First Report of Phytophthora cryptogea on Osteospermum spp. in South Africa.

Abstract

Osteospermum is the largest genus of the tribe Calendulea of the Compositae (Asteraceae) and has a center of diversity in South Africa with approximately 40 known species in the Cape Province (3). This indigenous plant genus is also a popular floricultural crop grown in South Africa because of drought and high temperature tolerance. Two diseased Osteospermum sp. samples were submitted by commercial nurseries to the Stellenbosch University Plant Disease Clinic. Both samples showed similar symptoms including black lesions on the lower stem, petioles, and the base of lower leaves. Isolations were made from roots, stems, and petioles following surface disinfestation of the lesions and plating on potato dextrose agar (PDA), water agar, PARP, and PARPH (1). The only microorganism that was consistently isolated from all lesions was a species of Phytophthora, from which single hyphal tip cultures were prepared. The cultures were identified morphologically as Phytophthora cryptogea Pethybridge & Lafferty. Morphological characteristics and cardinal growth temperatures followed Lévesque and de Cock (2). Distinctive morphological characteristics included abundant hyphal swellings in sterile soil extract and hemp seed water cultures, nonpapil-lated, persistent, internally proliferating, obpyriform sporangia having mean dimensions of 36 × 26 μm and mean length/breadth ratios that ranged from 1.30 to 1.53 (1). Cultures had a petaloid pattern on PDA and no growth at 35°C that distinguished it from Phytophthora drechsleri (1). Analyses and comparison of the internal transcribed spacer (ITS) regions of isolate STE-U 6133 (GenBank Accession No. GI DQ479410) supported the P. cryptogea identification. STE-U 6133 has been deposited in the Stellenbosch University culture collection. Pathogenicity of isolate STE-U 6133 was tested on six plants of Osteospermum jucundum (Phill.) T. Norl. and O. ecklonis (DC.) Norl. in individual containers. Inoculum was grown on V8 agar for 7 days, and 10-mm culture disks were incubated in a filter (0.2 μm) sterilized 1% soil extract solution (1). The disks were vortexed vigorously and filtered through miracloth (Calbiochem Corp., La Jolla, CA) to remove agar disks and excess mycelium. The inoculum concentration was adjusted to 104 sporangia/ml and incubated at 4°C for 4 h. Six plants were inoculated with a 10-ml suspension of sporangia and zoospores by pipetting the suspension at the collar of each plant. Another six plants were treated with water as controls. The inoculated plants were kept at temperatures ranging from 22 to 25°C and were watered until saturation every second day. Initial symptoms were black lesions on the lower stem and petiole bases developing 14 to 18 days after inoculation, followed by wilting of some plants. Control plants remained healthy. Isolations from symptomatic plants yielded only P. cryptogea of which pure cultures were established, completing Koch's postulate. The experiment was repeated with similar results. To our knowledge, this is the first report of P. cryptogea as a pathogen of O. jucundum and O. ecklonis in South Africa. P. cryptogea has been reported as a pathogen on Osteospermum spp. in other countries (4).