Reduction of Onion Yields by Chlorothalonil1

Abstract Ten weekly applications of mancozeb, chlorothalonil, or alternate applications of mancozeb and chlorothalonil were made on ‘Granex 33’ and ‘Texas Early Grano 502’ onions ( Allium cepa L.). Disease incidence was nil in all treatments, including the unsprayed control. There was no significant difference between the untreated check and mancozeb treatment for all measured yield components. Alternating chlorothalonil with mancozeb significantly decreased marketable yield and size of bulbs. Chlorothalonil alone reduced weight and size of marketable bulbs by 44 and 34%, respectively. Alternate applications of chlorothalonil and mancozeb reduced these values by 27 and 23%, respectively. Response of bulb size to fungicides was affected by both cultivar and harvest date. Chlorothalonil had no influence on the rate of maturation, but it did contribute to yield reduction.