A New Wilt of Cyclamen Caused by Phytophthora tropicalis in Germany and the Netherlands
Authors: Gerlach W. W. P. , Schubert R. 
Published: May 2007

Unusual symptoms were observed on cyclamen (Cyclamen persicum Mill.) in Germany and the Netherlands in 1997. Symptoms began with a change of leaf color from dark green to olive green and an unspecified flagging of leaves, followed by a yellowing of the margins of older leaves and then yellowing of entire leaves. Corms, sectioned lengthwise, were firm, cream colored, without discoloration of vascular bundles, but with some browning at the base and dark brown discolored roots. Eventually, the plants died. Five isolates (one from Netherlands, two from Northern Germany, and two from Southern Germany) of a Phytophthora species recovered from diseased corms of cyclamen were heterothallic, all A1 mating type, and occasionally formed a few chlamydospores in culture. Sporangia were commonly produced in umbellate clusters. Sporangial shape was naviculate to limoniform with a tapered base. They were papillate, occasionally bipapillate, caducous, with a pedicel of about 1.5 times of the sporangial length. Sporangial dimensions were 52 × 26 μm, with a length to width ratio of 2:1. Cardinal temperatures were 10 and 35°C; optimum growth occurred at 30°C. The pathogen was originally described as a taxonomically nonclassified species closely related to the Phytophthora palmivora complex (2). Based on isolate ATCC No. 76656, received from Uchida (1), we recognized that the above-mentioned cyclamen pathogen exhibits morphological characteristics typical of Phytophthora tropicalis. This taxon has been proposed by Aragaki and Uchida (1) for a number of Phytophthora capsici-like isolates (3) which differ from the type isolate of Phytophthora capsici Leonian. Pathogen identity was further confirmed by cloning subgenomic DNA fragments. Oligonucleotide primers and polymerase chain reaction were employed to amplify the internal transcribed spacer regions ITS1/ITS2 including the 5.8S subunit of the rRNA gene repeat. Both 809-bp products obtained from the American and the European P. tropicalis isolates H 778-1 (Isle of Oahu, Hawaii; host: Dianthus caryophyllus L.) and 066 (Bavaria; host: Cyclamen persicum) respectively, reveal identical nucleotide sequences with the exception of two single bp changes (EMBL accession numbers AJ299734 and AJ299733). In public electronic databases, there were no similar sequences with other Phytophthora spp. Using a sand-corn meal mixture added as inoculum to the planting substrate, pathogenicity of the European P. tropicalis-isolates was shown on Cyclamen persicum, Epipremnum aureum (Linden et André) Bunting, Dianthus caryophyllus, partially on Diascia vigilis Hilliard et B. C. Burtt and Hedera helix L. No pathogenicity was observed on Lycopersicon esculentum Mill., Capsicum annuum L., Cucurbita pepo L., and Cucumis sativus L., representing important hosts of P. capsici, and not on Carica papaya L., a typical host for P. palmivora (Butler) Butler. All data described here confirm the identity of the new cyclamen pathogen as P. tropicalis. The uniformity of the mating type, A1, suggests that the pathogen was inadvertently introduced at one point into Europe and possibly was distributed on seedling plants of cyclamen, since apparently no other host of this pathogen has been observed so far in Europe.

References: (1) M. Aragaki and J. Y. Uchida. Phytopathology 82:1164, 1992. (2) E. Idczak et al. Nachrichtenbl. Deut. Pflanzenschutzd. 50:1, 1998. (3) G. R. A. Mchau and M. D. Coffey. Mycol. Res. 99:89, 1995.