The effect of particle pretreatments on the biodeterioration of wood-plastic composites (WPCs) was investigated. WPCs made from untreated and pretreated frond midrib particles of date palm were used. Before the addition of a coupling agent, the wood particles were pretreated or extracted with either cold or hot water and mixed with polypropylene to produce panels, which were then superficially inoculated with an ascomycete’s fungus Trichoderma harzianum T6776. The WPC surfaces were studied using scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX) measurements. In comparison with the control, an intensive growth of T. harzianum hyphae was found over the WPC surface manufactured from untreated date palm midrib particles with the colonies clearly visible. The pretreatments of date palm particles reduced the growth of T. harzianum in comparison with the control and untreated particles. The results suggested that particle pretreatments could be a suitable way to limit the growth of molds over WPC surfaces made from date palm midrib.