Fusarium wilt of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is caused by Fusarium oxysporum Schlecht. (emend. Snyd. & Hans.) f. sp. niveum (E.F. Sm.). The disease was first described by Smith (1894) from South Carolina and Georgia in the United States. It is now well-established throughout the watermelon-growing regions of the world. Once a field is infested, the Fusarium wilt pathogen may survive for many years and can seriously limit watermelon production in those fields. Long-term crop rotation (5 to 10 years) and leaving the soil fallow for several years helps reduce the pathogen population in the soil, but generally does not eradicate the pathogen. Even in cases where the pathogen is reduced to noneconomic levels, reintroduction via contaminated soil or seed is possible. Over the years, attempts to control Fusarium wilt have focused on several approaches, including soil polarization (Martyn

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A new phytotoxin, stemphyloxin I, C21H3205, was isolated from cultures of the pathogenic fungus Stemphylium botryoswn f. sp. lycopersici. The toxin is a tricyclic compound possessing a most unusual /8-ketoaldehyde group. Injection of stemphyloxin I into a tomato leaflet caused unlimited necrotic spots and a loss of turgor, which at higher toxin concentration wilted the whole compound leaf. (8,9). Typical symptoms consist of light to dark brown or grayish spots 1 to 10 mm in diameter. A rapid wilting of the foliage often takes place in advanced stages of disease development (9). S. botryosum isolates of different host specificity can also infect alfalfa (2) or lettuce (6). Differential effect of phytotoxic compounds produced by this pathogen on susceptible and resistant alfalfa clones (3) or lettuce cultivars (1) has been reported. The former and latter observations as well as the nature of the disease syndrome on tomato plants suggest that toxic metabolites excreted by S. botryosum f. sp. lycopersici may be involved in pathogenesis.In a preliminary study we have found that culture filtrates of S. botryosum f. sp. lycopersici contain several phytotoxic compounds which are capable of producing symptoms normally associated with the disease. The present work describes the isolation, chemical characterization, and biological activity of one of these phytotoxins. Tomato plants were grown in the greenhouse at 23 + 2°C. The plants were inoculated at the age of 6 to 8 weeks by spraying the leaves with a conidial suspension of about 6 x 104 conidia/ml. Sucrose (1%) was added to the conidial suspension. The inoculated plants were kept for 48 h in a moist chamber at 23 ± 2°C in the dark. They were then transferred back to the greenhouse. The first visible symptoms appeared as irregular spots 3 to 4 days after inoculation.Green and white cell cultures of L. esculentum Mill. cv. San Marzano were obtained from Dr. J. Gressel, The Weizmann Institute of Science (13). The cells were grown in 100 ml fresh B-5 liquid medium (4) containing 0.1 ml/l kinetin and 0.5 mg/l naphthalene acetic acid sodium salt (13) Incorporation of radioactive amino acids into macromolecules of tomato cell cultures was carried out according to a modified procedure of Zilkah and Gressel (14). Exponentially growing cells were fitered through Nybolt um, GG47) under aseptic conditions and diluted with fresh medium to give a final pack volume of 10%1o. For each treatment, five 0.5-ml aliquots 23 www.plantphysiol.org on May 11, 2018 -Published by Downloaded from

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