การควบคุมโรคเน่าดำของกล้วยไม้สกุลช้างด้วยสารกำจัดเชื้อรา

Abstract

Black rot disease in orchid is a common yet an important disease in commercial orchid cultivation. Orchid industry is highly affected by black rot disease and which causes massive yield losses. In this research, black rot disease of Rhynchostylis gigantea from four commercial orchid nurseries (Mae Tang, Saraphi, Mae Rim and Mueang from Chiang Mai, Thailand) during the rainy season (July to September) of 2019 was investigated. The survey revealed the pathogen of black rot discase in Rhynchostylis gigantea and successful controlling measures of the disease. The prominent symptoms of black rot in Rhynchostylis gigantea were dark lesions on root tips, stems and roots causing crown rot, stem rot and root rot. Leaf spot symptoms were first appeared as irregular yellow spots developed in to water-soaked lesions followed by softened brown and black rot lesions. Hence, black rot causes the leaf fall and slow plant growth. Severe infections affect the flowers by initiating the small lesions on flowers and later turns in to water-soaked spots that rapidly expand into large, wet, translucent rots. One hundred fifty-symptomatic samples were randomly collected from selected nurseries. One hundred eighty-five-Pythiaceae isolates were obtained from hyphal tip isolation of symptomatic samples. All isolates were tested for the pathogenicity through detached leaf methods and Twelve-Pythiaceae isolates showed strong pathogenicity. The fungal pathogens were classified morphologically into three genera viz. nine isolates of Phytophthora, two isolates of Pythium and one isolate of Phytopythiun. The mycelial growth and the sporulation ability of the isolates were studied on six different media; Potato dextrose agar (PDA), Potato carrot agar (PCA), V8 juice agar (V8A), Cornmeal agar (CMA), Carrot agar (CA) and Oatmeal agar (OA). The results showed that higher mycelial growth and aerial mycelia production of 12 isolates were obtained using PDA, V8A, OA and PCA. Furthermore, PDA and V8A were found to be the best media for sporulation of nine Phytophthora isolates. Four Phytophthora sp. isolates: MU-OR3, MU- OR5, MT-OR7 and MR-OR9 showed strong pathogenicity with symptomatic rates of 67.372- 69.940 % after five days of inoculation. The plant root inoculation from zoospore suspension of above selected isolates induced stem rot, softening, and root rot by more than 50% of symptomatic rates within 30 days. These isolates were classified as level 3 severity. The isolates MU-OR3, MU- OR5, MT-OR7 and MR-OR9 were identified by morphology together with multi-gene phylogeny. The primers of internal transcribed spacer (ITS) and cytochrome c oxidase subunit I (COX1) were ITS5/ITS4 and OomCoxI Levup /Fm85mod respectively. Based on taxonomy and phylogeny, P. palmivora was identified and confirmed as the causal agent of black rot disease on R. gigantea. Moreover, this is a novel host record of P. palmivora causing black rot on R. gigantea in Chiang Mai, Thailand. The sensitivity of Phytophthora palmivora isolates: MU-OR3, MU-OR.5, MT-OR7 and MR- OR9 towards the selected fungicides was tested using poisoned food technique. Higher efficacy towards the inhibition of the pathogen was obtained from metalaxyl, dimethomorph and chlorothalonil. Mean ECso values of the mycelial growth inhibition of metalaxyl, dimethomorph and chlorothalonil are 0.25+ 0.03, 0.38t 0.03 and 1.93 - 0.43 ug/ml respectively. To investigate the control of black rot disease, seven fungicides were used at the recommended concentrations. The protective activity was tested by spraying the fungicide 24 h before the inoculation of P. palmivora (MU-OR7) using mycelial plug method and the curative activity was tested by spraying the fungicide 24 h after the inoculation. The leaves of 1-2-year-old R. gigantea were treated separately to assess the protective activity and the curative activity. The results found that the disease severity of black rot disease on orchid leaves was dramatically reduced by metalaxyl and dimethomorph with the application of 24h before the inoculation of recommended concentrations. The percentage inhibitions of disease lesions by metalaxyl and dimethomorph after 72 hr of inoculation are 69.63% and 70.37 %. In our study, metalaxyl exhibited both curative and protective activities on detached orchid leaves. However, metalaxyl shows higher protective activity and less effective curative activity.