Survey, morphology and white mold disease of country bean (Lablab purpureus L.) caused by Sclerotinia sclerotiorum (Lib.) de Bary in-relation to soil physico-chemical properties and weather conditions in Bangladesh

A survey was conducted to document the incidence of white mold disease in country bean production areas at Bogra, Pabna, Jessore, Sylhet, Hobigonj and Gazipur districts in Bangladesh during 2016–17. A total of 115 farmers’ fields was surveyed from early vegetative to end of harvesting stages. Pathogens were collected from diseased stems, racemes and pods. Soil samples were collected from the same fields. White mold disease was identified based on symptoms in the field. White mold disease infection was recorded in 100% of the surveyed fields in Shahjahanpur upazila of Bogra district, while approx. 90, 80, 60 and 20% of the fields were infected in Bahubal of Hobigonj; Golapganj of Sylhet; Ishordi of Pabna and Monirampur of Jessore, respectively. White mold was not found in any of the fields surveyed in Joydebpur, Gazipur. Disease incidence and severity were higher in Shahjahanpur of Bogra and Bahubal of Hobigonj district. Moderate disease incidence and severity were observed in Golapganj of Sylhet. Pathogen infection was higher in the fields with loamy fine sandy soil with pH of 5.11–6.22. Globose to irregular black sclerotia developed in infected plant parts and the sclerotia germinated and produced apothecia in the fields after rain. Hyaline, branched, well developed and septate mycelia developed on potato dextrose agar medium. The hyphae were 2.0–7.2 μm in width. After five days of incubation, the subsurface mycelial cells swelled and formed dark black sclerotia. The sclerotia were 1.5–6 mm in width and 2–17 mm in length. Sclerotia germinated, and brown colored round to globose shaped apothecia developed at the end of a long stipe. The length and diameter of apothecia were approximately 4–21 mm and 2–8 mm, respectively. Asci were hyaline and cylindrical in shape, while the ascospores were elliptical. The size of ascus and ascospore was 68–155 × 4.5–6.01 μm and 4.7–6.94 x 4–6.2 μm, respectively. Calcium oxalate was formed on the wall surface of the asci. Disease incidence was negatively correlated with pH and calcium content in the soil while apothecia formation was positively correlated with rainfall and humidity.