Rice Diseases Series (Part IV) – Rice Tungro Virus
The cultivated Asian Rice(Oryza sativa L.), being a highly domesticated crop, has several major pathogens due to its long history of dependence on human beings. Early breeding efforts during the Green Revolution focused more on grain yield, and that was rightly so because of the pressing worldwide problem on poverty and population growth. In the past two or three decades, however, disease resistance has been included as one of the major objectives of breeding programs worldwide.
This will be the last in our series of major rice diseases. Previously we discussed bacterial blight, a major disease caused by a bacterium; rice blast, a fungal disease; sheath blight and sheath rot, the two sheath diseases caused by fungal pathogens. Now, we will discuss a disease caused by a virus, the simplest organism able to possess a genetic material.
Rice tungro is the most economically important viral disease of rice in the Philippines. Tungro is an example of a disease caused by a viral complex since two viruses are needed for transmission and infection. Hence, rice tungro is often described as a disease complex.
Tungro is one of the most destructive rice diseases in Southeast Asia with outbreaks affecting thousands of hectares (ha). In the 1940s, long before the causal organism have been identified, outbreaks of the disease in major rice growing areas in the Philippines reduced the overall national yield to 1.4 million metric tons annually, nearly the amount of our 2008 rice import.
From 1960 to 1989, major outbreaks were recorded in areas planted to early varieties recommended by the Department of Agriculture. In the 1990s, sporadic outbreaks in Mindanao affected a total of almost 10,000 ha, and in Davao del Norte alone, a yield loss of almost P11,000,000 was recorded in 1993.
Recent outbreaks in the past three years were recorded by Pioneer Hi-Bred researchers and agronomists in Cagayan Valley (Cagayan, Isabela), Panay Island (Iloilo, Capiz), Central Mindanao (Midsayap and Kabacan areas), and Southern Mindanao. Estimated yield losses were as high as 90 percent and whatever grains left would not fetch a high price due to poor grain quality.
CAUSAL AGENTS AND MODE OF TRANSMISSION AND INFECTION
The tungro virus disease complex is an excellent example of co-evolution between the virus complex and the vector, although it is not clear whether the insect vector is benefiting from the presence of the virus.
Tungro virus disease cannot spread without an insect vector. The disease is transmitted by leathoppers, particularly the green leafhopper (GLH), Nephotettix viriscens (Distant). The disease complex is associated with two different viruses required for transmission and infection – rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV).
RTBV particles are rod-shaped and contain double-stranded deoxyribonucleic acid (DNA) as genetic material. RTSV particles, on one hand, are isometric and contain polyadenylated single-stranded ribonucleic acid (RNA) as genetic material. RTBV or RTSV alone or in combination as virus complex have been shown to have different reactions to transmission, and exhibiting symptoms. Refer to the table which shows that RTSV is required for efficient transmission while RTBV is required for manifestation of symptoms.
The insect acquires the virus by feeding on the plant for a short time, and can transmit the virus immediately after feeding. The virus does not remain in the vector’s body. After feeding on a diseased plant, the insect can transmit the virus for about 5-7 days, and the insect can become infective again after re-acquisition feeding.
SYMPTOMS OF INFECTION
Tungro affects the plant at any growth stage, most severely during the vegetative stage where symptoms are more pronounced. Leaf discoloration begins from the tip and spreads down the blade to the lower leaf portion. Leaves may also exhibit mottled or striped appearance. Stunting is also evident as well as reduced tillering. Flowering is delayed and consequently, maturity. Panicles are small, not well exserted, and are partially sterile. Grains are covered with dark brown blotches, and grain quality is poor. Other indications are the presence of GLH adults, nymphs, and eggs.
Tungro incidence is favored by presence of virus sources such as infected ricefields in adjacent localities and wild Oryza (rice) relatives. Uncontrolled presence of GLH and other hoppers may trigger an outbreak. The presence of virus sources, population and composition of the vector, age and susceptibility of the host plant, and synchronization of these three factors promote the incidence of the disease.
Tungro is particularly prevalent in communities which practice non-synchronous planting because such practices provide a continuous source of inoculum. Shortage of irrigation water may also promote tungro incidence because farmers are forced to plant asynchronously.
MANAGEMENT PRACTICES IN MINIMIZING INFECTION
Effective management of tungro disease in hybrid rice, however, is limited by lack of resistant hybrid rice varieties, lack of symptoms during early development of the disease, and vector adaptation to insect resistant varieties. Farmers start noticing the disease when the symptoms appear, and usually at this point, the disease has already spread and the GLH has already reproduced.
Currently, there are no hybrid varieties resistant to tungro. Existing resistant inbred lines such as the Matatag series are provided only as stop-gap varieties and may not yield as high as what farmers usually plant. However, these Matatag lines usually yield more that the farmers’ varieties under tungro disease pressure.
So it is important to detect the disease as early as possible. Some of the signs to look for are non-uniform growth or patches of irregular growth in the field, presence of GLH, and presence of the disease in neighboring fields. Observe the field diligently and ratoon or cut off infected plants.
Stubbles and plant debris should be removed after harvesting by plowing and harrowing to eliminate the inoculum. Farming communities should be organized so that planting would be synchronous to at most within one month of the general planting schedule of the locality.
There are no chemicals available that can be used to directly control the virus. However, the insect vector can be controlled. Lannate, a DuPont product, is an effective chemical control for GLH and other hoppers.
Virus / Virus complex Transmission by GLH Symptoms on plant
RTBV not transmissible mild symptoms
RTSV efficiently transmissible no clear symptoms
RTBV + RTSV efficiently transmissible severe symptoms