Diseases
Peronospora manshurica (Naum.) Syd. - Downy Mildew of Soybean
Systematic position.
Class Oomycetes, order Peronosporales, family Peronosporaceae, genus Peronospora.Synonym.
Peronospora sojae Lehman et Wolf.Biological group.
Biotroph.Morphology and biology.
Sources of the infection are the infected seeds and vegetation residues of cultural and wild-growing soya. Weak bloom of sporification is formed on affected cotyledons turning later yellow and falling down. At local affection of adult plants, light green spots are visible on the upper side of leaves, gradually turning brownish; grayish-violet felt bloom of sporification appears on the lower side of leaves. If beans are affected, then the bloom develops inside beans more often. Diffuse form of Peronosporosis causes development of thick bloom of sporification on all leaves and petioles. Beans are deformed on such plants, seedless, with plentiful fungal sporification. Both forms of the disease cause development of dense yellowish-gray mealy bloom of oospores on seeds. Conidiophores are dichotomously ramified. Conidia are unicellular, rounded-ovoid, colorless, 17.7-28.9 x 15.2-20.3 mkm in size. Oospores are rounded, pale-greenish, 30.4-50.7 mkm in diameter. In spring the fungus developes from oospores only. Their viability keeps 1.5 years on seeds and 1 year on vegetation residues. During the vegetation period the fungus gives 4-6 generations of conidia. The incubation period lasts 4-15 days.Distribution.
The disease meets almost everywhere in the countries of Europe, America, Asia, in some countries of Africa. In the former USSR Peronosporosis is recorded in Ukraine, Moldova, and in some regions of Russia.Ecology.
The fungus sporification is formed at temperature 18-20.C and higher and at relative air humidity 95-100%. Hot and dry weather retards development of the disease. Heavy downpours do not promote distribution and development of the disease, washing off spores of the pathogen.Economic significance.
The fungus affects cultural and wild-growing soya, causing reduction of assimilating surface of leaves, resulting in yield decrease by 40% and reduction of oil contents to 1.07%; seed germination decreases by 30%, and weight of seeds is reduced by 5-50%. Control measures are maintenance of crop rotation, autumn plowing, early terms of sowing, destruction of wild-growing soya, seed dressing, treatments of plants during vegetation, and use of resistant varieties.Related references
Zaostrovnykh V.I., Dubovitskaya L.K. 2003. Harmful organisms of soya and system of phytosanitary optimization of its crops. Novosibirsk: Kemerovo SKHI, Dal.nevostochnyi GAU, 528 p. (in Russian).Zhukovskaya S.A., Serebrennikova N.I., Kulikova L.S. 1990. Diseases of soya in the Far East. Zashchita rastenii (Moscow: Agropromizdat) 10: 42-45 (in Russian).