SCIENCE CITY OF MUÑOZ – While humankind is defending itself from the coronavirus disease, rice plants are also in a battle with different kinds of viruses that farmers have to watch, experts from the Philippine Rice Research Institute warned.
These viruses, PhilRice said, can result in significant yield losses when not properly managed. “Worst, rice plants infected by some of these viruses are not treatable,” it added.
Dr. Jennifer T. Niones, plant pathology expert of the Philippine Rice Research Institute of the Department of Agriculture (DA-PhilRice), said that there are four types of viruses, which farmers must be aware of to minimize farm threats. These viruses include tungro, dwarf, grassy stunt, and ragged stunt.
Tungro virus is an extremely damaging rice disease in South and Southeast Asia bringing in 70-100% yield loss. Rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV) cause this disease.
RTBV symptoms include mild stunting and mild yellowing, while RTSV -infected plants are quite stunted. Rice plants with both RTBV and RTSV display mottled and yellow to orange leaves, and severe stunting. Rice plants infected during early growth stages will not produce panicles. If attacked in later stages, panicles may develop but with low grain fill.
Niones said tungro is transmitted by six leafhopper species with the rice green leafhopper (GLH) as the most important carrier. GLH, which is usually abundant in irrigated rice fields, transmits the virus more efficiently than other vectors.
“After feeding on an infected plant for 30min, GLH can immediately transmit the virus to a healthy plant. Leafhoppers transmit RTSV and RTBV for approximately 4 and 7 days, respectively,” Niones said.
As infected plants cannot be treated, farmers must consider preventive measures especially if their area had past incidences of tungro infestation.
The crop management expert recommended tungro-resistant varieties such as NSIC Rc 118 and Rc 120. Synchronous planting is also encouraged as this practice reduces the food sources available for insect pests to survive on and multiply. Farmers may also schedule their planting when GLH population is at its lowest.
“Farmers must not also spray in seedbed when no tungro and few GLH are present. Insecticides should not be used repeatedly over long periods to maintain the population of viruses’ natural enemies and preserve the natural balance of insect populations,” she said.
For previously infected fields, farmers must immediately plow the stubbles after harvest to destroy the eggs and breeding sites of GLH.
First observed in the rice fields of Midsayap, Cotabato, rice dwarf virus (RDV) reduces yield by 50-80%, especially when it strikes down at vegetative stage. Infected plants show pronounced stunting, increased tillering, and shortened darker green leaves with fine chlorotic specks.
Infected plants usually survive until harvest time but rarely produce panicles. Panicles are usually of poor quality, and grains are unfilled. When damaged during seeding stage, rice plants do not produce grains.
RDV spread can be prevented through plowing of the fallowed rice field and synchronous planting. Applying insecticide to rice seedlings before transplanting can also be an option. Areas infected during the previous cropping season should be immediately plowed.
Grassy stunt virus
Rice grassy stunt virus (RGSV) gives headache to the rice farmers for it inhibits panicle production through stunting and yellowing of the plant. This virus is commonly spread by nymph and adult brown planthopper (BPH). However, BPH eggs do not transmit the virus. BPH feeding on infected stubbles for at least 30min transmits the virus.
Symptoms of the virus develop from 10-12 days after infection. RGSV-infected hills manifest severely stunted plants, excessive tillering, very upright plant growth, and grassy and rosette appearance of plants.
Leaves are yellowish-green that are shorter and narrower than normal appearance, and have many small rusty spots or patches, which merge into blotches. Leaves that remain yellow even when applied with sufficient nitrogen fertilizers is also a symptom.
RGSV frequently affects field where year-round and continuous rice growing is practiced. Although plants can be infected in all growth stages, infection usually happens during tillering stage.
Occurrence of RGSV is not widespread, but it can be severe when BPH is present in the field. The virus can be managed through planting BPH-resistant rice varieties such as NSIC Rc 222, 298, and 308 and synchronous planting. Populations of BPH should also be immediately managed. Infected fields should be plowed right after harvest to reduce the virus source.
Rice ragged stunt (RRSV), also transmitted by BPH, can bring up to 80% yield losses by causing partially exerted panicles, unfilled grains, and plant density loss. The leaves of infected plants show ragged appearance.
BPH contacts the virus by feeding on an infected plant within 24 hours, which can be transmitted to other plants within 6 hours of infection. Early instar nymphs of BPH transmit the virus more efficiently than its older stages. However, BPH eggs cannot transmit the virus.
RRSV symptoms include severe stunting during early crop stages, green leaves with darker than the normal color that appears to have jagged uneven edges, and appearance of yellow to yellow-brown leaves that twist into spiral shapes at the base of leaf blades. The veins that develop on leaf blades and sheathes are usually swollen, pale yellow, or white to dark brown. The underside of leaf blades and the outer surface of leaf sheaths have galls. Infected crops will also display delayed flowering and incomplete panicle emergence.
Like the tungro virus, RRSV-infected plants cannot be treated so preventive rather than direct-control measures should be implemented. Planting BPH-resistant varieties such as NSIC Rc 222, 298, and 308 is the most defensive measure. Synchronous planting and plowing of infected stubbles under the field after harvest are also encouraged.
Niones noted that to manage the four types of viruses, infected plants should be immediately removed once the disease is detected. They can be prevented by practicing fallow period at least one month between each cropping to reduce the pests’ food supply; reducing their populations. Resistant varieties should also be onsidered but insecticide should only be applied when needed.
Rice planting for rainy season is set to start in the later part of May. –