Collar and Root Rot of Olive Trees Caused by Phytophthora megasperma in Sicily.

Abstract

Olive (Olea europea L.) is grown on about 154,000 ha in Sicily (southern Italy). In the summer of 1999, a few 3-year-old olive trees with decline symptoms were observed in a recently planted commercial orchard in the Enna province (Sicily). The trees were propagated on wild olive (O. europea L. var. sylvestris Brot.) rootstock. Aerial symptoms, consisting of leaf chlorosis, wilting, defoliation, and twig dieback followed in most cases by plant death, were associated with root rot and basal stem cankers. A Phytophthora sp. was consistently isolated from rotted rootlets and trunk cankers using the BNPRAH (benomyl, nystatin, pentachloronitrobenzene, rifampicin, ampicillin, and hymexazol) selective medium. Pure cultures were obtained by single-hypha transfers. The species isolated from symptomatic olive trees was identified as P. megasperma Drechsler on the basis of morphological and cultural characteristics. All isolates were homothallic, with paragynous antheridia. The diameter of oospores varied from 28 to 42 μm (mean ± SE = 36.3 ± 0.4) when they were produced on potato-dextrose agar (PDA) and from 30 to 43 μm (mean ± SE = 37.8 ± 0.4) when they were produced in saline solution. Sporangia were non-papillate. Optimum and maximum temperatures for radial growth of the colonies on PDA were 25 and 30°C, respectively. At 25°C, radial growth rate was about 6 mm per day. The identification was confirmed by the electrophoresis of mycelial proteins on a polyacrylamide slab gel. The electrophoretic banding patterns of total soluble proteins and three isozymes (esterase, fumarase, and malate dehydrogenase) of the isolate from olive were identical to those of two isolates of P. megasperma obtained from cherry and from carrot in Italy and characterized previously (1). Conversely, they were clearly distinct from the electrophoretic patterns of four isolates of P. megasperma var. sojae Hildebr. from soybean (= P. sojae Kauf. & Ger.), from those of three isolates from asparagus tentatively identified as P. megasperma sensu lato (1) and from those of reference isolates of various species producing non-papillate sporangia, including P. cambivora (Petri) Buisman, P. cinnamomi Rands, P. cryptogea Pethybr. & Laff., P. drechsleri Tucker, and P. erythroseptica Pethybr. Pathogenicity of the isolate from olive was tested in the greenhouse at 18 to 25°C using 18-month-old rooted cuttings of olive cv. Biancolilla. Cuttings were inoculated on the lower stem by inserting a 3-mm plug taken from actively growing colonies on PDA into an incision made with a sterile scalpel. The wound was sealed with waterproof tape. Agar plugs with no mycelium were placed into the stem of cuttings used as a control. The bark was stripped and lesion areas were traced and measured 60 days after inoculation. The isolate from olive produced a brown necrotic lesion (mean size = 500 mm2) around the inoculation wound and was reisolated from the lesion. Conversely, the wound healed up on control plants. P. megasperma has previously been recognized as a pathogen of olive in Greece and Spain (3). However, this is the first report of P. megasperma causing root and collar rot of olive in Italy.