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 Or obanche cumana Wallr. The effectiveness of conidial inoculum of this s oil-borne fungus was determined in pots and root chamber trials with t he host-parasite association sunflower (Helianthus annuus L.)/O. cuman a. Conidia applied postemergence infected emerged parasite shoots befo re they reached their flowering stage and produced seeds. The high lev el of virulence of the fungus was proven by efficient Orobanche contro l at low inoculum doses. The preplant application of 10(4) unformulate d conidia at a density of 69 conidia/cm(2), which were incorporated in to 1 kg of soil, resulted in significant disease incidence on O. cuman a shoots as well as a reduced rate of emergence. The strong suppressio n of broomrape development after soil incorporation of higher doses of conidia led to 89% increase in dry matter of sunflower, compared to t he fungus-free control. Effective control by O. cumana was due to the susceptibility of early underground developmental stages to the pathog en. The mortality of tubercles increased significantly to 85% after in oculation with the fungus. The number of parasitic attachments on sunf lower roots was reduced due to mortality of the seed's germ tube. In a ddition, suppression of seed germination was observed. Hence, each dev elopmental stage of O. cumana represents a potential target for F. oxy sporum f.sp. orthoceras attack. Orobanche cernua Loefl. grown on tobac co (Nicotiana tabacum L.) was also susceptible to the pathogen. In con trast, biotypes of O. aegyptiaca Pers. on tomato (Lycopersicon esculen tum Mill.) are nonhosts of the fungus. (C) 1998 Academic Press.