Abstract
The course of development of late blight disease in potato was studied in the district of Lakhimpur, Assam, where potato is extensively cultivated on a commercial basis. The study was made in experimental plots treated with extracts of three weed plants, also utilized by the native people for food and medicine. Environmental factors influenced the incidence of the disease to a great extent. Once the pathogen gets a suitable climate of mild rainfall, with high humidity and low temperature, it takes epidemic form. Extracts of Alternanthera sessilis and Commelina benghalensis reduced the intensity of the disease to a considerable extent.