Occurrence and Chemical Control Techniques of Rice Black-streaked Dwarf Disease in Rongshui County

Riwei HUANG Rongqing MO Changhua TANG Zishuai SHANG

Abstract The chemical control technologies of southern rice black-streaked dwarf disease in hybrid rice were studied in Rongshui County, Guangxi, through a combination of investigation, observation and experiments. Three treatments were set up, namely, no water, shallow water and deep water, and each of which had a treatment area of 67 m2. Each treatment was sprayed once with 40 g of 200 million/g  Bacillus subtilis  wettable powder and 20-30 g of 25% pymetrozine wettable powder per 667 m2 in a mixture with 30 kg of water. On the day of spraying, 100 diseased rice hills in each treatment were marked for field observation. The test results showed that the control of southern rice black-streaked dwarf disease should be done by draining the field water and spraying the mixture of  B. subtilis  and pymetrozine evenly, and a better effect can be achieved by keeping the field moist for 6-7 d after spraying.

Key words Rice; Black-streaked dwarf disease;  Bacillus subtilis ; Pymetrozine

Received: August 4, 2021  Accepted: October 9, 2021

Riwei HUANG (1957-), male, P. R. China, technician, devoted to research about hybrid rice breeding and seed sales.

*Corresponding author.

Southern rice black-streaked dwarf disease is a rice virus disease that has been widespread in southern rice regions of China since the 1990s. Since 2009, the occurrence in the rice area of Rongshui County in Guangxi has been increasing year by year, resulting in reduced rice production in some townships or no harvest in some rice fields. For this reason, the occurrence and prevention and control technologies of southern rice black-streaked dwarf disease have attracted attention from plant protection departments and seed distribution and promotion units. After years of investigation and research, a complete set of comprehensive measures has been taken for vectors, occurrence conditions, and prevention and control technologies of southern rice black-streaked dwarf disease. In particular, a new and fruitful breakthrough has been made in chemical control technologies.

Occurrence of Black-streaked Dwarf Disease in Rice

Southern rice black-streaked dwarf virus (SRBSDV) is a new species of virus that is first identified and named by China to harm crops. It belongs to Fijiviruseoviridae of the reoviridae family, and its only virus vector is  Sogatella furcifera ［1］. The disease has the characteristics of a wide range of occurrence, strong outbreak, rapid spread and concealment of damage, and also has population harm and extreme danger. Once hosts are infected with the virus, they can carry the virus for life, and the transmission efficiency is high. If rice is infected at the seedling stage and before the tillering stage, and it cannot heading normally and will suffer from total crop failure almost. If the disease is infected at the jointing and booting stage, the yield loss will be 30%-50%.

The occurrence of  S. furcifera  is related to climatic conditions

In Rongshui County,  S. furcifera  has a higher incidence at lower hills and flatlands with an altitude of about 400 m than higher mountainous areas with an altitude of 500 m. Rongshui Town, Yongle Town, Hemu Town, Sirong Township, Huaibao Town, Sanfang Town, Wangdong Township are located in flat hilly areas, and have a higher incidence than other mountainous towns and townships with higher altitudes. High temperature and high humidity or high temperature and dry weather conditions provide favorable environmental conditions for the occurrence of  S. furcifera . The proportion of short-winged females in high humidity and high temperature is higher, and there are fewer short-winged males; and the proportion of long-winged adults in each generation is high in high temperature and dry conditions. In the second half of 2017, due to long-term high temperature and high humidity, the occurrence of  S. furcifera  in Rongshui County and even the entire South China rice area, especially in Huazhou City, Guangdong Province, Beiliu City, Guangxi and other places, caused serious outbreaks of rice black-streaked dwarf disease and thus a large-scale reduction in production. According to the meteorological data of Rongshui County over the years, the average monthly temperature of Rongshui from May to September is above 25 ℃ for a total of 153 d. According to the condition that the insect period of each generation of  S. furcifera  lasts about 26 d at a temperature of 25 ℃［2］, 6-7 generations can occur in one year in Rongshui County, posing a great threat to early rice tillering period and middle and late rice production. In the years when  S. furcifera  occurred severely, the occurrence of rice black-streaked dwarf disease was correspondingly severe.

The occurrence of  S. furcifera  is related to variety and crops for rotation

Two-line varieties are more infected than three-line varieties, and three-line varieties are heavier than conventional varieties［3］. The selection of insect-resistant and disease-resistant varieties is an important prevention and control measure. For example, Zhongzheyou No.8 and Zhongzheyou No.10 have fluff on the leaves of plants, so once the rice planthoppers are close to the fluff of seedlings, the insect body will be stabbed, which protects the seedlings. They are highly resistant to rice planthoppers, and southern rice black-streaked dwarf disease did not occur in more than 10 years of planting. The incidence of mid-season rice and late rice is much higher than that of early rice. The reason is that the whole growth period of mid-season and late rice is under high temperature and high humidity or high temperature and dry climate conditions, which is completely consistent with the environmental conditions that  S. furcifera  is prone to occur. The outbreak of  S. furcifera  is a precursor to the outbreak of black-streaked dwarf disease in southern rice.

The occurrence of rice black-streaked dwarf disease is related to the number of pathogenic virus

According to observations, in local fields or partial hills where black-streaked dwarf disease occurs in early rice, if no field prevention and control measures are taken when late rice is transplanted, the occurrence in late rice will be more serious, while the incidence of disease will be extremely low if agricultural control measures such as eradication of surrounding weeds and removal of infected rice straws and rice piles have been carried out.

Effects of different insect population densities on rice yield structure

Insect population density is the most important factor that determines the extent of damage to rice by  S. furcifera . The population density of  S. furcifera  has a very significant positive correlation with the yield loss of rice［4］.

S. furcifera  has green tendency

Rice fields with improper water and fertilizer management, a high amount of nitrogen fertilizer, and dark green leaves tend to attract  S. furcifera , leading to the occurrence of black-streaked dwarf disease.

Symptoms of Rice Black-streaked Dwarf Disease

Onset at seedling stage

The heart leaves grow slowly, and the leaves are short and wide, stiff and erect. The leaves are dark green, with irregular milky white disease protrusions on veins［5］, which later turn dark brown. The roots are short and rusty. The plants are short, and the growth and development are stopped. They often die out early.

Onset at tillering stage

Symptoms first appear in new tillers. Diseased plants are obviously dwarfed, and there are more tillers. The main stem and early tillers grow out short, diseased ears, which are usually neck-wrapped ears, with no seeds or very low seed setting rate.

Onset at jointing stage

There are two types of symptoms of diseased leaves, one of which is the white spot type, in which dashed yellow-white stripes parallel to leaf veins are found on the leaves or leaf sheaths, which is most obvious at the base, and the other is the twist type, in which heart leaves are drawn out as twisted nicks, the flag leaves are short, the necks of ears are shortened and are stuck, the seed setting rate is not high, and the yield is low.

Infection at ear stage

Ears head incompletely, and the flag leaves are folded at the base.

New Drug Control Technologies of Rice Black-streaked Dwarf Disease

Taking comprehensive agricultural control measures

S. furcifera  is migratory. There are more southeast winds in spring and summer, during which  S. furcifera  migrates northward from the rice area in Southeast Asia with the airflow［6］. A larger population that migrates causes more serious damage, and the occurrence of black-streaked dwarf disease is inevitable thereby. After 2009, the occurrence of black-streaked dwarf disease in Rongshui County has been increasing year by year, and it is necessary to adopt comprehensive agricultural control measures to prevent and control rice black-streaked dwarf disease. The selection of disease-resistant and insect-resistant varieties, the implementation of key technologies of seed treatment and the prevention and control in seedling fields have significant effects on the prevention and control of southern rice black-streaked dwarf disease. Eliminating virus-carrying rice straw in the field, eradicating weeds around rice fields, reducing the number of insect sources and pathogenic virus, strengthening field management, increasing the application of phosphorus and potassium fertilizers, promoting early growth and rapid development of rice, improving disease resistance and timely controlling the first generation nymphs that breed in rice fields to eliminate the source of insects before spreading the virus are all effective measures yielding twice the result with half effort.

Adopting a combination of investigation, observation and experiment to explore drug control technologies of rice black-streaked dwarf disease

Every year in the early, mid and late growing seasons, related scientists should conduct investigations and research and guide drug prevention and treatment in villages where black-streaked dwarf disease occurs, and collect data on different drugs, different dosages, different administration methods, different dosage times, and different dosage time. In 2016, some early rice fields in Xiatan Village, Yongle Town were infected. Farmers used anti-viral disease drugs for 4 consecutive times, but the effect was not good and it became more and more serious. We selected a kind of 200 million/g  Bacillus subtilis  wettable powder for farmers to control the disease, and after farmers used the drug, we found in follow-up investigation and observation that the infected rice plants in different hills or in the same field showed two different morphological manifestations, one of which was normal growth, and the other still did not resume growth. In order to explore the real reason, we repeated field observations and found that the above two phenomena appeared in uneven fields. The rice hills on the convex surface returned to normal growth after spraying, while the rice hills on the concave surface grew slowly or were unable to recover, which gave us a great inspiration that the effect of drug application may be related to the amount of water in the field. In 2017, an experiment of irrigation depth was carried out in the diseased field of early rice in Youzhatun, Rongshan Village, Yongle Town. Three treatments were set up in the experiment, namely, no water (water in the fields was drained while keeping the field moist), shallow water (2-3 cm), and deep water (5-6 cm). Each treatment had an area of 67 m2, and set with no repetition, and various treatments were in contrast to each other. The tested variety was Longliangyou 1377, the sowing period was April 2, and the transplanting period was April 27.   The control drugs were  B. subtilis  (Daowubing) and pymetrozine, and the dosage was 200 million/g  B. subtilis  WP 600 g+25% pymetrozine WP 360 g for every hectare, which was sprayed in a mixture with 450 kg of water. The spraying date was May 16, and the medication was administered once. On the day of spraying, 100 diseased rice hills were marked in each treatment for field observation. The observation results are shown in Table 1.

Test Effects

In this study, we verified the information captured in a large number of field surveys and observation records in the past, and explored effective methods for the prevention and treatment of rice black-streaked dwarf disease with drugs. At 6 d after spraying  B. subtilis  and pymetrozine, the diseased plants began to grow normally, 97% of the rice hills in the dry area, 52% in the shallow water area, and 9% in the deep water area, recovered; at 12 d after spraying, rice hills that returned to normal growth in the three treatments accounted for 98%, 67%, and 21%, respectively; and no new rice hills found to return to normal growth in the three treatments 18 d after spraying. It could be seen from the three observation cycles that the three treatments with different irrigation depths were also different in the speed at which diseased plants returned to normal growth. Because the field was dry and only maintained at a moist state, the roots, stems, and leaves of rice were infiltrated with the drugs, which killed the pathogen, and the aeration of the soil was greatly increased, which led to fast root growth and development, a large number of white roots exposing on the mud surface, and correspondingly accelerated leaf growth speed. The numbers of new heart leaves in the three treatments of no water, shallow water and deep water were 3.3, 2.8 and 2.6 leaves, respectively.

The control effect of the water-free treatment area failed to reach 100%, which might be related to the uniformity of spraying. In particular, the less the liquid drug in contact with the base of the rice hills, the worse the effect. The effective period of control with  B. subtilis  was 6-12 d, with 6 d being the best. The spraying must be uniform. Except for the leaves, the base of the rice hills should be attached as much drugs as possible, which is an important operation link to improve the efficacy of drug control and must not be ignored.

Conclusions

Southern rice black-streaked dwarf disease can be controlled by draining the water of rice fields and spraying the mixture of  B. subtilis  and pymetrozine evenly, and keeping the field moist for 6-7 d after spraying.

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