CULTURAL AND MORPHOLOGICAL CHARACTERISATION OF PHYTOPHTHORA ISOLATES ASSOCIATED WITH ROOT ROT OF OLIVE TREE ME SANCHEZ-H ERNANDEZ, M MUNOZ-GARCIA and A TRAPERO Dep. Agronomia. ETSIAM. Universidad de Cordoba. Apdo. 3048.14080 Cordoba. Spain. Background and objectives A wilting and death of young trees is severely affecting new orchards of olive tree (Olea europaea L.) in southern Spain. Since aerial symptoms are non-specific, Verticillium wilt and root, rot diseases, abiotic factors, and even pest injuries, have been associated with this problem. During 1996-97, a disease survey conducted in 80 affected olive plantations revealed the importance of the root rot component. Climatic conditions during the survey period, mainly the very high rainfall rate,! determined that soils in the olive orchards remained waterlogged for a long-time. In these conditions, incidence of root rot increased, being associated with tree death in around 65% of the total number, of plantations surveyed. Two fungal species, preliminarily identified as ,Phytophthora' megasperma and Cylindrocarpon destructans, were isolated from diseased roots in 94% and 6% of the orchards, respectively [1]. This work is dealing with the cultural and morphological characterisation of the P. megasperma isolates obtained from rotten roots. Materials and methods Stock cultures of P. megasperma isolates were maintained on Carrot-agar medium (CA) at 20 C in darkness and sub-cultured at 4 weeks intervals. Growth rate and colony morphology tests were carried out at different temperatures (5,10,15, 20, 25, 30 C) in 9 cm diameter Petri dishes containing 20 ml of CA. Sporangia of every isolate were produced in Petri dishes containing blocks, of a CA culture and non-sterile soil extract. Plates were incubated for 2-3 days in the dark at 15 and 20 C. Length and breadth of 25 sporangia were measured for each isolate, using only newly formed mature sporangia. Sexual structures were measured on CA cultures of 7-12 days. Antheridia, oogonia and oospores size was achieved by counting 50 units of each mature structure. Oospore wall thickness was also determined. All sexual and asexual structures were measured following their removal to microscope slides and staining with lactophenol-acid fuchsin. Pathogenicity of isolates was tested on waterlogged artificially infested soils [1]. Results and conclusions Colony morphology of all isolates was best determined by growing them on CA at 20 C. Isolates consistently fell into two morphological types. The predominant type, called the A-group comprised 36 isolates from a total of 52 isolates tested. These isolates produced colonies with a flat or slightly petaloid growing pattern, with a powdery appearance when oospores were abundant. The remaining 16 isolates constituted the B-group. These isolates produced colonies with a clearly petaloid pattern (rosette). Moreover, comparison of growth rate at different temperatures separated the isolates in two groups in agreement with the morphological groups described. Both types of isolates were pathogenic to young olive trees in artificial inoculations. Nevertheless, it was not possible to asign any distribution area particular to any fungal group along the field area surveyed. The morphological characterisation of asexual and sexual structures and growth rate of the A-group seems to confirm the assignation of the isolates to the large-oogonia P. megasperma group [2]. Asexual characteristics of B-group were similar to those from P. megasperma, but sexual behaviour differed. B-group of isolates were mainly self-sterile, likely heterothallic. In addition, a number of B-group isolates were weakly self-fertile, developing only amphigynous antheridia. This results, suggest that B-group of Phytophthora isolates from olive tree could constitute a heterothallic variety into the P. megasperma complex or they could be assigned to a different species. Currently pairing tests and molecular characterisation are in process. References 1. Sanchez-Hernandez ME, et. al. 1997. Proceedings 10th. Congress of the Mediterranean Phytopathological Union, pp.79-82. 2. Hansen EM, Maxwell, DP, 1991. Mycologia 83,376-81.