An oligonucleotide microarray for the detection of some fruit-tree viruses was designed and its theoretical detection limit was assessed using Cy3-labelled oligonucleotides. The real sensitivity of the microarray was compared for different kinds of fluorescently labelled targets: (a) cDNA and PCR amplified targets, (b) PCR amplified targets labelled using three different labelling methods. In the first case (a), the number of viral cDNA molecules was below the assessed detection limit of the microarray and only PCR amplified targets were detected. A second comparison (b), done on 3 selected viruses, included indirect labelling, the direct incorporation of labelled-dUTPs, and the use of Cy3-labelled primer. The targets labelled most intensively were produced by the Cy3-primer labelling (2 of 3 viruses) or by the indirect labelling method (1 of 3 viruses), the weakest signal showed targets labelled directly (all 3 viruses). The use of Cy3-primer labelling involved the simplest preparation and the lowest cost, however occasional weak cross-hybridization appeared. The indirect labelling method was of the highest specificity. The probes hybridizing near the 3-end of the targets showed the lowest intensities of fluorescent signal.

Biology Centre of the Academy of Sciences of the Czech Republic Institute of Plant Molecular Biology Branisovska 31 370 05 Ceske Budejovice Czech Republic

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