Diseases

Olpidium brassicae Woron., Pythium debarianum Hesse, and Rhizoctonia solani Kuehn. - Blackleg of Cabbage.

Systematic position.

Olpidium brassicae Woron. belongs to the class Chytridiomycota, order Spizellomycetales, family Olpidiaceae, genus Olpidium. Synonyms: Chytridium brassicae Woron.; Asterocystis radicis de Wild.; Olpidium borzu de Wild.; O. radicis (de Wild.) Pascher. Pythium debarianum Hesse. belongs to the class Oomycota, order Pythiales, family Pythiaceae, genus Pythium. Rhizoctonia solani Kuhn. (teleomorph - Pellicularia filamentosa) belongs to Mitosporic Fungi (traditionally order Agonomycetales (Mycelia sterilia), genus Rhizoctonia).

Biological group.

Highly specialized parasites of Cabbage.

Morphology and biology.

Olpidium brassicae Woron. Vegetative body is ameboid, turning into zoosporangium inside host plant cells. Zoosporangium having a thin pallid cover, spherical, 12-120 microns in diameter. In host plant the cell can form to 12 zoosporangia. In the spring the zoospores go to the surface of host plant through aerotropical channels. Zoospores are spherical, 3 microns in diameter, with one flagellum being about 17 microns in length. Retracting flagellum, the zoospore crusts and pours its content as a lump of protoplasm with one nucleus into an epidermical plant cell or into the deeper located cell of primary bark. Thus a new infestation of plants occurs. This cycle of asexual reproduction takes several days. When the sexual reproduction is taking place, the zoospores leaving different zoosporangia merge in pairs, forming biflagellate zygote that fastens to surface of a host cell after a dormant period, crusts and turns into cyst. Cysts are pallid or bright-yellow, 8-25 microns in diameter, with thick warty coat and dense protoplasm. After a dormant period the cyst germinates into numerous primary zoospores being a source of infection. The cabbage is infected at the period of cotyledon or first leaf appearance in hotbeds mainly at superfluous humidity. Plants lose turgor, turn yellow, and usually perish. Root neck becomes soft, black and thin, usually rots. Infects cabbage and other cruciferous species, and also flax, cucumbers, tomato, salad, tobacco, and other plants.
Pythium debarianum Hesse. Hyphae branchy, 5 microns in diameter, with septae in old cultures. Zoosporangia are spherical-ovate, apical or intercalary, 15-26 (19 on the average) microns in diameter, germinate into infection tubes or zoospores. Oogonia smooth, apical or intercalary, usually spherical, 15-28 (21 on the average) microns in diameter. Antheridia are 1-6 per oogonium, monocline or dinocline. Oospores smooth, apleurotic, 15-20 (17 on the average) microns in diameter, sprouting with germinative tube. Infects seedlings of cabbage, beet, peas, pumpkin, cotton plant, soybean, amber cane, lupine, tobacco, string bean, garden radish, potato, clover, vetch, tomato, maize, and many other plants. Usually the underground parts of young plants being weakened under unfavorable conditions become infected. The disease symptoms are the dying-off and blackening bark of the inferior part of stem that becomes thin, usually rots. The above-ground part of plant lags in growth, turns yellow, fades, and usually perishes. The disease develops especially intensively in cold and wet seasons, when the rootage is forming slowly, and roots are thin and weak owing to lack of air.
Rhizoctonia solani Kuhn. Mycelium forms plexuses and black sclerotia being adherent to the underground parts of plants. Hyphae are brown, pallid in places, strongly branching at right angles. Distance between mycelial septae is 22.5 to 195 microns, mycelium width is 6 to 10 microns. Sclerotium width is 1.5 mm and less, they winter in the soil and on plant residues. The disease develops in cold acid soils with superfluous humidification. Infects over 230 species of plants, such as potato, tomato, cabbage, garden radish, beet, alfalfa, string bean, lupine, clover, lentil, flax, and others. The disease is specific in forming the brown dense mycelium and sclerotia on root surface. The infected tissue gets brown. The plant perishes.

Ecology.

Disease development is promoted by abrupt daily fluctuations in temperature, long-term lowering temperatures (13.C and less), high soil humidity (80% and higher) and air humidity (90-95%), soil pH about 5-7.6, and lack of light.

Distribution.

Blackleg disease is widely distributed in the Western Europe (Poland, Germany, Sweden, Denmark), Boreal America. In the former Soviet Union the disease is marked everywhere in agricultural regions. The blackleg is marked in Abkhazia, Kuban, in the Caucasus, and also in Northwest and Central Russia, in Ukraine and Byelorussia. Big losses in cabbage sprouts are marked in Arkhangelsk, Sakhalin, Pskov, Leningrad, and Nizhnii Novgorod Regions.

Economic significance.

In some unfavorable years the Blackleg produces destruction of cabbage plantlets to 50-60%. Productivity is reduced by 30-40%. Control measures are as follows. Blackleg pathogens belong to the same ecological type, so the common protective measures are such as inactivation of the pathogens by steam or chemical preparations in soil indoors, seeds dressing by chemicals; maintenance of optimum temperature (12-15.C) and humidity (not higher than 75%) in hotbeds; regular soil loosening; hotbeds airing; maintenance of sowing norm; application of the necessary amounts of mineral fertilizers; duly singling and careful rejection of sprouts.

Related references.

Bilai V.I., ed. 1988. Microorganisms Ц originators of plant diseases. Kiev: Naukova dumka. 550 p. (in Russian).
Gutsevich S.A. 1935. Disease of cabbage "blackleg". Proceedings of the Leningrad Society of naturalists 58(1): 69-82 (in Russian).
Hawksworth D.L., Kirk P.M., Sutton B.C., Pegler D.M. 1995. Ainsworth & BisbyТs Dictionary of the fungi. CAB International. 616 pp.
Kalashnikov K.Ya. 1941. Blackleg of cabbage. Garden and truck garden. Kiev, N. 3: 21-22 (in Ukrainian).
Osnitskaya E.A. 1969. Pests and diseases of potato and vegetables. In: Polyakov I.Ya., Chumakov A.E., eds. Pests and diseases distribution on agricultural crops in RSFSR in 1968 and the forecast of their appearance in 1969. Moscow: Rosselkhozizdat: 123-144 (in Russian).
Pystina K.A. 1973. Approaches to ecology and geography of Pythium Pringsh. species. In Proc. VI symposium of mycologists and lichenologists of Baltic republics. Riga: Latvian State University: 52-60 (in Russian).
Tupenevich S.M. 1946. Control of blackleg and plasmodiophoroid diseases of cabbage sprouts. Agricultural Bulletin 5: 11-12 (in Russian).

© Khlopunova L.B.