Abstract

Isolations from diseased squash roots revealed the presence of Alternaria tenuis, Aspergillus niger, Fusarium oxysporum, F.  solani and Rhizoctonia solani.  The last two fungi were more  frequent than any of the other fungi. Pathogenicity tests proved that squash plants were highly vulnerable to attack by Fusarium solani andRhizoctonia solani in descending order, during the pre- and post-emergence stages. Isolates No.1 of F. solani and No.2 of R. solani were the most virulence pathogen and significantly retard the morphogenesis of survived squash plants. The in-vitro antagonistic effect of Trichoderma harzianum on root-rot pathogens revealed the presence of clear antagonistic action between them. The highest mean inhibition values were 78.86 and 41.94% RI against F. solani  and  R.  solani, respectively. T. harzianum also exhibited a mycoparazitation associated with high level of growth reduction by its filtrate on the two pathogens. In vitro, benlate fungicide was found to be very toxic to the root-rot pathogens. Untreated squash seeds grown in the infested soil (positive control) with F. solani and R.  solani showed higher percentage of infection. The plants grown under this treatment were significantly shorter with lesser weight than the corresponding figures of the other treatments. Soil or seed coated with T. harzianum and benlate had a significant lower percentage of infection (pre, post, dead plants and developed plants), significantly longer in height and better plant growth parameters. T. harzianum applied as soil or seed treatments enhancing the total microbial flora of squash rhizosphere at the first 30 days of plant growth and greatly increased the total fungal counts comparing with the corresponding figures of negative and positive controls. As the density of T. harzianum reached 99.3% and 100.0% in cultivated plants soil infested with F. solani and R. solani respectively, the other genera or species of fungi become greatly reduced or disappeared.

Key words: squash, root-rot disease, Saudi Arabia.