Tungro

Damage to plants

• leaf discoloration - partially filled grains

Signs and symptoms

• discoloration begins from leaf tip and extends down to the blade or the lower leaf portion

• infected leaves may also show mottled or striped appearance - stunting

• reduced tillering

• delayed flowering, which may delay maturity - panicles small and not completely exserted

• most panicles sterile or partially filled grains and covered with dark brown blotches

Factors favoring disease development

• presence of the virus sources

• presence of the vector

• age and susceptibility of host plants

• synchronization of the three above factors

• all growth stages of the rice plant specifically the vegetative stage

Symptoms

Tungro in the field

Confirmation

There are some serological tools to detect tungro viruses. These are Latex agglutination test, Enzyme Linked Immunosorbent Assay or ELISA, and Rapid Immunofilter Paper Assay or RIPA. The presence of the vector Nephotettix spp. is indicative of the disease.

Problems with similar symptoms

The yellowing of the plant and its stunted height is often confused with 1) physiological disorders such as nitrogen and zinc deficiencies and water stress; 2) pest infestation such as stem borer infestation, planthopper infestation, and rat damage; and 3) other diseases such as grassy stunt virus disease and orange leaf disease.

Why and where it occurs

Tungro incidence depends on the availability of the virus sources, population and composition of the vector, age and susceptibility of host plants, and synchronization of the three factors mentioned.

Causal agent

Tungro virus disease is transmitted by leafhoppers, wherein the most efficient vector is the green leafhopper, Nephotettix virescens (Distant). The disease complex is associated with rice tungro baciliform virus (RTBV) and rice tungro spherical virus (RTSV). RTBV cannot be transmitted by leafhoppers unless RTSV is present. Insects could acquire the virus from any part of the infected plant. After acquiring the virus, the vector can immediately transmit to the plants.

RTBV particles are rod-shaped and 100-300 nm in length and 30-35 nm in width. It contains DNA of 8.3 kb. RTSV particles are isometric and 30 nm in diameter. It has a polyadenylated single-stranded RNA of about 12 kb.

Host range

Under artificial (experimental) conditions, rice tungro viruses infect Echinochloa colona (L.) Link, E. crus-galli (L.) P. Beauv. ssp. hispidula (Retz.) Honda, Eleusine indica (L.) Gaertn., Dactyloctenium aegyptium (L.) Beauv., Ischaemum rugosum Salisb., Hydrolea zeylanica (L.) Vahl, Hemarthia compressa (L. f.) R. Br., Fimbristylis miliacea (L.) Vahl, Leersia hexandra Sw., and Panicum repens L. Only rice and some wild relatives are infected under natural conditions.

Life cycle

Mechanism of damage

The insect acquires the virus by feeding on the plant for a short time in an 8-hr acquisition access period (minimum of 30 minutes). It can transmit the virus immediately after feeding. Either or both viruses can be transmitted during a 1 hour inoculation access period (minimum of 7 minutes).

The virus does not remain in the vector’s body. After feeding on a diseased plant, the insect can transmit the virus for about 5 days and the longest is about a week. The insect becomes reinfective after re-acquisition feeding.

When is damage important?

Tungro virus disease affects all growth stages of the rice plant specifically the vegetative stage.

Economic importance

Tungro is one of the most damaging and destructive diseases of rice in countries in Southeast Asia. Outbreaks of the disease can affect thousands of hectares in many countries. Plant infected with the virus at the early crop growth stage could have as high as 100% yield loss in severe cases.

The damage caused by the disease depends on the variety used, the plant stage when infection occurs, the virus particles, and the environmental conditions.

Management principles

There are three limitations of effective tungro management: 1) the absence of symptoms at early growth stage of the disease development, 2) lack of resistant varieties to the tungro viruses, and 3) vector adaptation on GLH-resistant variety.

Planting of resistant varieties against tungro virus disease is the most economical means of managing the disease. There are resistant varieties from the Philippines, Malaysia, Indonesia, India, and Bangladesh, which are available.

Among the cultural management practices, adjusting the date of planting is recommended. Likewise, observing a fallow period of at least a month to eliminate hosts and viruses and vectors of the disease and plowing and harrowing the field to destroy stubbles right after harvest in order to eradicate other tungro hosts are also advisable.

Selected references

• Department of Agriculture (DA) and Philippine Rice Research Institute (PhilRice). 1997. Rice tungro virus disease. DA, Elliptical Road, Diliman, Quezon City and PhilRice, Maligaya, Muñoz, Nueva Ecija. 26 p.

• International Rice Research Institute (IRRI). 1983. Field problems of tropical rice. Manila (Philippines): IRRI. 172 p.

• Ling KC. 1972. Rice virus diseases. Manila (Philippines): International Rice Research Institute. 134 p.

• Mariappan V, Yesuraja I, Gomathi N. 1998. Viral, mycoplasmal and bacterial diseases of rice and their integrated management. In: Paul SM, editor. Pathological problems of economic crop plants and their management. Jodhpur (India): Scientific Publishers. 47 p.

• Nyvall RF. 1999. Field crop diseases. Iowa State University Press, USA. 1,021 p.

• Ou SH. 1985. Rice diseases. Great Britain (UK): Commonwealth Mycological Institute. 380 p.

• Reissig WH, Heinrichs EA, Litsinger JA, Moody K, Fiedler L, Mew TW, Barrion AT. 1986. Illustrated guide to integrated pest management in rice in tropical Asia. Manila (Philippines): International Rice Research Institute. 411 p.

• Tiongco ER, Chancellor TCB, Villareal S, Magbanua MGM, Teng PS. 1996. The effect of roguing as a tactical control measure for tungro disease. In: Chancellor TCB, Teng PS, Heong KL, editors. Rice tungro disease epidemiology and vector ecology. Manila (Philippines): International Rice Research Institute. p 86-91.

• Tiongco ER, Sebastian LS, Bajet NB. 1998. Manual on virus and virus-like diseases of rice in the Philippines. Department of Agriculture Philippine Rice Research Institute, Maligaya, Muñoz, Nueva Ecija. 18 p.

Contributors

Suparyono, JLA Catindig, PQ Cabauatan, and HX Troung