Diseases

Black Leg of Sugar Beet - Disease is caused by different fungi, such as Pythium debarianum Hesse., Fusarium spp. Li: Fr., Phoma betae (Tode) Desm.

Taxonomic position.

Pythium debarianum - Class Oomycota, order Pythiales, family Pythiaceae, genus Pythium;
Fusarium spp. - Class Deuteromycetes, order Moniliales, family Tuberculariaceae, genus Fusarium;
Phoma betae - Class Ascomycota, order Diaporthales, family Valsaceae, genus Phoma (=Plagiostoma).

Biological group.

Facultative parasites and facultative saprotrophs.

Morphology and biology.

Black leg of Sugar Beet is a complex disease appearing in a combination of adverse soil and conditions of shoot development with the subsequent infection of plants by various microorganisms, in particular fungi of genera Fusarium, Phoma, Pythium etc. The most active agent is Pythium debarianum. It affects the underground part of shoots in initial phases of their development only, causing dying and blackening of the lower part of stalk, also thinning and rotting. The aboveground part of the affected plant lags behind in growth, turns yellow, fades and frequently dies off. Disease development is particularly intensive in cold and damp periods when the root system is formed slowly; some parts of the roots die off because of lack of air in damp ground, becoming a source of infection. After formation of the second pair of true leaves the plant becomes resistant to the disease. However, at the end of beet vegetation various kinds of root deformation are appreciable, such as constriction of collar, stripline scab, branching, and malformation. The fungus forms white felt mycelium with poorly ramified zoosporangiophores bearing spheric-ovate zoosporangia (15-26 microns in diameter) that germinate, yielding sprout tubes or zoospores. Zoosporangia can form conidia in absence of liquid droplet moisture. On vegetation residues the fungus also forms oospores that are smooth, apleurotic, 12-20 microns in diameter, germinating by sprout tube. Oospores can be kept in the ground for a long time. The fungus infects seedlings of beet, cabbage, peas, pumpkin, cotton, soy, sorghum, lupine, tobacco, string bean, garden radish, potato, clover, vetch, tomatoes, corn, and many other plants. Typically, Phoma betae gets in the ground with the infected seeds, affecting more developed shoots, mainly the aboveground part of caulicle of primarily weakened plants of sugar beet. Symptoms are similar to these for Pythium debarianum. In its cycle of development the fungus forms arachnoid dark-colored mycelium, where the black points (pycnidia) are formed,100-400 microns in diameter, spheric-flat, light to dark brown; fine unicellular ovate, sometimes almost spherical, colorless conidia are formed inside the pycnidia, 3.5 . 6.5 x 3 - 4 microns in size. The conidia surrounded by mucus appear from pycnidia as long tapes, infecting young plants. Fungi of the genus Fusarium, such as, F.beticola Fr., F.javanicum K., F.moniliforme Sch., F.oxysporum Sch., cause decay of roots of beet sprouts and affect mainly fine lateral roots and the lower part of the main root, causing dry rot of light-brown color. The rotting is visible as hyaline or brown spots or dark strokes (fasciae), located along root. Later the spots spread to the top part of root and upward along stalk. Constriction is formed on caulicle; the root rots, darkens, and becomes thin. Cotyledons and true leaflets wither, turn yellow. Such shoots frequently perish. If the top of the main root is affected, then strong development of lateral rootlets occurs; shoots recover and form ramified root. Beet shoots infected by fusariosis are covered with white arachnoid mycelium with pink or yellowish tint. Conidiophores are well developed, simple or ramified. Macroconidia are formed on simple or ramified conidiophores, being usually crescent, variously curved, having 3-5 septae. Microconidia are formed on long cylindrical conidiophores, numerous, oval-cylindrical, 10.8 . 18.6 x 1.5 - 3 microns in size. Chlamydospores are uni- or bicellular, numerous, colorless. Beet, soy, cotton, potato, clover, wheat, cucumbers, flax, hemp, tomatoes, and many other plants are affected.

Distribution.

Disease is distributed everywhere sugar beet is cultivated. The strongest development of the Black Leg is recorded in Central Black Earth region (Belgorod, Voronezh, Kursk, Tambov Regions); in Orel, Nizhnii Novgorod, Samara Regions, in Bashkiria and Tatarstan; in a number of Regions of Ukraine (Vinnytsya, Khmelnytskyi, Poltava, Chernihiv, Ivano-Frankivsk, Lviv Regions); in Altai, Kirghizia; Baltic States, Byelorussia, and the Northern Caucasus.

Ecology.

Agents of the Black Leg of Sugar Beet develop and affect shoots at raised temperatures and air humidity; in heavy damp soils; in thickened crops; and at deep placement of seeds. P. debarianum develops at optimum temperature 16-19.C, soil acidity 4.7-7.3 pH, soil humidity over 60%. Agent of fusariosis develops at optimum temperature 18-27.C (minimum 10.C, maximum 35.C); optimum air humidity 40-70%. Development of phomosis of beet is favored with high air humidity of (60-80%) and temperature 20-24.C. Infecting agents are kept in the ground on vegetation residues and in seeds.

Economic significance.

The Black Leg is an extremely nocuous disease for shoots of sugar beet. In the presence of a combined adverse conditions of weather, soil, etc. for sprout development the destruction of plants reaches 100%. The disease results in decrease of root yield by 40-50%, of sugar yield by 11% to 40.5%. Control measures include crop rotation, deep autumn plowing of fields, application of fertilizers, seed dressing, treatment of soil to keep it in friable condition, duly thinning and removal of weak plants.

Reference citations:

Al.khovskaya T.F. 1969. Fungal diseases of sugar beet in Chu River valley of Kirghizia. PhD Thesis. Tashkent, 19 p. (in Russian).
Dotsenko A.S. 1985. Mechanism of appearance of rots of sugar beet roots and ways of restriction of their nocuity in Kirghizia. In: Zubenko V.F., ed. Effective measures of protection of sugar beet from diseases at industrial technology of its cultivation. Kiev: VNIS, p. 37-41. (in Russian).
Gorlenko M.V. 1980. Phytopathology. Leningrad: Kolos, 318 p. (in Russian)..
Hawksworth D.L., Kirk P.M., Sutton B.C., Pegler D.M. 1995. Ainsworth & Bisby.s Dictionary of fungi. CAB International. 616 p.
Orekhova V.A. 1985. Control of Black Leg of Sugar Beet in Altai Territory. In: Zubenko V.F., ed. Effective measures of protection of sugar beet from diseases at industrial technology of its cultivation. Kiev: VNIS, p. 45-48. (in Russian).
Popova I.V. 1968. Diseases of sugar beet. In: Polyakov, I.Ya., ed. 1968. Distribution of pests and diseases of grown crops in the USSR in 1967 and the forecast of their appearance in 1968. Moscow: Rossel.khozizdat, p. 94-96. (in Russian)..
Popova I.V. 1969. Diseases of sugar beet. In: Polyakov I.Ya., Chumakov A.E., eds. Distribution of pests and diseases in agricultural crops in the RSFSR in 1968 and the forecast of their appearance in 1969. Leningrad: VIZR. p. 116-119. (in Russian).
Pozhar Z.A. 1959. Diseases of sugar beet sprouts. In: Savchenko E.N., ed. Beet Growing. V. 3. Kiev: Ukrsel.khozgiz (VNIS), p. 385-410. (in Russian).
Pozhar Z.A. 1963. Diseases of sugar beet. In: Polyakov I.Ya., Chumakov A.E., eds. Distribution of pests and diseases in agricultural crops in the USSR in 1962 and the forecast of their appearance in 1963. Leningrad: VIZR. p. 218-225. (in Russian).

© Khlopunova L.B.