Short DNA gene sequences (oligonucleotides) from the ribosomal spacer regions of bacterial and fungal pathogens were used to identify and monitor economically important apple diseases. The oligonucleotides or probes were attached to a nylon membrane by an amine modified linker arm and arranged in a precise pattern to form an array for detecting five pathogens corresponding to five apple diseases. Initially the specificity of the probes was determined by hybridizing pure cultures of the pathogens to the probes. The DNA array correctly identified Botrytis cinerea, Penicillium expansum, Podosphaera leucotricha, Venturia inaequalis, and Erwinia amylovora and eliminated closely related species. When the array was used to monitor V. inaequalis ascospores collected from spore traps located in orchards, it confirmed the presence of ascospores as predicted by the disease forecasting model. Preliminary tests to quantify P. leucotricha populations using grayscale values was effective to 20 conidia per leaf disk. The DNA array is a promising new detection system for accurate identification of several pathogens in a single test with the potential for being a new tool for epidemiological studies.