Scientific journal
European Journal of Natural History
ISSN 2073-4972

RHIZOCTONIA И GAEUMANNOMYCES SPECIES COMPOSITION UNDER THE CONDITIONS OF WESTERN SIS-CAUCASIA

Zhalieva L.D.
Western Sis-Caucasia root rot pathogen population has been studied at Krasnodar Lukyanenko Research Institute of Agriculture for three decades. It is presented by the following genera: Alternaria, Pythium, Helminthosporium, Wojnowicia, Fusarium, Cercosporella, Gaeumannomyces, Rhizoctonia, which have various occurrence frequencies on winter wheat. The share of Rhizoctonia fungi has been reported to exceed in frequency all other pathogens. The Rhizoctonia fungi, causing root rot on winter wheat, include several species: Rhizoctonia solani Kuhn. Teleomorph - Thanatephorus cucumeris (A. B. Frank) Donk, Rhizoctonia cerealis Van der Hoeven. Teleomorph - Ceratobasidium cereale D. Murray & L. L. Burpee (Anastomosis - AG-D.) и Rhizoctonia oryzae Teleomorph - Waitea circinata var oryzae  (Anastomosis WAG-0); Rhizoctonia zeae Teleomorph - Waitea circinata var. zeae (Anastomosis WAG-Z), and р. Gaeumannomyces - G. gramynis var tritici, G.graminis var avenae и  G. gramynis var graminis.

Rhizoctonia fungi have been widely studied on grasses, since they tend to cause serious damage to the plants covering golf-links. On grain crops these fungi have been observed in the USA by D.M. Weller et al. (2002), R.J. Cook (1981), R.W. Smiley and D.E. Wilkins (1992), A. Ogoshi (1987),  in Australia by J.S. Gill, K. Sivasithaparam and  K.R.J. Smet-Fem, in Germany (especially on malting barley), in Turkey by E. Demirci, C. Eken and H. Zengin  (on sorghum), in Korea by Dong-mei Li, Ke-qiang Cao (2001), in Japan by T. Tsukiboshi and T. Kimigafukuro (1993). They have mostly identified Rhizoctonia solani, and less often Rizoctonia cereals or Rhizoctonia oryzae.

In the Ukraine in the 1970-s root rots on grain crops have been considered to be caused by Rhizoctonia solani, but more recently the pathogen have been proved to be Rizoctonia cereals. According to the Russian researchers Dolzhenko, Zdrazhevskaya, Burkova et al. (2003), in the Northern Caucasia root rots on grain crops are caused by Gaeumannomyces, Fusarium and Pseudocercosporella sp. Vlasova, Nikitina and Zhukova (All-Russian Plant Protection Institute) reported in 1995 that in Rostov region root rots on grain crops were caused by Fusarium-Pseudocercosporella fungi.

In recent years Rizoctonia sp. have been occurring on winter wheat more and more often, while in the 1970-s this pathogen did not even damage cereal crops. Z.A. Bochkareva and L.N. Tarasenko reported in 1974 that root rots were caused by fungi belonging to Fusarium culmorum, F. sporotrichiella, Ophiobolus graminis, Wojnovicia graminis and Helminthosporium sativum.

Gaeumannomyces fungi have first been described in Australia in 1852 (154 years ago).

S.D. Garrett (1981), H.E. Nilsson and J.D. Smith (1981), D. Hornby (1998), R.J. Cook and D.M. Weller (1987) are among those numerous scientists who have been studying fungi belonging to this genus.

According to Garrett, grain crop diseases caused by these fungi are spread wherever these crops are grown in temperate and arid zones under irrigation. This disease has been admitted the most devastating disease damaging cereals.

The material has been collected during route inspections carried out jointly with regional and local plant protection stations. Pathogens were isolated in accordance with the conventional methods adopted in mycology using selective media.

Our studies carried out at KNIISH in the 1980-s revealed a small share of Rhizoctonia sp. (2-5%) in the complex of pathogens causing root rots. In 1990 their occurrence frequency in the northern part of the region (farm "Rossia" Pavlovskij area) averaged 3%; in the central part (farm "Rodina" Ust-Labinsk area) - 5.1-13.1%, in the southern sub-mount part (farm "Nasha Rodina" Gulkevichskij area) - 1%.

Our research has shown that the share of Rhizoctonia fungi in the Western Sis-Caucasia has been steadily growing compared to other pathogens. In Krasnodar region the share in 2000 accounted for 16%, in 2001 - 26.2%, in 2002 - 33%, in 2003 - 36.5% in 2004 - 38% and in 2005 - 40.5%. In the last years the rate of growth has slowed down from 10.2% in 1999-2000 to 2.4% in 2004-2005.

The visual symptoms of Rhizoctonia disease on cereal crops include thick brown coating of mycelium and sclerotium covering the root surface. The damaged tissue gets brown. This type of damage is also characterized by distinct eye spot. Ellipse-shaped light spots emerging at the base of leaf sheath and straw have distinct dark-brown edges, which help to distinguish them from Cercosporella spot disease. The spots which appear mainly on leaf sheaths may reach 15-25 mm in length. The major sources of primary infection are usually Rh. solani sclerotia accumulated in soil and mycelium contained on plant residues. The sclerotia maintain their viability in soil for two years. The major role in disease development belongs to the fungus mycelium, which is characterized by intensive growth. Under favorable climatic conditions (darkness, humidity up to 95% and air temperature 28-30oC) the infection quickly spreads to the upper parts of plant, including leaf blades, and even to the neighboring plants. Mycelium fragments may be dispersed to long distances by wind and cause new infection.

According to the data of the International Rice Growing Institute, there exist no sources of immunity. Rice varieties, which are resistant to Rhizoctonia at the sprouting stage, may become susceptible at the stage of maturation and vice versa. In the Russian Federation the problem of winter wheat resistance to Rhizoctonia fungi has not been studied, neither were studied the strains of the fungus causing Rhizoctonia root rot on cereals.

In our studies Rhizoctonia fungi have been isolated on root rot damaged winter wheat plants grown after all studied previous crops but for fallow. In Rostov region the frequency of these fungi on plants grown after winter wheat varied between 1 and 19.5%, after peas - between 0.5 and 5.5% and after corn - between 3 and 13% depending on the climatic and geographical conditions.

Fungi belonging to this genus were isolated on winter wheat plants starting from the germination stage and through to the stage of full grain ripeness. At the tillering stage their share among other pathogens varied between 0.5-16% and at the grain filling - 17.5-38.5% depending on the year conditions. The Rhizoctonia fungi, causing root rot on winter wheat, include several species: Rhizoctonia solani Kuhn. Teleomorph - Thanatephorus cucumeris (A. B. Frank) Donk, Rhizoctonia cerealis Van der Hoeven. Teleomorph - Ceratobasidium cereale D. Murray & L. L. Burpee (Anastomosis - AG-D.) и Rhizoctonia oryzae Teleomorph - Waitea circinata var oryzae  (Anastomosis WAG-0); Rhizoctonia zeae Teleomorph - Waitea circinata var. zeae (Anastomosis WAG-Z), and р. Gaeumannomyces - G. gramynis var tritici, G.graminis var avenae и  G. gramynis var graminis.

Root rot causing fungi belonging to Gaeumannomyces genus are widely presented in the Western Sis-Caucasia. They can cause root rot, foot rot or mixed type rot disease on winter wheat. We have observed that the pathogen better survives in dry summers. We have isolated G. gramynis var tritici, G.graminis var avenae and G. gramynis var graminis on winter wheat growing in the Western Sis-Caucasia. Although, their occurance and frequency flactuated depending on the year conditions, location and even preveious crop.

We have isolated various strains of these fungi and examined their pathogenicity to 35 varities developed in Krasnodar, Odessa and Zernograd. Resistant varieties have not been detected.

Currently, we continue studying biological properties of Rhizoctonia and Gaeumannomyces fungi, as well as the necessity and availability of means to control root rot on winter wheat.

The article is admitted to the International Scientific Conference "Higher education institution science prospects"; Sochi (Dagomys), September 20-23, 2007; came to the editorial office on 01.08.07