Mode of Infection of Phyllosticta maculata on Banana as Revealed by Scanning Electron Microscopy

The initial infection stages of Phyllosticta maculata on banana were studied using scanning electron microscopy. Conidial germination on the banana leaf surface commenced within 3h postinoculation to produce a long and slender germ tube. The hyphae developed secondary branches and mostly grew randomly across the leaf surface. Appressoria were formed at the apex of the germ tubes within 18h postinoculation and were variable in shape. A layer of an extracellular matrix surrounded the appressoria at the pathogenhost interface. On the fruit surface, conidia germinated to produce predominantly swollen germ tubes which functioned as lateral appressoria together with some slender ones. These germ tubes were formed within 3h postinoculation. There was no stomatal penetration apparent on the leaf; instead, direct penetration through the cuticle with and without the formation of appressoria was observed. Cuticular degradation on the leaf surface was evident with a circular, darkened area around the point of penetration by hyphae or appressoria. The significant role of pycnidia and conidia in the epidemiology of the disease was further demonstrated in naturally infected leaf samples.