A Brief Outline of Infectious Diseases of Olive

Virus infections of olive (Olea europaea), to which littte attention has been paid up to a relatively recent past, are surprisingly widespread, as shown by: (i) the very high presence (above 50% in average) of double-stranded ribonucleic acids (dsRNAs) in the plants analysed in the course of field surveys carried out especially in the Mediterranean and Middle Eastern countries; (ii) the identification in these plants of 15 different viruses with diverse taxonomic allocation. Infections are generally symptomless. When shown, symptoms consist of deformations of fruits and leaves and of foliar discolourations ranging from chlorosis to bright yellowing. “Bumpy fruits” and the “Leaf yellowing complex” are the only two diseases whose viral aetiology seems to be convincingly ascertained. Virus identification is not based on biotests (mechanical transmission to herbaceous hosts is unreliable and there are no differential woody indicators available) nor on immunoenzymatic assays (ELISA), which are also unreliable, but on nucleic acid-based techniques (various RT-PCR protocols). The economic impact of infections has not been determined although recent reports indicate that some viruses seem to affect the yield and the quality of the oil. For an ultimate answer, a comparison needs to be done between selected and sanitazied accessions and their infected counterparts. Equally scanty is the information on the epidemiology of olive-infecting viruses, except for three necroviruses (OLV-1, TNV-D and OMMV), whose transmission through soil, direct or mediated by Olpidium brassicae, has been experimentally ascertained. Olive latent virus 1 (OLV-1) and Cherry leafroll virus (CLRV) are transmitted through seeds and seedlings and, like all the other viruses, with propagating material (nursery productions), which is the major responsible for their worldwide distribution. Viral infections have been detected in 22 countries in the five continents. Preventive control through certification schemes is desirable. One of such schemes designed and implemented in Italy, is based on the pomological and sanitary selection and sanitation of mother stocks.