The successful cultivation of edible mushroom is, among other factors, dependent on the type and composition of the growth substrate. An investigation on the growth response of P. ostreatus to different substrates treated with used automobile oil was conducted in Calabar, Cross River State, Nigeria. The test mushroom was cultured on three substrates [Dry banana leaves (BL), sawdust (SD) and Dry banana leaves/sawdust composite (BL/SD)] treated to six levels of concentration (0ml, 20ml, 40ml, 60ml, 80ml, 100ml per 3.5kg of substrates) of used automobile engine oil. Initiation of mycelial growth was within 7 days post inoculation (dpi) in all the tested substrates. Total colonization of untreated substrates was achieved in BL, BL/SD and SD in 21, 28 and 35 days post inoculations. There was a negative correlation between growth response of P. ostreatus and concentration of used automobile oil. Of the three substrates, banana leaves/sawdust composite showed greatest resistance to the effect of oil treatment. Generally, the length of mycelia added to the substrate during colonization was highest in the 2nd and 3rd weeks of spawn run. Higher concentrations of used oil produced reduced area of pileus. Pileus areas of 3.37cm2 and 45.51 cm2 were produced on banana leaves substrates treated with 100ml and 20ml of spent oil respectively. The number of fruit bodies produced by the oil- treated substrates progressively declined with higher concentrations. Mean number of fruit bodies recorded for SD, BL/SD and BL were 2.60, 5.09, 6.10 and 0.95, 1.04, 0.3 for 20ml and 100ml oil treatment respectively. The result for thickness of pileus and length of stipe followed similar pattern. The fresh weight obtained for the three substrates were significantly different (P<0.05) irrespective of the level of oil concentration except at 100ml where the fresh weights of the fruit bodies obtained from BL/SD and SD were comparable (P<0.05). All samples treated to oil concentrations beyond 20ml were comparable (P<0.05) in the dry weight of P. ostreatus obtained.
