Historical-record approach:
Mango malformation is one of the oldest recorded disorders of mango, first noted from Indian orchards well before the disease's causal agent (later linked to Fusarium species) was understood. Early plant-pathology surveys in Bihar documented deformed vegetative and floral shoots in mango as far back as the late 19th century.
Among the given years, 1891 corresponds to this earliest documented observation of the disorder, predating the later, more detailed 20th-century studies (1935, 1948) that examined its cause and spread.
Therefore, mango malformation was first reported in 1891.