Abstract Fusarium oxysporum is a soil-borne fungal pathogen that causes wilt diseases in a wide variety of plants. Fusarium oxysporum poses a significant threat to agriculture due to its ability to cause severe yield losses in a variety of crops. Management of Fusarium wilt is challenging because the fungus persists in the soil for many years. and there are limited effective chemical control options. This research is designed to study the effect of Trichoderma harzianum in reducing the effect of Fusarium oxysporum f. sp. lycopersici (Fol) on tomato plants. Fusarium oxysporum was isolated from diseased tomato plants and identified by using morphology and ITS sequencing of DNA. Three biological treatments were applied to examine the effect of Trichoderma harzianum on controlling disease wilt caused by Fusarium oxysporum. The results related to disease severity showed that the control plants appeared healthy according to disease severity scale of 1–2 with an average of (1.3) in contrast to plants treated with Fol which showed diseased plants according to disease severity scale of 3–4 with an average of (3.7). While the plants treated with Fol and to which Trichoderma harzianum was added showed healthy plants according to disease severity scale of 1–2 with an average of (1.8).
Khalaf Alhussaen (2025) studied this question.