Management of leaf spot and anthracnose diseases of Rauwolfia Serpentina (l.) Benth. Ex Kurz

Abstract

Rauwolfia serpentina (L.) Benth. ex Kurz is a medicinal plant. The plant is locally known as “Sarpagandha” also known as Indian snakeroot. During the tenure of 2007 to 2013, two types of symptoms viz., leaf spot and anthracnose were recorded from leaves of R. serpentina. Leaf spot disease incidence was started from February and gradually increased up to July. Leaf spot symptom showed lowest disease severity (1) in the month of February and the highest (9) in the month of July. Whereas anthracnose disease incidence was started from March and gradually increased up to September. In case of anthracnose symptom, the lowest disease severity (2) was recorded in the month of March and the highest (9) was recorded in the month of September. Temperature showed noticeable effect on disease development. Rainfall and humidity also showed slight effect on disease development. A total of fifteen fungal species representing 12 genera were found to be associated with leaf spot and anthracnose diseases of R. serpentina collected from field of Botanical garden, Curzon Hall Campus, Dhaka University; Lawachara, Sylhet; Botanic garden, Chittagong University campus and Mymensingh Agricultural University campus of Bangladesh. The isolated fungi were Alternaria alternata (Fr.) Keissler, Aspergillus flavus Link ex Fr., A. niger van Tieghm, Colletotrichum gloeosporioides (Penz.) Sacc., Curvularia lunata (Wakker) Boedijn, Fusarium sp.1. Fusarium sp. 2, Macrophoma sp., Nigrospora sphaerica (Sacc.) Mason, Penicillium digitatum Saccardo, P. italicum Whemer, Pestalotiopsis guepinii (Desm.) Stay, Pseudocercospora libenbergii (Syd.) Deighton, Rhizopus stolonifer Vuillemin. Toney and Trichoderma viride Pers. Pseudocercospora libenbergii is a new record for Bangladesh. Pathogenecity test of the isolated fungi were done following ‘Detached leaf technique’ and ‘Spraying of spore suspension’ on healthy plants. Among the isolated fungi Colletotrichum gloeosporioides was found to be pathogenic to R. serpentina. Ten fungicides viz., Amcogin 50 WP, Autostin 50 WDG, Capvit 77 WP, Dithane M 45, Greengel 72 WP, Hayvit 80 WG, Oxivit 40 SC, Ridomil Gold MZ, Sulcox 50 WP and Tilt 250 EC at 100, 200, 300, 400 and 500 ppm concentrations were tested which are responsible for leaf spot and anthracnose disease of R. serpentina respectively. Autostin and Tilt showed complete growth inhibition of both the fungi at all the concentrations used. Similarly ethanol leaf extracts of ten plants viz., Adhatoda vesica Nees, Azadirachta indica A Juss, Citrus limon L., Curcuma longa L, Lantana camara L, Moringa oleifera Lam., Ocimum sanctum L., Psidium guajava L., Thuja orientalis L. and Vitex negundo L., were selected to evaluate their efficacy at 5, 10, 15 and 20% concentrations against P. liebenbergii and C. gloeosporioides. Out of the ten plant extracts A. indica and C. limon showed complete radial growth inhibition of both the fungi at 20% concentrations. Four antagonistic fungi viz., Aspergillus flavus Link, A. fumigatus Fresenius, A. niger van Tiegh. and Trichoderma viride Pers. were isolated from field soil of R. serpentina by serial dilution method and selected to evaluate their antagonistic potentiality against P. lebenbergii and C. goeosporiodes following dual culture colony interaction, volatile and non-volatile metabolites. In Dual culture method, A. niger showed highest (66.97%) inhibition of test fungus C. gloeosporioides. Volatile metabolites of A. niger showed maximum 43.77 % inhibition of P. lebenbergii. In dual culture method T. viride showed maximum (84.28%) inhibition of test fungus C. gloeosporioides. Volatile metabolites of T. viride showed higher (77.64%) inhibition against test fungus. Non-volatile metabolites of antagonistic fungi showed that A. flavus inhibited 94.42% radial growth of test fungus followed by T. viride 90.90%, A. niger 86.13% and A. fumigatus 73.73%. A. flavus, A. niger and T. viride may be exploited commercially as a biocontrol agent against anthracnose disease of R. serpentina. Field experiment was conducted to evaluate the efficacy of two selected fungicides and two plant extracts against leaf spot and anthracnose diseases of R. serpentina in Botanical research garden, Department of Botany, University of Dhaka. Both the fungicides Autostin 50 WPG and Tilt 250 EC and leaf extracts shows effective management of the disease over untreated check. However, among the treatments Autostin and Tilt at 100 ppm concentration and A. indica A. Juss and Citrus limon L. at 10% concentration was found significantly superior in controlling the disease severity, PDI (Percent disease index) and increasing number of healthy leaves. Number of healthy leaves per plant was highest 79.55 in 2016 and 52.00 in 2017.