Turnsole (Chrozophora tinctoria) is a common spring-summer weed in chickpea (Cicer arietinum) and other dry-land crop production areas in southern Spain. Under field conditions, this weed often develops a general wilt and eventual death associated with a vascular discoloration of stems and roots. Diseased turnsole plants frequently occurred together with chickpea plants affected by Fusarium wilt caused by Fusarium oxysporum f. sp. ciceris, a major disease of chickpea crops in southern Spain (1). Isolations from roots, stems and leaf petioles of turnsole plants consistently yielded F. oxysporum, and it was morphologically similar to F. oxysporum f. sp. ciceris. To test the pathogenicity of this fungus, germinated and previously surface-sterilized seeds of turnsole and chickpea cultivar Blanco Lechoso were planted in a greenhouse soil mixture artificially infested with four isolates of F. oxysporum from turnsole and two isolates of F. oxysporum f. sp. ciceris, one of them inducing the wilt syndrome and the other causing yellowing (1). Pathogenicity tests were conducted following the standard inoculation method used for F. oxysporum in chickpea (1). Inoculated and control plants were maintained in a greenhouse at 15 to 30°C. Isolates of F. oxysporum from turnsole caused wilt symptoms and death of turnsole plants within 2 months, but chickpea isolates did not affect turnsole. Conversely, chickpea plants were affected only by the two isolates of F. oxysporum f. sp. ciceris from chickpea. All diseased chickpea and turnsole plants exhibited typical vascular discoloration. F. oxysporum was consistently reisolated from the vascular tissues of roots, stems, and leaf petioles of affected plants. Based on these results, the fungus causing wilt of turnsole was identified as a forma specialis of F. oxysporum different from the chickpea wilt pathogen. Since the Fusarium wilt diseases of turnsole and chickpea are caused by different pathogens, the occurrence of F. oxysporum causing wilt of turnsole in the field can not be used to forecast Fusarium wilt of chickpea, but it may be considered as a potential biocontrol agent of this weed under field conditions. This is the first report of F. oxysporum causing wilt of turnsole.
Reference: (1) A. Trapero-Casas and R. M. Jiménez-Díaz. Phytopathology 75:1146, 1985.