Histopathological studies of the process of Phytophthora palmivora infection in oil palm

Abstract

The predominant disease affecting oil palms in Colombia is bud rot, a decay of the growing leaf tissues that may eventually reach the meristem. Previous studies have suggested many biotic and abiotic causes, but Koch’s Postulates have recently confirmed Phytophthora palmivora as the primary pathogen of bud rot. In this paper, we describe the infection process in immature leaflets inoculated with zoospores of P. palmivora using light microscopy and scanning electron microscopy. Initially, zoospores were attracted primarily to leaf trichomes, where some developed appressorium-like structures and subsequently penetrated the tissue. Germinated zoospores were also observed penetrating the stomata or intercellular spaces. After penetration hyphae colonized the intercellular leaf spaces, followed by intracellular invasion, penetrating to the vascular bundles and then spreading throughout the host tissue. Between 12 and 24 h after inoculation, hyphae emerged from the surface of the leaflet through the stomata, trichomes and intercellular spaces. Within 36 and 48 h, sporangia, chlamydospores, and newly released zoospores were observed, starting a new cycle of secondary infections. This sequence was consistently observed in repeated experiments. We suggest that in the field collection of moisture at the base of emerging leaflets stimulates the release of zoospores and infection. Colonization is restricted once the leaflet emerges, leaving dormant chlamydospores and hyphae in small necrotic lesions. Successive rain events stimulate the formation of new generations of sporangia, zoospores and infections, which, if frequent enough, may completely colonize the bud area. Subsequent colonization of infected tissues by opportunistic bacteria and fungi increases the severity of bud rot symptoms. The information presented here provides an overview of histopathological aspects of the early infections process of P. palmivora in detached immature leaflets.