Several experiments were conducted to evaluate the effectiveness of Alternaria alternata fungus isolates on powdery mildew disease on squash and to diagnose them according to the taxonomic keys of the Alternaria spp fungus genera. They were propagated to conduct experiments on them. The experiments showed a significant effect of the two isolates on the growth and germination of powdery mildew disease in squash. The two isolates also had a significant effect on reducing the formation of conidiophores. Powdery mildew disease was sprayed with a suspension of the two A. alternata fungus isolates, as it gave the greatest effect on the germination of powdery mildew disease, in addition to the number of conidiophores, followed by the spray treatment with reproductive units when compared to the control. The plants were also sprayed with the filtrate of the A. alternata fungus isolate, as it gave a tangible effect on the germination of powdery mildew disease, in addition to the number of conidiophores when measured by the control. Time also significantly affected the germination and number of conidiophores for powdery mildew disease.