First Report of Phytophthora citrophthora Causing Fruit Brown Rot of Feijoa in Italy.

Abstract

Feijoa (Feijoa sellowiana) is native to South America. In the early 20th Century it was introduced into Sicily (southern Italy), where it is grown as an ornamental plant and for its fruits. In 1985 a Phytophthora brown rot of feijoa fruits was reported in the province of Syracuse (eastern Sicily) (2). Several species of Phytophthora, including P. citricola, P. citrophthora, and P. nicotianae, were recovered from soil samples taken from trees with infected fruits. These species were experimentally inoculated on detached feijoa fruits and all incited symptoms of brown rot. However, only P. citricola was isolated from naturally infected fruits. In early autumn 1999, an outbreak of Phytophthora brown rot of feijoa fruits was observed in the Syracuse province, in the same site where the disease had been first recorded. P. citricola (95% of the isolates) and P. citrophthora (5% of the isolates) were recovered from symptomatic fruits. The species were identified on the basis of morphological and cultural characters according to Erwin and Ribeiro (1). The P. citricola isolates formed colonies with a distinctive chrysanthemum pattern on potato-dextrose agar (PDA), had an optimum temperature for radial growth of 25°C, and were homothallic with paragynous antheridia and spherical oogonia (mean diameter of oogonia= 20 μm). Sporangia, which were produced only in water or saline solution, were semi-papillate (often with two apices) and variable in shape. The P. citrophthora isolates formed petaloid colonies on PDA, had an optimum temperature of 25°C, and produced noncaducous, papillate (frequently bipapillate), ovoid to limoniform sporangia. They did not produce gametangia. The identification of both species was confirmed by the electrophoresis of mycelial proteins on polyacrylamide slab gel. The electrophoretic patterns of total proteins and four isozymes (alkaline phosphatase, esterase, malate dehydrogenase, and superoxide dismutase) of the P. citricola and P. citrophthora isolates from feijoa were identical to those of reference isolates of these two species from various other hosts. Conversely, they were clearly distinct from the electrophoretic patterns of reference isolates of P. cactorum, P. capsici, P. nicotianae, and P. palmivora. The random amplified polymorphic DNA patterns of the P. citrophthora isolates from feijoa obtained by polymerase chain reaction (RAPD-PCR) were compared with those of reference isolates of other species of Phytophthora and those of P. citrophthora isolates obtained from citrus trees with symptoms of trunk gummosis and root rot, grown in the immediate vicinity of feijoa trees. DNA was extracted and analyzed following previously described procedures, using 16 decamer primers (3). The RAPD-PCR patterns of the P. citrophthora isolates from feijoa were identical to those of the isolates from citrus but were distinct from those of reference isolates of the other species of Phytophthora, suggesting that inoculum of P. citrophthora may have originated from infected citrus trees. P. citricola is known as a causal agent of fruit brown rot of feijoa and guava (Psidium guajava), a closely related species (1). Conversely, this is the first report of natural infections of P. citrophthora on feijoa fruits.