Instituto di Patologia Vegetale, Universita degli Studi, 40126 Bologna, Italy

Background and objectives
Fig mosaic (FM) is an economically important disease which occurs wherever the common edible fig (Ficus carica) grows. The causal agent of FM is transmitted by grafting and in nature is vectored by the eriophyid mite Aceria (=Eriophyes) ficus. Its host range appears to be narrow, mainly restricted to species of the Ficus genera [1].

Very little is known about the real identity of the disease aetiological agent. In recent times, however, large, ovoid, double membrane-bound bodies or particles (DMPs) have been reported in cells of symptomatic leaves by various authors. These DMPs are considered to be FM-inducing pathogens and may represent, together with similar DMPs associated with other eriophyid mite-borne plant diseases of unknown aetiology, a new group of plant viruses with unique traits unlike any previously reported [2].

A fig tree showing the typical leaf mosaic symptoms was recently found in Emilia-Romagna (northern Italy). The foliage was also heavily infested by A. ficus. This source of FM was studied with electron microscopy, and colonies of the resident population of mites were used for transmission tests in an attempt to acquire more knowledge about the nature and characteristics of the disease aetiological agent.

Materials and methods
For ultrastructural examinations, samples of leaf tissues were fixed in 4% glutaraldehyde in 0.05 M sodium phosphate buffer pH 7, and post-fixed in buffered 1% osmium tetroxide. The samples were then stained in bulk overnight in 0.5% uranyl acetate in distilled water, dehydrated in a graded ethanol/acetone series, embedded in Spurr's resin and finally sectioned with an ultramicrotome. Thin sections were double-stained in 2% aqueous uranyl acetate and lead citrate, and then viewed with a Philips CM 10 transmission electron microscope at 600 kV operating voltage. Attempts were made to transmit the pathogen from the FM-affected tree to healthy fig seedlings and other potential host-plant species. Small fig leaf pieces, microscopically examined and found to be infested with A. ficus but not with any other pests, were placed on the tip of test plants at the two to three-leaf stage. These plants were then grown in a greenhouse at temperatures of 25-27C during the day, 18-20C at night and a 15-h photoperiod. They were protected with insect-proof cages, kept under moist conditions for a few days to facilitate the eriophyid survival and feeding, and observed for symptom development.

Results and conclusions
Electron microscopy observations of symptomatic leaves of the selected FM source revealed the presence of DMPs, 150-200 nm in diameter, in the cytoplasm of parenchyma cells. Transmission trials with A. ficus confirmed the transmissible nature of the FM pathogen from fig to fig. In other plant species, it was shown that a disease-causing agent was also mite-transmitted to periwinkle (Catharanthus roseus).

Virus-like symptoms were obtained on a periwinkle plant after an incubation period of 40 days. This plant showed chlorotic spotting, mosaic and yellowing of the leaves and malformation of the laminae, and the flowers were also subsequently observed. The infectious nature of the pathogenic agent was further confirmed by graft-transmission tests. The syndrome on periwinkle closely resembled that of the naturally mosaic-affected fig tree and the symptomatology reported in other regions for FM. Electron microscopy of diseased periwinkle leaf tissues clearly demonstrated the occurrence of DMPs, morphologically similar to those present in the original FM source used in this study. No other known pathogen was detected in the plant. Results of the present experiments indicate that a new host plant for the FM-inducing agent, that is not a member of the Moraceae family, has been found. This could be an important base for future investigations on FM aetiology.

1. Slykhuis JT, 1980. In Harris KF, Maramorosch K, eds, Vectors of Plant Pathogens. Academic Press, pp. 325-356.
2. Ahn KK, Kim KS, Gergerich RC et al., 1996. Journal of Submicroscopic Cytology and Pathology 28, 345-355.