Early infection process of Damasonium minus by the potential mycoherbistat Rhynchosporium alismatis

Rhynchosporium alismatis is a fungal pathogen that promises to be a suitable biological control agent to manage the rice weed, starfruit (Damasonium minus). Light and scanning electron microscopy were used to study the infection process. Conidial germination, appressorium formation, and penetration by R. alismatis were compared at 25 and 30°C on starfruit leaves, and in separate experiments, conidial germination and appressorium formation were compared at both temperatures on cellophane paper. Appressorium formation was significantly (P < 0.005) higher at 25 than 30°C after 8 h on leaves and after 10 h on cellophane paper. Sessile appressoria were frequently observed on leaves, while no such structures were observed on cellophane paper. Penetration through stomata appeared to be a random event. Subepidermal hyphae were frequent after 24 h, and conidial formation was observed 48 h after inoculation of leaves at 25°C. Deformation of the leaf surface around the appressoria indicated signs of physical pressure. Fast conidial germination, penetration, and infection is an advantage, since the fungus is not exposed for long periods of time to adverse environmental conditions. The formation of conidia on the leaves can lead to further disease spread.