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During autumn 1998, chlorotic mottling, yellowing, and stunting symptoms were observed on cucumber (Cucumis sativus L.) plants in an experimental plot, in Algarve, southern Portugal. The first symptoms appeared 3 weeks after planting, associated with a heavy infestation of Bemisia tabaci (Gennadius). Plants affected early produced few and small fruits. Analysis of double-stranded RNA (dsRNA) extracted from symptomatic cucumber leaves revealed the presence of two dsRNAs of about 8 and 9 kbp, not present in healthy cucumber plants. Reverse-transcription polymerase chain reaction (RT-PCR) using dsRNA or total RNA extracts as template and the oligonucleotide primers 410L and 410U (1), specifically amplified a fragment of expected size of the HSP70-homolog gene of Cucurbit yellow stunting disorder virus (CYSDV). The RT-PCR-amplified fragment was sequenced (Acc. No. AF287474) and showed 99 …