Farmer&;s lung is caused by

Farmer&;s lung - systemic and pulmonary reactions resulting from inhalation of dust from hay mould, mouldy straw, threshing dust, or by persons who have become hypersensitive to antigens in the dust, most often associated with inhalation of spores of micromonospora faenia or thermoactinomyces vulgaris. Ref: Baveja textbook of microbiology; 4th edition