Survey Area Four districts of Rajshahi and Rangpur division which are mid-western and northern region of Bangladesh were selected for the survey study. Natore, Bogra and Jaoypurhat districts of Rajshahi Division and Gaibandha district of Rangpur Division were designated for survey. A total of seven (7) Thana/ Sub-station were surveyed of four selected districts. The selected region has a subtropical monsoon climate characterized by wide seasonal variations in rainfall, moderately warm temperatures and high humidity. Three seasons are generally recognized: a hot, humid summer from April to June; Rainy monsoon season from June to October and a cool, dry winter from October to March. In general, maximum summer temperatures range 32ºC-38ºC. April is the warmest month of the country. January is the coldest month, when the average temperature for most of the country is 10ºC. The northern part is mainly agriculture-based area, which is 23% of total area of Bangladesh and 24% of total population live here (Banglapedia, 2012). The mass population of northern Bangladesh is under poverty line, struggling hard for a better living standard. Recently, medicinal plants are commercially cultivated (Ashwaganda, Aloevera, Kalmegh, Tulsi, Basak and Satamuli) in the district of Natore, Bogra, Gaibandha, Joypurhat, Rangpur and Naogoan to fulfill the demand of Pharmaceuticals, Ayurvedic and Kabirazi uses. In order to study the disease incidence (%) of selected medicinal plant was surveyed in summer, rainy and winter seasons in the selected region of Bangladesh during 2014-2015.
Sample collection, isolation of fungi and estimation of disease incidence Leaf blight, leaf spot, root rot, stem rot, powdery mildew, tuberous rot and collar rot samples were collected during field survey. All the samples with the symptoms of disease were collected in separate plastic bags. The name of plant, places and time of samplings were recorded. Samples were taken into the laboratory (Lab.) and the fungi were isolated by using the standard moist blotter method (SBM) and Agar plate methods (APM). The survey was conducted on the basis of the random sampling method. The disease incidence (%) was calculated by following formula.
Disease incidence (%) = Number of plants affected / Total number of plants observed x 100
Identification of fungi Fungi was identified on the basis of morphological and microscopic characteristics. Pure cultures of these fungi were prepared and maintained on potato dextrose agar (PDA) slants. Plant pathogenic agents of powdery mildew could not grow on the most commonly used nutrient media in lab conditions. So, this fungus was identified as such, according to microscopic structures and classified on several related keys and references (Barnett and Hunter, 1999). Pathogenicity tests were carried out in field conditions and the fungi were re-isolated from these plants and identified. The causal organism, morphological and microscopic characteristics of the pathogen and symptoms of the diseases were examined and recorded during experimentation.
Statistical analysis All data were analyzed by Duncan’s Multiple Range Test (DMRT) using the computer-based program SPSS (SPSS Inc., Chicago, IL, USA).