Plant responses during the pathogen infection and the pathogen control reflect its strategies
to protect its cells. This work represents the Alternaria cerealis MT808477 as a phytopathogen causing
leaf spot disease in tomatoes. A. cerealis was identified morphologically and genetically by 18SrRNA,
and its pathogenicity was confirmed by light and scanning electron microscopy. Trichoderma harzianum
has the ability to control A. cerealis MT808477 by stimulating various cell responses during the
controlling process. The cell behavior during the biological control process was observed by analyses
of total phenol, flavonoids, terpenoids, antioxidant, malondialdehyde and antioxidant enzymes
(catalase and peroxidase). The extracts of infected tomato leaves were tested against plant and human
pathogenic microorganisms. Results showed that the biological control process activates the defense
cell strategies by increasing the plant tolerance, and activation of plant defense systems. The total
phenol, flavonoids, terpenoids, antioxidant and malondialdehyde were increased after 48 h. Catalase
and peroxidase were increased in infected tomato plants and decreased during the biological control
process, reflecting the decrease of cell stress. Leaves extract inhibited the growth of nine plant and
human pathogenic microorganisms. Biological control represents a safe and effective solution to
phytopathogens that decreases plant cell stress by stimulating various defensive agents.