Recent advances in the biocontrol of Orobanche (broomrape) species

Parasitic broomrapes (Orobanche spp.) are major uncontrolled weeds in the Mediterranean regions of Europe and the Near East causing major losses to vegetable, grain legume, and sunflower crops. Selective herbicides alone cannot provide persistent, season-long control of these parasites, and much methyl bromide is used for their control, where affordable. Thus they are excellent targets for biocontrol. The recent progress by the COST 816 Orobanche working group in this area is reviewed herein. Natural infestation by the fly Phytomyza orobanchia of seed capsules of Orobanche crenata parasitising faba bean halved Orobanche seed production while inundative releases of adults reduced it to 5% of viable seeds. The fungi Fusarium arthrosporioides E4a and F. oxysporum Eld, as well as strains of bacteria were isolated from diseased, juvenile, Orobanche flower stalks. They are pathogenic to O. aegyptiaca, O. crenata and O. ramosa on most vegetable crops. A F. oxysporum f. sp. orthoceras was specifically pathogenic to O. cumana on sunflowers. All were used in various experiments with a modicum of success. Methods were developed to formulate isolated mycelia, which could eventually allow the use of transgenic hypervirulent pathogens in asporogenic (deletion) mutants (as a failsafe against spread). Mycotoxins were also isolated from different Fusarium and other fungal species that kill Orobanche, and are being considered for direct use, or to augment other strategies. All three Fusarium spp. used have been transformed with gus and/or gfp genes allowing tracing their movement in the environment, and opening the way to future transformations to hypervirulence.