Abstract
The present report dwelt upon the observations on downy mildew of wheat and its causal fungus, Phytophthora macrospora.
As for the causal fungus, only the oospore had been known until TASUGI discovered for the first time its conidial stage on rice in 1927. He reported in detail the morphological character of the fungus in 1930, and he stated that the causal organism should be transfered to Nozemia from Sclerospora.
The author happened to find the conidial stage of the fungus on wheat-plants badly affected by the downy mildew after a flood at Ikeda, Prov. Tokachi, Hokkaido in 1937. The conidia produced on wheat leaves in the fields were 57.5-97.5μ in length and 30.0-65.0μ in width; average 74.6×46.8μ. When these leaves were soaked in water, the spores elongated to 65.0-112.5×32.5-55.0μ; average 87.37×43.75μ. The latter measurements were correspondent to those reported by TASUGI about the conidia discovered on affected rice plants. Oogonia formed in the tissues of affected wheat were 55.0-100.0×50.0-95.0μ; mostly 62.5-75.0×56.5-72.5μ; average 69.12×65.13μ. Antheridia paragynous, 22.5-41.25×12.5-22.5μ; average 27.75×15.15μ. Oospores, . 42.5-72.5×42.5-70.0μ; average 60.25×56.67μ. Zoospores 12.5-16.25×10.0-12.5μ; average 14.17×10.8μ. The conidia were produced abundantly at 11°-24°C.
According to the morphological characteristics of the conidia it was concluded that the causal organism undoubtedly belongs to Phytophthora. Having compared the conidia and oospores formed on wheat and rice with those on other affected cereal crops and wild grasses, the author came to the conclusion that Sclerospora macrospora SACC., S. Kriegeriana P. MAGN, , S. Oryzae BRIZI are one and the same species.
Hereby the author propose to change the nomenclature of the causal organism as follows:
Phytophthora macrospora (SACC.) S. ITO et I. TANAKA n. comb.
=Sclerospora macrospora SACC. Hedw. XXIX, 155, 1890: Syll. Fung. IX, 342, 1891-BERL. Monogr. Peronosp. 148, 1903.
=Sclerospora Kriegeriana P. MAGN. Verh. Ges. Deutsch. Nat. u. Aerzte, 67; Vers. zu Lübeck, 1895, 100-SACC. et SYD. in SACC. Syll. Fung. XIV, 461, 1899.
=Sclerospora Oryzae BRIZI, La Peronospora del Riss, in Nature, X, 168, 1919-TROTT. in SACC. Syll. Fung. XXIV, 65, 1926.
