Biological weed control in rice with a strain of Colletotrichum gloeosporioides (Penz.) Sacc. used as a mycoherbicide

Abstract

An indigenous fungal pathogen of the native leguminous weed, northern jointvetch ( Aeschynomene virginica [L.] B.S.P.) was found to cause an anthracnose disease at endemic levels annually on its host in Arkansas rice and soybean fields without apparent impact on competitive ability or dispersal of the weed. The pathogen was isolated in pure culture and identified as a host-specific strain of Colletotrichum gloeosporioides, an intensively studied ‘group species’ which causes many anthracnose diseases and consists of species and strains representing wide latitudes of hosts specificity and virulence on both economic crops and wild plants in the tropical and temperate regions of the world. An understanding of anthracnose disease aetiology gained from the literature and confirmed by empirical studies with the weed and fungus revealed that the pathogen was specific and potentially devastating to its weed host, but was constrained in natural situations by poor capacity for dissemination of inoculum. Its spores, although produced in abundance, develop late, are sticky and dispersed mainly by splashing rain or insects, mechanisms that are suppressed on annual weed plants interspersed with annual crop plants. For the fungus to be effective as a biological control agent (i.e. as a mycoherbicide), complete coverage of a field with inoculum at appropriate times and levels to compensate for the pathogen's poor capacity for dissemination would be required. A multidisciplinary team of plant pathologists, weed scientists, veterinary scientists and fermentation scientists investigated the practical potential of the mycoherbicide concept with this host-pathogen combination and found it to be biologically feasible, registerable and commercially practical. The fungus is marketed as living dry spores for use on a comparatively small area weed problem and in a crop-management programme that relies heavily on chemical pesticides. Extension of this concept to control other weeds or other pests seems possible and practical.