1School of Agriculture, Crop and Animal Production, University of Thessaly, Volos 383 34 Pedion Areos, Greece; 2Benaki Phytopathological Institute, 145 61 Kifissia, Greece

Background and objectives
In June 1995, a severe new leaf spot disease incited by the fungus Pyricularia oryzae was identified on young Ctenanthe oppenheimiana and C. setosa plants, grown as foliage ornamentals. These plants had been imported to Greece from Brazil via the Netherlands in May 1995 [1]. The present study was undertaken in order to elucidate the origin of primary inoculum, and the susceptibility and the host range of various cultivated plants within the family Marantaceae to attack by this fungus.

Materials and methods
The virulence of P. oryzae isolates from Ctenanthe (Brazilian origin) and rice plants (Greek origin) to 14 plant species of Marantaceae was tested on excised leaf blades, floated upside down on sterilized water in petri dishes. Inoculations were made by pipetting 5 droplets of conidial suspension (5x104 spores/ml) on each leaf tissue, replicated three times. Control leaves were inoculated with sterile water droplets. Petri dishes were incubated at 24-30C with a photoperiod of 12 h. Seven days later, the percentage of inoculation points with infection symptoms and the severity of the disease were assessed on the basis of the following categories of symptoms: A = well-formed, yellowish-white, transparent lesions, 8-10 mm in diameter, covered with fungal fructifications; B = light-green specks, 1-2 mm in diameter, scattered at the inoculation points; C = no visible infection symptoms. lsozyme variation of the above Ctenanthe and rice isolates of the fungus was compared. For this, freeze-dried mycelia from cultures grown in potato-sucrose broth and ground in STEB extraction buffer were used. Electrophoresis of the supernatants in non-denaturating polyacrylamide gels was applied. Visualizations of esterase (EC and lactate dehydrogenase (EC isozymes were performed using standard techniques [2].

Results and conclusions
The response of the 14 tested plant species within the Marantaceae to inoculation with P. oryzae isolates from C. oppenheimiana was as follows: two Calathea (C. amabilis, C. ornata), two Ctenanthe (C. oppenheimiana, C. setosa), Maranta leuconeura and Stromanthe sanguinea species were highly susceptible (category A). Calathea insignis, Calathea rufibarba and Ctenanthe pilosa exhibited a hypersensitive reaction (category B), whereas, five Calathea species (C. lietzei, C. makoyana, C.veitchiana, Calathea sp. 'Freddie', Calathea sp. 'Misto'), were immune (category C). On the other hand, the rice isolates of P. oryzae were found to be avirulent on most of the tested plants within the Marantaceae, and only on some species caused symptoms of category B (hypersensitive response). Analysis of esterase and lactate dehydrogenase isozymes showed different banding patterns for rice (Greek origin) and Ctenanthe (Brazilian origin) isolates of the fungus.

The results of this work provide strong evidence of the existence in P. oryzae of highly pathogenic forms specialized to infect cultivated plants within the Marantaceae. Plant species of the above family, however, seem to accommodate resistance genes which, by genetic manipulations, may be utilized to combat rice blast disease.

1. Pappas AC, Vioutogiou I, 1996. Plant Disease 80, 463.
2. Shaw CR, Prasad R, 1970. Biochemical Genetics 4, 297-320.