Sporulation of Fusarium oxysporum f.sp. lycopersici on stem surfaces of tomato plants and aerial dissemination of inoculum.
Plants exhibiting symptoms of wilt and xylem discoloration typical of Fusarium wilt caused by F. oxysporum f.sp. lycopersici were observed in greenhouses of cherry tomatoes at various sites in Israel. The lower stems of some of these plants, however, were covered with a pink layer of macroconidia of F. oxysporum which resembled the sporulating layer on stems of tomato plants infected with F. oxysporum f.sp. radicis-lycopersici (causal agent of crown and root rot disease). Monoconidial isolates of F. oxysporum from diseased plants were assigned to vegetative compatibility group 0030 of F. oxysporum f.sp. lycopersici and identified as belonging to race 1 of F. oxysporum f.sp. lycopersici. The possibility of coinfection with F. oxysporum f.sp. lycopersici and F. oxysporum f.sp. radicis-lycopersici was excluded by testing several macroconidia from each plant. Airborne propagules of F. oxysporum f.sp. lycopersici were trapped on selective medium in greenhouses in which plants with a sporulating layer had been growing. Sporulation on stems was reproduced by inoculating tomato (cvs. Marmande and Rehovot 13) plants with races 1 and 2 of F. oxysporum f.sp. lycopersici. This phenomenon has not been reported previously with F. oxysporum f.sp. lycopersici and might be connected to specific environmental conditions, e.g., high humidity. The sporulation of F. oxysporum f.sp. lycopersici on plant stems and the resultant aerial dissemination of macroconidia may have serious epidemiological consequences. It is concluded that sanitation of the greenhouse structure, as part of an holistic disease management approach, is necessary to ensure effective disease control.