CONDITIONS FOR SYMPTOMATOLOGICAL DIFFERENTIATION OF BACTERIAL CANKER, SPOT, AND SPECK ON TOMATO SEEDLINGS

Abstract

Bacterial canker, spot, and speck of tomatoes (Lycopersicon esculentum Mill.) caused by Corynebacterium michiganense (E.F.S.) Jensen, Xanthomonas vesicatoria (Doidge) Dows., and Pseudomonas tomato (Okabe) Burk., respectively, were symptomatologically differentiated on 2- to 3-week-old spray-inoculated seedlings only under conditions of 87–97% relative humidity and 23–28 °C temperature. The numerical threshold of infection of both C. michiganense and P. tomato was 1 × 106 cells/ml and that of X. vesicatoria was 1 × 103 cells/ml. Preinoculation host injury and an inoculum concentration of 1 × 108 cells/ml were most favorable for high incidence of the diseases.Characteristic symptoms incited by the canker organism were (1) small whitish pimple-like spots developing into raised blister-like lesions on the lamina, (2) elongated swellings on veins, and (3) cankers on the hypocotyl. The distinctive symptoms of the bacterial spot disease were (1) small greenish-yellow to brown leaf spots, (2) large yellow blotches becoming necrotic and producing a severe blight effect on leaves, and (3) light-brown streaks on the hypocotyl. The distinguishing symptoms of the speck disease were discrete dark-brown spots and occasional marginal necrotic areas on leaves and cotyledons. On cotyledons, both C. michiganense and X. vesicatoria produced identical minute whitish flaky spots often with greenish centers. Sometimes these spots coalesced and resulted in wrinkling of the surface of the cotyledon.