In countries bordering the Mediterranean sea, olive trees serve as host to a fairly high number of plant parasitic nematodes. Several of these nematodes are sedentary endoparasitic forms, many of which are recognised as pathogens to olive. Distribution, pathogenicity and host-relationships, are discussed. Currently, measures for practical nematode control in olive are limited to production nematode-free propagating material. INTRODUCTION The leading olive-producing countries in the Mediterranean basin are Spain, Italy and Greece. Some 750 million trees are grown over approximately 8.5 million ha, about 97% of which are in Mediterranean countries (COI, 1991). Several plant-parasitic nematodes have been found associated with olive trees wherever they are grown, including Meloidogyne spp., Pratylenchus spp., Heterodera mediterranea Vovlas, lnserra and Stone, Helicotylenchus spp. and Criconemella xenoplax Raski (Abrantes and Santos, 1991; Lamberti and Vovlas, 1993; Nico et al., 1998; Pena-Santiago, 1990). The effects of some plant parasitic nematodes on plant growth and yield, are largely the result of the disruption these organisms cause to the normal process of plant root growth and soil exploration for both water and nutrients. Nutrient deficiencies resulting from failure of the plant root system to explore and exploit the soil adequately can also be major consequences of plant parasitic nematode attack. This paper report a review of the distribution, parasitic habits and pathogenicity on olive roots of the main plant parasitic nematodes attacking this tree. MATERIAL AND METHODS Samples of olive feeder roots, together with rhizosphere and bulk soil, were collected with a shovel from fue upper 20 cm of soil from olive nurseries and orchards in several olive growing afeas from Italy and Spain. For identification, specimens were glycerol-infiltrated and preserved by conventional methods (Seinhorst, 1966). Segments of infected olive roots were fixed in a formaldehyde-chromoacetic solution, dehydrated in tertiary butyl alcohol series and embedded in paraffin (Johansen, 1940). ROOT-KNOT NEMATODES (MELOIDOGYNE SPP.) Root-knot nematodes (Meloidogyne spp.) are economically important pathogens and distributed worldwide. They are sedentary obligate endoparasites and parasitize thousands of different plant species inducing the formation of knots or galls on roots of susceptible host plants (Fig. 1). The main root-knot nematode species attacking olive trees in fue Mediterranean basin are Meloidogyne arenaria (Neal) Chitwood, M. incognita (Kofoid & White) Chitwood, M. javanica (Treub) Chitwood and M. lusitanica Abrantes et Santos. They occur with a sporadic distribution in nurseries and established orchards, and cause heavy galling (Fig. 1) and growth retardation (Lamberti and Baines, 1969a; Nico et al., 1998; Sasanelli et al., 1997). lnfections by these root-knot nematode species induce cell formation and Proc. 4th IS 00 Olive Growing Eds. C. Vitagliano & G.P. Martelli A~t. u~... ~I!i; TI/UI/ ?M? 857 stelar destruction in olive roots (Fig. 1). Preferred feeding sites are primary phloem or adjacent undifferentiated parenchymatic cells of fue perycicle. In response to fue feeding, host tissue undergoes pronounced morphological and physiological changes. Feeding cells become hypertrophied tuming in multinucleate giant cells, possibly as a result of the introduction of secretions of oesophageal glands. Giant cells are essential for a successful host-parasite relationships, acting as a metabolic sink towards photosynthates so that plant growth and yield are affected. ROOT -LESION NEMA TODES (PRATYLENCHUS SPP.) Root-lesion nematodes are migratory endoparasites of the root cortical parenchyma in which they cause large necrotic afeas which may coalesce and affect most of fue root (Fig. 2). Nematode damage to the root system can result in destruction of cortical parenchyma which may cause sloughing-off of the tissue and severe necrosis. Nematode population density as well as environmental conditions affect symptom expression. Surveys of olive nurseries and orchards in countries bordering the Mediterranean basin demonstrated the wide distribution of these nematodes. The species most frequently encountered were Pratylenchus penetrans (Cobb) Chitwood et Oteifa and P. vulnus Allen et Jensen (Lamberti and Baines, 1969b; Nico et al., 1998). P. vulnus (Fig. 2) was shown to be a potential olive pathogen in artificial inoculations of several olive cultivars (Arbequina, Ascolana, Manzanilla, Picual) (Lamberti and Baines, 1969b; Nico et al., 1998). CYST -FORMING NEMA TODES . Cyst forming ne~atodes are characterized by fue cyst stage which can persist in the so11 for several years m the absence of a host planto These very destructive nematodes are distributed worldwide. The cyst-forming nematode, H. mediterranea, has been found recently in commercial 'Manzanilla' olive orchards established in sandy soils in southem Spain (Castillo et al., 1998). Sedentary juvenile stages and adult females showed a semiendoparasitic feeding habit and did not completely penetrate olive roots. Histopathological studies of infected olive roots showed that nematode feeding induce syncytial formation thus eliciting cell alterations in fue cortex, endodermis, pericycle, and vascular parenchyma of the roots. The large nematode population and the anatomical alterations of infected roots suggest that parasitism of olive roots by H. mediterranea may contribute to the decline of olive seedlings in new plantations or in nurseries. MIGRATORY AND SEDENTARY ECTOPARASITIC NEMATODES Several species of Helicotylenchus (H. dihystera, H. digonicus, H. pseudorobustus, H. erythrinae and H. oleae) have been observed associated with root necrosis, which are considered capable of affecting olive cree growth under particular growing conditions. Species of Xiphinema are also common in olive rhizosphere, and X. elongatum has been shown to affect plant growth in Egypt (Diab and El Eraki, 1968). The biology of X. diversicaudatum is under study to determine weather or not it is a vector of olive nepoviruses. In addition, a number of rather specialised sedentary ectoparasitc nematodes attack olive: a biotype of the citrus nematode Tylenchulus semipenetrans Cobb, Gracilacus peratica Raski, and Ogma rhombosquamatum (Mehta & Raski) Andrassy infect olive trees in Italy (Inserra et al., 1980; Vovlas and Inserra, 1981; Vovlas and Inserra, 1976), and Trophotylenchulus saltensis Hashim in Jordan (Hashim, 1983). However, convincing evidence of their pathogenicity is stilllacking. Three species of sedentary-ectoparasite reniform nematodes (Rotylenchulus macrodoratus Dasgupta, Raski et Sher, R. macrosomus Dasgupta, Raski et Sher and R. reniformis Lindford et Oliveira) associated with olive crees have been studied in detail (Badra and Khattab, 1980; Vovlas and Inserra, 1976), but evidence of actual plant damage has not been obtained. The nematode survives in the soil in the juvenile and adult male stages. Irnmature females penetrate roots and become established in the endodermis