Abstract
Conidial germination, hyphal elongation and penetration of Leveillula taurica (Lev.) Arn. on the leaf surface of peppers were observed using both optical and electron microscopes.
Germination of conidia of L. taurica reached the maximum level of about 70% within 2 to 3hr after inoculation. Each conidium bore a germ tube at the tip of which an appressorium was formed at 3hr after inoculation. Conidial germination initiated in most cases from the base of conidia, in some cases from the tip of conidia and in rare cases even from the middle of conidia. There was only one germ tube in any case of conidial germination. Almost all of the germ tubes initiated to form appressoria within 3hr after inoculation. There were two types of appressoria formed on the pepper leaves: namely, one was a fist-like appressorium in shape formed immediately after conidial germination, while the other was a pier-like appressorium formed on extended hyphae. The latter showed a tendency to bifurcate two or three times. Neither type of appressoria bore haustria, but seemed to play the role of fixing the germ tube or hypha to the leaf surface. The hyphae initially grew in a winding fashion, forming appressoria a few times and penetrating through the stomata by branched hyphae in most cases or, in rare instances, by main axial hyphae on the lower leaf surface. After the first penetration, the hyphae elongated, branched at a right angle to the parent hyphae and penetrated further through the stomata by the tips of the branches. After penetrating two or three times the hyphae continued to develop in a straight line, reaching about 2, 700μm in length 6 days after inoculation. In this fungus, the colonies were fan-shaped, and not circular or oval as in the case of some ecto-parasitic powdery mildews.
