Alternaria Leaf Spot Disease on Cucumber: Susceptibility and Control Using Leaf Disk Assay
Results obtained in the present research using leaf disk assay indicated significant differences in susceptibility of tested cucumber cultivars to Alternaria cucumerina-infection. Hasan cultivar was the most susceptible, but Rocket cultivar was the least susceptible. Assessment of treatment efficacy with fungicides and Trichoderma harzianum against the disease using leaf disk assay indicated the presence of significant differences between the treatments with 4 types of new, low-residual fungicides or 2 forms of T. harzianum and the control treatments with a blank formulation of invert emulsion or sterile distilled water. Treatments with fungicides just before the disease inoculation demonstrated that Score® and Switch® completely inhibited appearance of the disease-lesion on treated cucumber leaf-discs when applied at a rate of 0.35% (V/V) and 0.20% (W/V), respectively. Treatments with the same fungicides at the same application rates 24h after the disease inoculation also completely inhibited the disease-lesion appearance when Score® was applied. It suppressed the disease-lesion diameter to 2.5mm or reduced it by 83.5% relative to control when Switch® was applied. Moreover, application of formulated conidia of T. harzianum in invert emulsion at a concentration of 2.0X108 conidia/ml significantly suppressed the disease-lesion diameter on treated cucumber leaf-discs. Such application decreased the lesion diameter to 4.5 and 6.5mm or reduced it by 70.4 and 57.2% relative to control when used just before or 24h after the disease inoculation, respectively. It is, thus, concluded that the results obtained on susceptibility of cucumber cultivars to the disease infection and its control should be confirmed on whole plants under field conditions before being recommended to be applied by the farmers and extension agents.