The potential of Fusarium oxysporum f.sp. orthoceras as a biological control agent for Orobanche cumana in sunflower

Fusarium oxysporum Schlecht. f.sp. orthoceras (Appel & Wollenw.) Bilay was evaluated for its potential to control the root-parasitic weed Orobanche cumana Wallr. 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 annuus L.)/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 efficient Orobanche control at low inoculum doses. The preplant application of 10(4) unformulated conidia at a density of 69 conidia/cm(2), which were incorporated into 1 kg of soil, resulted in significant disease incidence on O. cumana shoots 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 by O. cumana was 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 seed's germ tube. In addition, suppression of seed germination was observed. Hence, each developmental stage of O. cumana represents a potential target for F. oxysporum f.sp. orthoceras attack. Orobanche cernua Loefl. grown on tobacco (Nicotiana tabacum L.) was also susceptible to the pathogen. In contrast, biotypes of O. aegyptiaca Pers. on tomato (Lycopersicon esculentum Mill.) are nonhosts of the fungus. (C) 1998 Academic Press.