Tungro rice disease alert up in Bicol
February 17, 2010 12:29 am
By Danny O. Calleja
LEGAZPI CITY, Feb. 16 – The shortage of water owing to the El Niño phenomenon that is hitting several parts of the country may create another problem for the agriculture sector—tungro infestation on rice plantations, according to the Department of Agriculture (DA).
Tungro is one of the most destructive rice diseases in Southeast Asia with outbreaks affecting thousands of hectares and records of the DA show that 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 the country’s 2008 rice import.
From 1960 to 1989, major outbreaks were recorded in areas planted to early varieties recommended by the DA. In the 1990s, sporadic outbreaks in Mindanao affected a total of almost 10,000 hectares and in Davao del Norte alone, a yield loss of almost P11 million was recorded in 1993.
Recent outbreaks in the past three years recorded by the DA in Cagayan Valley, Panay Island, Central Mindanao and Southern Mindanao resulted in an estimated yield losses as high as 90 percent and whatever grains left would not fetch a high price due to poor grain quality.
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.
It cannot spread without insect vectors like leafhoppers, particularly the green leafhopper (GLH) and 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 while RTSV particles are isometric and contain polyadenylated single-stranded ribonucleic acid (RNA) as genetic material.
The insects acquire the virus by feeding on an infected plant for a short time and can transmit the virus immediately after feeding for about five to seven days.
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 and stunting is also evident as well as reduced tillering while flowering is delayed and consequently, maturity.
Panicles are small, not well exerted and are partially sterile. Grains are covered with dark brown blotches and its quality is poor. Other indications of infections 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.
The disease is particularly prevalent in communities which practice non-synchronous planting because such practices provide a continuous source of inoculum. Shortage of irrigation water also promotes tungro incidence because farmers are forced to plant asynchronously.
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 especially now that some ricelands are being threatened by shortage of water due to the dry spell, a DA advisory received here over the week said.
Bicol may not be severely affected by the drought as according to the Philippine Atmospheric, Geophysical and Astronomical Services Administrtion (PAGASA), the region has been under near normal rainfall condition during the past days.
But just the same, ricefields in the region would experience water shortage due to an expected early start of summer season this year that is why a tungro alert is up, the DA advisory said.
It said some of the signs of the disease 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. Farmers should observe the field diligently and ratoon or cut off at once infected plants, it advised.
To eliminate the inoculums, it said stubbles and plant debris should be removed after harvesting by plowing and harrowing.
Moreover, 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, Lannate, a DuPont product is an effective chemical against GLH and other hoppers,” it added. (PNA) LAP/LQ/DOC/cbd
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