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- Disease Survey of Rice in Lao PDR and Control of Sheath Diseases
Disease Survey of Rice in Lao PDR and Control of Sheath Diseases
Thesis Abstract:
The objectives of this study were to survey rice diseases and to study in detail the important rice disease in Lao PDR. Survey and collection of rice diseases in major rice production fields; identification of the rice disease causal agents; and study on the pathogenicity, epidemic, etiology, and effective methods for controlling sheath blight diseases were carried out. The rice disease survey and collection in three provinces of Lao PDR (north, central, and south) during rainy and dry seasons for 2015–2016 were conducted. The diseases found were sheath blight, sheath spot, blast, brown spot, narrow brown spot, and bacterial leaf blight. The focused diseases in this study were sheath diseases. Characteristics of sheath disease pathogen were grouped by using morphological characteristic, molecular technique, and pathogenicity test. Nineteen isolates of Rhizoctonia were characterized. From these, seven isolates were Rhizoctonia solani and 12 isolates were R. oryzae. The R. solani mycelia on potato dextrose agar (PDA) were light brown to brown, sclerotia brown color, and globose to irregular shape. On the other hand, R. oryzae mycelia on PDA were brown and orange brown and sclerotia globose to irregular shape, dark brown or light-yellow color. The PCR amplification of ITS1-5.8S-ITS2 of Rhizoctonia spp. DNA was performed. The PCR products obtained were about 750 base pairs for all isolates. The sequence bases of PCR products were compared to the GenBank database. The result showed that isolates SMN1-5, SMCl-2, and VNN were similar to R. solani strain RSR 8 (KF570302) with 98 percent identity, R. solani strain AG1-lAKA (KJ577141) with 99 percent identity, and R. solani isolates RASC8 (JF701711) with 97 percent identity, Rhizoctonia isolate VHN1-3 found similar to R. oryzae isolate VC 241 (KT362102) with 99 percent identity, Rhizoctonia isolates SMN1-7 and VHN had 99 percent similarity to R. oryzae isolate VC241 (KT362102), and R. oryzae isolate VC51 (KT 362129), respectively. The pathogenicity of all Rhizoctonia isolates were tested in vitro. The result revealed that the isolates VSV1-2 of R. oryzae inoculated on rice sheath produced largest lesion average wound area of 303 mm2 compared with isolate SMN1-5 of R. solani with average area of 191 mm2. All 19 isolates were pathogenic to rice cultivar. This study indicated that R. solani can invade the sheath and leaves of rice cultivar (Tadokham 8). However, R. oryzae invaded only sheath.
Control of sheath blight was experimented in laboratory and greenhouse. On dual culture test between Trichoderma spp. and R. oryzae, it was found that three isolates, T18, T13 T. asperellum, and T. koningii 67 showed high percentage of inhibition against mycelium of pathogen, compared to others. The percentages of inhibition were 88 percent, 81 percent, and 81 percent, respectively. For R. solani, the inhibition mycelia were 41 percent, 37 percent, and 36 percent, respectively. Result on mycoparasitism indicated that the hypha of Trichoderma spp. coiled pathogen hypha leads to the death of the parasitized fungus. The result of greenhouse trials demonstrated three effective isolates of antagonists in suppressing two isolates, R. oryzae and R. solani. T. asperellum T18 and T13 were applied as the best method to control R. oryzae by foliar spray. The wound area lesion of sheath spot were 15 mm2 and 25 mm2, compared to control treatment; and percent of disease suppression were 92 percent and 87 percent, respectively. T. koningii 67 was applied as the best method to control R. oryzae by root dip treatment. The wound area lesion of sheath spot was 24 mm2 and percent of disease suppression was 87 percent. T. asperellum T18 was applied as the best method to control R. solani by soil treatment. The wound area lesion of sheath blight was 67 mm2 and percent suppression of R. solani disease was 61 percent. T. asperellum T13 and T. koningii 67 were applied as the best method to control R. solani by soil treatment, root dip, and foliar spray. The wound area lesion of sheath blight were 70 mm2 and 96 mm2, compared to control treatment and percent of disease suppression, which were 60 percent and 50 percent, respectively. Integrated control by using Trichoderma spp. and fungicides on R. oryzae showed that T. asperellum T18 with captan, T. koningii 67 with captan, and T. asperellum T13 with mancozeb expressed percent disease suppression of 75 percent, 74 percent, and 73 percent, respectively. Application of fungicides (captan and mancozeb) alone was not different to combination treatment with percent disease suppression of 76 percent and 71 percent, respectively. Interestingly, integrated use between Trichoderma spp. and fungicides against R. solani resulted clearly that T13 with mancozeb, T18 with captan, and T18 with mancozeb showed percent disease suppression of 75 percent, 71 percent, and 71 percent, respectively. Whereas the application of fungicides (mancozeb and captan) alone obtained the percent disease suppression of only 57 percent and 53 percent. This indicated that antagonist fungi Trichoderma spp. T13, T18, and 67 can be applied with mancozeb and captan for control sheath diseases. However, field experiments are needed to confirm greenhouse experiment.
