Development of a non-chemical mycoherbicide gibbatrianth for controlling trianthema portulacastrum (horse purslane) a notorious agricultural weed: A review

Horse purslane (Trianthema portulacastrum L.) is considered the most troublesome agricultural weed in tropical and sub-tropical areas of the world, including the US, Mexico, Northern and Southern states of India and Pakistan by virtue of its infestation in various vegetable and agricultural fields. Foliar phytopathogenic fungi, the fungi that cause foliage diseases in plants, offers a tremendous opportunity to develop as bioherbicides/mycoherbicides. Globally, a total of 13 fungal pathogens belonging to 10 genera of Ascomycota, all except Myrothecium verrucaria, are reported on horse purslane from India. Of these, Gibbago trianthemae, a foliar pathogen that causes leaf spots and stem blight, has been evaluated for its biocontrol potential to control horse purslane non-chemically in the USA, India and Mexico. The studies carried out on Gibbago-Trianthema system has revealed that this pathogen has all the desirable characteristics that makes it a suitable candidate to be developed as a mycoherbicide for horse purslane. The formulation of the fungal pathogen (G. trianthemae) consisting of conidial suspension + surfactant Tween 20 has been named Gibbatrianth. G. trianthemae, belongs to Dothideomycetes, a class under the phylum Ascomycota (formerly classified in Hyphomycetes, Deuteromycota). It is a phaeodictyoconidial fungus, easily cultured on potato dextrose agar and trianthema dextrose agar at 25oC. It produces non-beaked, dark, muriform conidia having both transverse and longitudinal septa, and developing singly through porogenous development. Each conidium germinates producing several germ tubes, and a germ tube in turn produces another conidium at its tip. Germination of conidia in this host-specific pathogen on leaves occurs within 6 to 12 hours post spraying, causing infection through stomata by producing peg like structures called appressoria. This pathogen is a hemibiotroph similar to mycoherbicidal fungus Colletotrichum upon which maximum bioherbicides are available in the market. Gibbatrianth is the first and the only mycoherbicide developed for controlling horse purslane globally. There is a huge scope for commercial exploitation of Gibbatrianth, a nonchemical herbicide, for controlling horse purslane worldwide, including India, for getting organic vegetables, free from the carcinogenic chemicals.