First Report of Stolbur Phytoplasma in Avocado in Spain

Since its introduction to southern Spain in the late 1970s, avocado (Persea americana Mill.) has become an alternative irrigation crop to more traditional and less productive dry land crops such as olive, almond, and grapevine. Avocado orchards in this region currently cover an area of 7,500 ha, producing 58,000 tons of fruit in 2000, with 42,000 tons exported annually to the European Union. In summer 1999, symptoms similar to those caused by phytoplasmas were observed in a plot of avocado cv. Hass. Symptoms of leaf roll, leaf veinal chlorosis with the leaves becoming small and abnormally red, and dwarfing were irregularly distributed on affected trees. Host species of phytoplasmas, such as Lavandula officinalis and Thymus officinalis, are found (1) in orchards surrounded by Mediterranean forests. Leaves from eight symptomatic plants taken from affected plots and leaves from two symptomless plants taken from a healthy plot were collected in May, July, and October 2000, and analyzed for phytoplasma infection by polymerase chain reaction (PCR) assays. DNA for PCR was prepared from leaf petioles, midribs, or trunk phloem by phytoplasma-enrichment fraction according to Daire et al. (2). The stolbur phytoplasma was detected in trees by PCR using stolbur-specific nonribosomal primer pair stol 4 f/r (3) or by nested PCR with 16S rDNA primers pairs P1/P7 and fU5/rU3. Phytoplasmas were detected only in samples collected in July. Phytoplasmas were detected by universal primers in all symptomatic samples analyzed in July, whereas stolbur-specific primers gave positive results in only 75% of the symptomatic samples. Restriction fragment length polymorphism (RFLP) analysis with enzymes AluI and Tru9I confirmed the phytoplasma belonged to the stolbur group. To our knowledge, this is the first description of a phytoplasma disease of avocado trees in Spain. Stolbur is the most important vector-borne disease caused by phytoplasma in several crops. In Spain, it has been identified in various crops and weeds such as grapevine, pear, tomato, carrot, pepper, chicory, and strawberry, and appears to be ubiquitous in herbaceous plant host in several families. Rigorous control of stolbur in avocado should be implemented as soon as possible to avoid further disease development and subsequent economic damage to this industry. References: (1) A. Batlle et al. Eur. J. Plant Pathol. 106:811, 2000. (2) X. Daire et al. Ann. Appl. Biol. 121:95, 1992. (3) X. Daire et al. Eur. J. Plant Pathol. 103:507, 1997.