Plant Protection Inst., Kostinbröd 2230, Sofia, Bulgaria
Man in 't Veld
Plant Protection Service, P.O. Box 9102, 6700 HC, Wageningen, Netherlands
Limonium (statice or sea-lavendar, family Plumbaginaceae) is grown in the Netherlands as a perennial (Limonium sinense) or annual (Limonium sinuatum) crop. Plants have tufted leaves and numerous clustered flowers of different colors and are used for flower arrangements. In August 2000, we received diseased plants of L. sinense cv. Diamond and L. sinuatum. Disease symptoms consisted of leaf wilting followed by plant collapse. The base of the leaves showed progressive necrotic areas that later turned dark brown to black. The cortex of the stem and roots was water-soaked and dark brown to black. Longitudinal sections of stems and roots of diseased plants displayed discoloration of tissues. Rotted root tissue was brown with a characteristic black margin. Rotted vascular tissues and other stem parts were also dark brown. Pith parenchyma turned gray-brown and had a firm, wet rot. In plants with advanced disease symptoms, a cavity in the stem parenchyma was observed. Isolations were made from sections of symptomatic leaves, stems and roots of both Limonium species on cherry and water agar (WA), followed by incubation at 20°C. Phytophthora sp. was isolated consistently from the base of leaves, stems, and roots of diseased plants and identification of isolates was based on morphological characteristics and by isozyme analysis (3). Observations of colony morphology and growth at 35°C were made on V8 agar. Mating type was determined in dual cultures with mating type A2 (P. nicotianae, P 1923 ) and A1 (P. nicotianae, PD98/8/10402). Sporangial features were observed from liquid cultures of the isolates (autoclaved soil-extract or sterile distilled water). All isolates formed colonies consisting of loose, fluffy aerial mycelia. Sporangia and chlamydospores were present in all fungal isolates and all isolates were able to grow at 35°C. Few sporangia were produced on solid media (WA and V8 juice agar), but were abundant in liquid cultures. Sporangia were borne singly or in simple sympodial sporangiophores (3 to 4 sporangia), and were ovoid/spherical, obturbinate with rounded base and had prominent papillae (some had two papillae). Sporangia measured 40 to 64 × 24 to 56 μm, (average 50.4 × 38.4 μm) and had an average length:breath ratio of 1.3:1. Chlamydospores were terminal and intercalary and measured 18 to 44 μm (average 31.6 μm). Hyphal swellings with hyphal outgrowths were present. Isolates of the fungus were heterothallic and produced oogonia and oospores rapidly and abundantly on V8 agar at 22°C only with the A1 mating type of P. nicotianae. We concluded that all isolates from Limonium had the A2 compatibility type. Antheridia were amphigynous. Oogonia were spherical and ranged from 20 to 30 μm, (average 27.5 μm). Oospores ranged from 18 to 27 μm, (average 23.1 μm). The observed characteristics are similar to those described for P. nicotianae. Isozyme analysis, using the dimeric enzymes malic enzyme (EC 126.96.36.199) and malate dehydrogenase (EC 188.8.131.52), revealed the presence of the Mdhp100 allele and the Mdh-2100 allele. Both alleles are characteristic for P. nicotianae (3). Based on morphological features and isozyme genotyping, isolates of Phytophthora from diseased Limonium plants could be assigned to P. nicotianae van Breda de haan (1). A report from Florida associated Phytophthora sp. with root rot of Limonium plants (2) but did not identify the species. According to the multi-decade records at the Netherlands Plant Protection Service (unpublished data) Phytophthora has never been observed on Limonium before. This is the first report of P. nicotianae associated with root rot and basal rot of Limonium plants in Europe.
References: (1) D. C. Erwin and O. K. Ribeiro. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society, St. Paul, MN. (2) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN. (3) W. A. Man in 't Veld et al. Phytopathology 88:922--929, 1998. (4) P. Oudemans and M. D. Coffey. Mycol. Res. 95:1025--1046, 1991.