In spring 2001, plants of the lettuce cv. Salad Bowl showing symptoms of a wilt disease were observed in several commercial plastic greenhouses near Bergamo, in northern Italy. Wilted plants were first observed during the spring and summer of 2001 when temperatures were between 26 and 35°C. Symptoms were observed in the same area and the same farms in March 2002, in concomitance with a period of high temperatures. Although the distribution of the disease was generally uniform, symptoms were more severe in the central part of the greenhouses where temperatures were warmest. Symptoms were first observed at thinning, when seedlings (30 days old) appeared wilted. Vascular tissues of affected seedlings appeared red or brown. Affected plants were stunted and developed yellow leaves and brown or black streaks in the vascular system. The vascular streaks in the yellow leaves extended from the crown and were continuous with a red-brown discoloration in the vascular system of the crown and upper taproot. Symptoms were typically not visible on the outside of the crowns or roots. Fusarium oxysporum was consistently and readily isolated from symptomatic vascular tissues on a Fusarium-selective medium (2). Seeds of cv. Salad Bowl were planted in steam-sterilized soil artificially infested with 1 × 104 CFU/g soil of each of two isolates of F. oxysporum obtained from infected plants. Plants grown in noninfested soil served as noninoculated control treatments. Plants (25 per treatment) were grown at 25 to 28°C in growth chambers. Wilt symptoms and vascular discoloration of the roots, crown, and veins developed 20 days after seeding for all plants grown in the infested soil, and F. oxysporum was consistently reisolated from infected plants. The pathogenicity test was conducted twice. To our knowledge, this is the first report of F. oxysporum on lettuce in Europe. A wilt of lettuce attributed to F. oxysporum f. sp. lactucae was reported in Japan in 1967 (3) and in the United States in 1993 (1), in the latter case the incitant organism was described as F. oxysporum f. sp. lactucum.
References: (1) J. C. Hubbard and J. S. Gerik. Plant Dis. 77:750, 1993. (2) H. Komada. Rev. Plant Prot. Res. 8:114, 1975. (3) T. Matuo and S. Motohashi. Trans. Mycol. Soc. Jpn. 8:13, 1967.
