SCANNING ELECTRON MICROSCOPIC STUDY OF THE INFECTION PROCESS OF PHYTOPHTHORA CAPSICI

The infection process of Phytophthora capsici on susceptible and resistant pepper cultivars were studied by scanning electronic microscopy. The development of P. capsici within 24 h post inoculation (hpi) was quite similar between susceptible cv. early Calwonder (EC) and resistant cv. Criollo de Morelos 334 (CM334). Zoospores encysted on the leaves and formed adhesive cysts at 3 hpi. Cyst germinated by producing two germ tubes at 12 hpi. The hyphae penetrated both abaxial and adaxial epidermis of pepper leaves without forming appressoria-like structures or via stomata even when they passed over it at 24 hpi. Up to the first three steps of infection, no apparent difference was observed between susceptible and resistant cultivars. However, differences were detected at the third day post inoculation (dpi) as mycelia grew out from the abaxial surface of the leaf of the susceptible cv. EC, but not from CM334. Small water soaked lesions appeared at 4 dpi, and sporulation occurred at 5 dpi on the susceptible cv. EC, while on the resistant cv. CM334, no disease symptom developed and no hyphae was observed on the leaf surface even at 12 dpi. The disease symptom and the sporulation appeared first on the abaxial leaf surface of cv. EC, followed by on adaxial leaf surface 24 h later. The new mycelia from leaves were observed both leaf surfaces through the ruptured epidermis, leaf veins and the stomata. The emergence of new mycelia through stomata appeared to be random rather than a preferred route. At 10 dpi the mycelium spread all over the surface of susceptible pepper leaf and the hyphal colony could be observed at 12dpi.