Selection of chemicals in systemic acquired resistance to rice grassy stunt virus

JOURNAL OF SCIENCE, Hue University, Vol. 67, No. 4A, 2011<br /> <br /> SELECTION OF CHEMICALS IN SYSTEMIC ACQUIRED RESISTANCE TO<br /> RICE GRASSY STUNT VIRUS<br /> Le Thanh Toan1 and Pham Van Kim2<br /> 1<br /> <br /> College of Agriculture and Applied Biology, Cantho University<br /> 2<br /> <br /> Vietnamese Phytopathological Society<br /> <br /> Abstract. The aim of this experiment is to select the best effective chemical to improve the<br /> health of rice grassy stunt disease infected rice plants. The experiment was conducted in the<br /> net-house of Plant Protection Department, College of Agriculture and Applied Biology<br /> (Cantho University), and was carried out with randomized complete design, 5 replications,<br /> 6 rice plants per replication. Rice grassy stunt disease was transmitted to rice plants at 7<br /> DAS (day after sowing) with 3 larvae of rice grassy stunt virus borne brown<br /> planthopper/tiller. Rice plants were induced by seed soaking in 24 hours and then leaf<br /> spraying at 10, 20, 30 and 40 DAS. The experiment had 4 induced resistance treatments:<br /> CuCl2.2H2O (0.05mM), K2HPO4 (20mM), oxalic acid (0.5mM) and combination of CuCl2<br /> (0.05mM) + K2HPO4 (20mM). Results showed that rice plants in all four treatments had the<br /> height higher than those of infected control. Besides, these chemicals limited the quantity of<br /> infected plants, maintained the length of floccules, increased the ratio of firm grain and<br /> yield per pot. Among them, oxalic acid had the best systemic acquired resistance.<br /> Keywords: rice grassy stunt disease, Rice grassy stunt virus, systemic acquired resistance.<br /> <br /> 1<br /> <br /> Introduction<br /> <br /> The epidemics of rice grassy stunt virus (RGSV) and rice ragged stunt virus (RRSV)<br /> have begun to cause serious damage in Vietnam since 2006. According to Plant<br /> Protection Department (2006), rice area seriously infected with RGSV and RRSV was<br /> 43,887.4 ha, with 14,527.1 ha destroyed in Summer Autumn 2006; and 62,224 ha<br /> infected with these viruses, with 2,321 ha destroyed in Autumn Winter 2006. Estimation<br /> of rice yield loss at the Mekong Delta was approximately 428,000 ton because of the<br /> direct damage of brown planthopper (BPH), RGSV and RRSV. This epidemic has<br /> continued to cause damage to Vietnam’s rice production, affect national food security<br /> and rice export. Many methods for limiting rice grassy stunt disease (RGSD) and rice<br /> ragged stunt disease (RRSD) have been applied in many places. The method of sowing<br /> at a time in accordance with lowest population of migratory BPH helped the farmers in<br /> restricting damage of virus diseases. In addition, other methods protecting young rice<br /> plants also had a part in reducing the damage. However, as there is no rice variety that<br /> 77<br /> <br /> can be tolerant of RGSV and RRSV while rice price is at high level and farmers often<br /> grow overlapping crops, RGSD and RRSD still occur in the field with different ratios.<br /> In this situation, to help farmers “live” together with RGSD and RRSD as well as to<br /> avoid future epidemics, the methods of improving the health of rice plant so as to<br /> overcome virus diseases can retain rice yield. Among these methods, systemic acquired<br /> resistance is feasible, cheap and safe for the environment.<br /> Worldwide, many scientists have successfully used systemic acquired resistance<br /> (SAR) to control plant virus diseases. Ghoshroy et al. (1998), Naylor et al. (1998),<br /> Chivasa and Carr (1998), Dong and Beer (2000) and Ahn et al. (2005) have successfully<br /> used chemicals namely cadmium, salicylic acid, salicylhydroxamic acid, 2,6dichloroisonicotinic acid, antimycin A, riboflavin and vitamin B1 to induce the<br /> resistance of tobacco plant against mosaic disease (Tobacco mosaic virus). Naylor et al.<br /> (1998) and Chirkov et al. (2001) succeeded in using chitosan to induce resistance in<br /> potato against Potato virus X (PVX). Mayers et al. (2005) also succeeded in using<br /> salicylic acid in SAR researches helping cucumber to resist Cucumber mosaic virus<br /> (CMV).<br /> These SAR achievements with virus diseases in tobacco, papaya, chilli, potato<br /> and cucumber mean that promising results can be expected if SAR method is applied to<br /> RGSD and RRSD in rice. If successful, we can introduce this method to help farmers<br /> limit damage when they cannot avoid BPH at the beginning of crop.<br /> To reach this aim, the study was conducted in the net-house to find the best<br /> effective chemicals in SAR to RGSD if the rice plants were infected early, at 7 days<br /> after sowing.<br /> <br /> 2<br /> <br /> Materials and methods<br /> <br /> The experiment was conducted in the net-house of Plant Protection Department, College<br /> of Agriculture and Applied Biology (Cantho University). The rice variety was OM2517<br /> – a BPH susceptible variety widely cultivated at the Mekong Delta.<br /> RGSV source was collected at Cantho city, purified to separate from RRSD to<br /> be used for the experiment. Clean BPHs, used for transmitting RGSD, were collected in<br /> the field and allowed to lay their eggs in pickerel weed’s sheaths (Momochoria<br /> vaginalis). The eggs hatched, the BPH larvae were not hold RGSV and RRSV<br /> (rechecking by ELISA). The BPH larvae 1-2 took RGSV in grassy stunt rice for 2 days;<br /> they were then fed on healthy rice to be latent. The BPH larvae 4-5 infected with RGSV<br /> were used for the experiment.<br /> The SAR chemicals were CuCl2 (concentration 0.05mM), oxalic acid (0.5mM),<br /> K2HPO4 (20mM). The way to treat chemicals was seed soaking for 24 hours and leaf<br /> spraying at 10, 20, 30 and 40 DAS.<br /> 78<br /> <br /> The experiment was carried out with randomized complete design, 10<br /> replications, 6 rice plants/replication, and 6 treatments namely (1) CuCl2 ; (2) oxalic<br /> acid; (3) K2HPO4; (4) combination of CuCl2 + K2HPO4; (5) a control with infected rice<br /> plants and (6) a control with healthy rice plants.<br /> RGSD was transmitted to rice plants at 7 DAS with 3 BPH larvae 4-5/tiller. The<br /> BPH transmitted for 2 days before it was killed. After the transmission, the rice plants<br /> were put in the net-house. The data collected were the ratio of infected plants and the<br /> length of rice plant. Yield/pot and elements of yield were recorded after harvesting. The<br /> elements of yield included the effective tillers per pot, the length of floccules, and the<br /> quantity of floccules.<br /> The data was analyzed using MSTATC software.<br /> <br /> 3<br /> <br /> Results<br /> <br /> 3.1<br /> <br /> The effect of chemical treatment to infected rice plants’ growth<br /> <br /> The data presented in Table 1 showed that the control with infected rice plants had high<br /> ratio of infected plants (68.3%). This result demonstrated that experiment’s inoculation<br /> was good. Meanwhile, chemical-treated plants had lower ratios, significantly different<br /> to the control with healthy rice plants. This result indicated that the chemicals had effect<br /> in limiting RGSD, compared to the control with infected rice plants (without using any<br /> chemical). Among the different chemical treatments, chemicals had the same ability in<br /> decreasing ratio of infected plants.<br /> Table 1. The effect of chemicals’ treatment to infected rice plants’ growth<br /> <br /> Treatments<br /> <br /> Beginning Increasing<br /> Ratio of<br /> Average<br /> time for<br /> rate of<br /> infected latent time infected<br /> height at<br /> plants’<br /> 35 DAI<br /> plants (%)<br /> (day)<br /> death (day)<br /> (%)<br /> <br /> CuCl2 0.05mM<br /> <br /> 35.0<br /> <br /> b<br /> <br /> 13.72<br /> <br /> b<br /> <br /> 20<br /> <br /> 23.6 b<br /> <br /> K2HPO4 20mM<br /> <br /> 26.7<br /> <br /> b<br /> <br /> 13.75<br /> <br /> b<br /> <br /> 43<br /> <br /> 23.2 b<br /> <br /> CuCl2 0.05mM + K2HPO4 20mM<br /> <br /> 36.7<br /> <br /> b<br /> <br /> 12.55<br /> <br /> b<br /> <br /> 42<br /> <br /> 19.4 b<br /> <br /> Oxalic acid 0.5mM<br /> <br /> 26.7<br /> <br /> b<br /> <br /> 13.75<br /> <br /> b<br /> <br /> 60<br /> <br /> 29.8 b<br /> <br /> Control with infected rice plants<br /> <br /> 68.3<br /> <br /> c<br /> <br /> 12.78<br /> <br /> b<br /> <br /> 21<br /> <br /> 0.0<br /> <br /> Control with healthy rice plants<br /> Significant level<br /> CV (%)<br /> <br /> c<br /> <br /> 0.0 a<br /> <br /> 0.00 a<br /> <br /> -<br /> <br /> 58.0 a<br /> <br /> *<br /> <br /> *<br /> <br /> -<br /> <br /> *<br /> <br /> 28.4<br /> <br /> 36.5<br /> <br /> -<br /> <br /> 35.1<br /> <br /> 79<br /> <br /> For average latent time, there are no significant differences between infected<br /> treatments. This result showed that chemicals did not affect the average latent time.<br /> Recording the beginning time for infected plants’ death, the control with infected<br /> rice plants was 21 days after inoculation (DAI), while others treated with K2HPO4 was<br /> 43 DAI, with combination of CuCl2 + K2HPO4 was 42 DAI and with oxalic acid was 60<br /> DAI – remarkably higher. For this parameter, the treatments namely oxalic acid,<br /> K2HPO4 and combination of CuCl2 + K2HPO4 were effective SAR chemicals; among<br /> them, oxalic acid was the most effective chemical in prolonging rice’s life.<br /> At 35 DAI, chemical treatments had the increasing rate of height significantly<br /> different from that of the control with infected rice plants.<br /> 3.2<br /> <br /> The effect of chemical treatment to infected rice plants’ yield elements<br /> <br /> The data presented in Table 2 indicated that there was a significant difference of<br /> effective tillers per pot between infected and non-infected treatments. Infected<br /> treatments had 21.5 – 34.9 effective tillers per pot, while the control with healthy rice<br /> plants had 52.3 effective tillers per pot. Therefore, RGSD influenced the quantity of<br /> effective tillers per pot.<br /> The length of floccules was decided by variety’s heredity factors, nutritional<br /> condition and pests. Among them, RGSD had impact on the floccules length of infected<br /> plants. Data presented in Table 2 indicated that chemical treatments improved health of<br /> the rice plant, so their length of floccules was longer than that of the control with<br /> infected rice plants, and equivalent to that of the control with healthy rice plants. This<br /> result showed that the chemicals could maintain the length of floccules. This was in<br /> accordance with Nguyen Thi Tam’s result (2008).<br /> The chemical treatments had quantity of floccules higher than that of the control<br /> with infected rice plants and lower than that of the control with healthy rice plants<br /> significantly (Table 2).<br /> Table 2. The effect of chemical treatment to infected rice plants’ yield elements<br /> <br /> Treatments<br /> <br /> Effective<br /> tillers/pot<br /> (pot)<br /> <br /> Length of Quantity Weight of<br /> floccules of floccules firm grain<br /> (cm)<br /> (floccules)<br /> /pot (g)<br /> <br /> CuCl2 0.05mM<br /> <br /> 30.1 bc<br /> <br /> 12.20 a<br /> <br /> 20.1 b<br /> <br /> 25.40 a<br /> <br /> K2HPO4 20mM<br /> <br /> 21.5 c<br /> <br /> 14.34 a<br /> <br /> 19.5 b<br /> <br /> 27.63 a<br /> <br /> CuCl2 0.05mM + K2HPO4 20mM<br /> <br /> 26.2 bc<br /> <br /> 13.21 a<br /> <br /> 19.0 b<br /> <br /> 28.01 a<br /> <br /> Oxalic acid 0.5mM<br /> <br /> 34.9 b<br /> <br /> 13.75 a<br /> <br /> 25.8 b<br /> <br /> 33.27 a<br /> <br /> Control with infected rice plants<br /> <br /> 27.0 bc<br /> <br /> 6.59 b<br /> <br /> 11.2 c<br /> <br /> 12.11 b<br /> <br /> 80<br /> <br /> Control with healthy rice plants<br /> Significant level<br /> CV (%)<br /> <br /> 52.3 a<br /> <br /> 15.61 a<br /> <br /> 48.4 a<br /> <br /> 31.13 a<br /> <br /> *<br /> <br /> *<br /> <br /> *<br /> <br /> *<br /> <br /> 13.2<br /> <br /> 5.9<br /> <br /> 13.6<br /> <br /> 16.6<br /> <br /> Weight of firm grain per pot of chemical treatments was equivalent to that of the<br /> control with healthy rice plants, but significantly different in comparison with that of the<br /> control with infected rice plants. Weights of firm grain per pot of these treatments were<br /> CuCl2 (25.40 g per pot), K2HPO4 (27.63 g per pot), CuCl2 + K2HPO4 (28.01 g per pot),<br /> oxalic acid (33.27 g per pot), the control with infected rice plants (12.11 g per pot) and<br /> the control with healthy rice plants (31.13 g per pot).<br /> The chemical treatments such as CuCl2, K2HPO4, CuCl2 + K2HPO4 and oxalic<br /> acid, at concentrations used in this experiment, were able to maintain the rice height<br /> better than the control with healthy rice plants. In addition, these chemicals also helped<br /> the rice plants in decreasing the ratio of infected plants, prolonging the survival time of<br /> infected plants, raising the quantity of floccules, maintaining the length of floccules,<br /> having a part in weight of firm grain per pot compared to the control with infected rice<br /> plants. This result demonstrated that CuCl2, K2HPO4, CuCl2 + K2HPO4 and oxalic acid,<br /> at concentrations used in the experiments, were effective chemicals in SAR to RGSD;<br /> among them, oxalic acid was more dominant in lengthening the rice plant’s live.<br /> Moreover, there were some tolerant tillers on an infected rice plant treated with oxalic<br /> acid (Fig. 1). This was a special phenomenon. This phenomenon rarely occurs on virusinfected-plants. As a result, oxalic acid was selected as one of the most effective<br /> chemicals.<br /> <br /> Tolerant tillers<br /> <br /> Typical RGSD-tillers<br /> <br /> Fig. 1. Rice plant at oxalic acid treatment<br /> <br /> This conclusion was in accordance with the results of Ngo Thanh Tri et al.<br /> (2011). The authors showed that CuCl2 and oxalic acid helped the rice plant in<br /> 81<br /> <br />