Preinfection behaviour of Phyllosticta musarum on banana leaves

Abstract

Light microscopy of the prepenetration process of Phyllosticta musarum, the banana freckle disease causal agent, on banana leaf was conducted. Germination of conidia started within 2–3 h after inoculation (AI) in a film of water at 25°C. More than 50 and 90% of conidia germinated at 12 and 72 h AI at 25°C, respectively. The germination of a conidium usually produced one germ tube, and occasionally two germ tubes, which were borne at the side of the conidium. However, only one germ tube developed into a mature appressorium. Appressoria were dark brown in colour and had different shapes. Most of them were round or slightly lobed. Appressorium formation initiated at 6 h AI, and the appressorial formation rate reached more than 50 and 80% at 24 and 48 h AI, respectively. A reddish-brown necrosis of host cells appeared under the appressoria as they were forming and was seen at 120 h AI. Most appressoria were formed in the grooves between adjacent epidermis cells. The pathogen seems to penetrate directly through the epidermal cuticle layer of the host by forming appressoria and infection pegs.