An integrated approach for understanding the high infection rates of olive viruses in Tunisia

This study was conducted to understand the high incidence of viruses previously reported in Tunisian olive groves. Forty three cultivars, native and introduced, under different cultivation regimes, were sampled in Tunisia, one of the countries with the highest levels of olive viruses. All olive-infecting viruses were tested. Total nucleic acids were extracted from 280 olive trees and their use as template for RT-PCR resulted in several non-specific products. Alternatively, dsRNAs were also extracted and 229 sampled trees showed dsRNAs molecules, suggesting the presence of viruses. However, RT-PCR tests using dsRNA as template, allowed to identify viruses in 209 samples. Olive mild mosaic virus (OMMV), Olive latent virus 1 (OLV-1), Olive leaf yellow associated virus (OLYaV) and Tobacco necrosis virus D (TNV-D) were detected. OLYaV was the most prevalent (63.0%), followed by OLV-1 (42.9%), TNV-D (21.80%) and OMMV (9.50%). The highest infection levels were observed in the northwest and centre east regions of Tunisia. Native cultivars presented higher virus incidences, mostly due to the most widespread cultivars in Tunisia: Chemlali (87%), Chetoui (94%), Meski (94%). Among the introduced cultivars, Picholine, Arbequina and Koroneiki presented the highest virus levels (88%, 79% and 64%, respectively). Considerable differences in virus presence were observed in samples from olive trees under different modes of management where levels were significantly (p>0.05) higher in intensive orchards, regardless of the region and cultivar. To our knowledge, this was the first time that OMMV and TNV-D were detected in Tunisia and that such a complete analysis was performed.