The Potential ofFusarium oxysporumf.sp.orthocerasas a Biological Control Agent forOrobanche cumanain Sunflower

Fusarium oxysporumSchlecht. f.sp.orthoceras(Appel & Wollenw.) Bilay was evaluated for its potential to control the root-parasitic weedOrobanche cumanaWallr. The effectiveness of conidial inoculum of this soil-borne fungus was determined in pots and root chamber trials with the host–parasite association sunflower (Helianthus annuusL.)/O. cumana.Conidia applied postemergence infected emerged parasite shoots before they reached their flowering stage and produced seeds. The high level of virulence of the fungus was proven by efficientOrobanchecontrol at low inoculum doses. The preplant application of 104unformulated conidia at a density of 69 conidia/cm2, which were incorporated into 1 kg of soil, resulted in significant disease incidence onO. cumanashoots as well as a reduced rate of emergence. The strong suppression of broomrape development after soil incorporation of higher doses of conidia led to 89% increase in dry matter of sunflower, compared to the fungus-free control. Effective control byO. cumanawas due to the susceptibility of early underground developmental stages to the pathogen. The mortality of tubercles increased significantly to 85% after inoculation with the fungus. The number of parasitic attachments on sunflower roots was reduced due to mortality of the seedʹs germ tube. In addition, suppression of seed germination was observed. Hence, each developmental stage ofO. cumanarepresents a potential target forF. oxysporumf.sp.orthocerasattack.Orobanche cernuaLoefl. grown on tobacco (Nicotiana tabacumL.) was also susceptible to the pathogen. In contrast, biotypes ofO. aegyptiacaPers. on tomato (Lycopersicon esculentumMill.) are nonhosts of the fungus.