areas of the leaf, especially on barley cultivars (Fig. 4 ). Due to sporulation of
the fungus on older leaves, the lesions may become olive brown in color. In addition
to wheat and barley, B. sorokiniana causes leaf spot in maize (Zea mays L.), rice
(Oryza sativa L.), rye (Secale cereale L.), and many others (Acharya et al. 2013 ;
Zhu et al. 2015 ).
Leaf infection by the pathogen may come from the seeds, roots or the air. If the
pathogen is in the soil, then the infection occurs through stomata on the hypocotyl
and progresses to the root, shoot and coleoptile. In the leaf, penetration of B. soro-
kiniana occurs through stomata and the epidermis. Warm weather favors the patho-
gen. Leaf surface wetness is also an important factor influencing infection (Kumar
et al. 2002 ; Agostinetto et al. 2015 ).
Bipolaris sorokiniana produces several phytotoxins (Bach and Kimati 1999 ;
Jahani et al. 2014 ) which affect many of the physiological and metabolic processes
in plants (Bashyal et al. 2012 ). Toxins of B. sorokiniana and other fungal pathogens
affect electron transport, ion transport, calcium uptake, chlorophyll and enzyme
activities (Liljeroth et al. 1994 ; Bach and Kimati 1999 ; Agrios 2005 ).
A major role of toxins is in pathogenicity. For instance, Liljeroth et al. ( 1994 )
suggested that toxins play a role in pathogenicity by killing or weakening plant cells
A.M. Al-Sadi