Abstract
I. Damping-off of China aster (Callistephus chinensis).
Seedlings are infected. The diseased part becomes brown, with water-soaked appearance, resulting in the damping-off. Though the causal fungus is a pythiaceous one closely related to Pythium megalacanthum, it differs from the latter in the production of conidia, absence of sporangia and the size of oospores. Therefore, the writers propose to give it a new name, Pythium megalacanthum var. Callistephi which may be described as follows: -
Mycelium hyaline, well developed in host tissue and in culture. Hyphae branched, 2.8-6.4μ in diameter, often septate in old culture. Conidia usually terminal and rarely intercalarly, hyaline and sphaerical, 23.1-44.4μ (av. 34.5μ) in diameter, germinating directly. Oogonia terminal, hyaline, sphaerical, usually covered with many spines (5.6-8.3×2.8-4.6μ), without spines 32.4-50.9μ (av. 41.6μ) in diameter. Antheridia terminal or intercalarly, hyaline, kidney-shaped, 11.1-21.3×8.3-17.6μ (av. 15.7×12.6μ), usually one, rarely two to an oogonium. Oospores sphaerical, almost filling the oogonium, 22.2-39.8μ(av. 33.1μ) in diameter, wall light yellowish brown, smooth, 0.9-2.8μ in thickness. Parasitic on Callistephus chinensis NEES.
The fungus grows well on oatmeal and potato agar. The range of the temperature of its growth lies between about 10°C and 29°C and the optimum 23°C. The most favorable pH of the medium for its growth seems to be pH 5.8-6.9.
In inoculation experiments, the seedlings of Callistephus chinensis, Lactuca sativa and Papaver Rhoes are severely infected, resulting in the typical damping-off. Zinnia elegans is somewhat less susceptible than the above-mentioned plants. Calendula officinalis and Antirrhinum majus are weakly attacked and Solanum Lycopersicum reveals only the root-browning symptom. No sign of infection is seen on Solanum melonga and Cucumis sativus.
II. Damping-off of Zinnia (Zinnia elegans.)
Seedlings are also infected, diseased parts becoming brown and damping-off as in the case of China aster. A pythiaceous fungus is easily isolated from the diseased part.
The fungous growth is best on potato agar, and well also on bean agar. On the other media it grows feebly or not at all. The optimum temperature for its growth seems to be about 24°C, the minimum and the maximum being 8°C and 35°C respectively. The optimum pH of the medium for its growth lies between pH 5.8 and 6.5.
In inoculation experiments, it attacks most severely the seedlings of Zinnia elegans and Callistephus chinensis, Brassica oleracea, Platycodon glaucum and Lactuca sativa, Antirrhinum majus, Calendula officinalis and Papaver Rhoes are also severely infected. Cucumis sativus, Solanum Lycopersicum and S. melonga are weakly attacked. No infection is observed in Pisum sativum.
Compared to several species of the genus Pythium where oogonia are covered with spines, it is closely related to Pythium spinosum, in morphological and physiological respects. But they differ somewhat in the pathogenicity and the size of conidia. So, the writers consider that their fungus is a strain of P. spinosum.
