Determination of seedborne fungal pathogens of onion (allium cepa l.), prevalence and control of purple blotch caused by alternaria porri (Ellis.) CIF

Abstract

This study was undertaken to establish important fungal diseases and seed borne pathogens of onions and to establish the potential of some plant extracts for the control of purple blotch caused by Alternaria Porri (Ellis.) CIF. A survey was carried out in the major onion growing areas of Kenya for prevalence of different diseases where disease incidence and severity was scored and diseased samples collected for pathogen isolation. Purple blotch and downy mildew (Peronospora destructor (Berk.) were the most prevalent diseases of onions in Kenya giving a mean incidence of 38.1 % and 29.5 % respectively. Purple blotch was highly prevalent in Perkerra (62.1 %) and in Longisa (53.8 %) whereas downy mildew (62.9 %) and rust (Puccinia allii) (63.05 %) incidence were high in North Kinangop and pink rot (Phoma terrestris) (12.85 %) in Kibirigwi. Aspergillus niger was the most common fungus found on onion seeds with a mean incidence of 35.2 % on agar and 15.5 % on blotter. However, there was no correlation between seed infection and incidence of the disease in the field, with respect to damping off of seedlings. At both Kisii and Kabete, purple blotch was observed in the field but was not detected on seed. Other pathogens observed were Stemphylium vesicarium and Cladosporium allii although the incidence was low (0.4 %). Although all varieties were found attacked by purple blotch, Bombay Red and Red Creole were moderately resistant (3.2) while Sivan and Texas Grano were resistant 2.0- 2.5). A crude extract of Solanum nigrum at all concentrations (0.1, 0.05, 0.025 g -I water), totally inhibited the growth of A porri, while extracts of Warburgia ugandensis (0. 1 g -I water) and Cleome gynandra (0.1 g -I water) inhibited 56.7 % and 44.8 % growth respectively. Spore viability studies in vitro indicated that S. nigrum extract is fungitoxic while that of C. gynandra is fungistatic and therefore have potential for controlling purple blotch.