Institute of Himalayan Bioresource Technology, Palampur 176, 0617 Himachal Pradesh, India

Background and objectives
Blister blight of tea, caused by the biotrophic fungal pathogen Exobasidium vexans, is the major disease in the principal tea-growing countries. The pathogen attacks harvestable tender shoots, inflicting enormous yield loss (30-40%) and quality deterioration even below the 35% disease threshold, causing yield losses [1]. Air-dispersed basidiospores are the sole infective propagules in the life cycle of the fungus, inducing the development of hypertrophid, coalescing and white powdery blisters in the susceptable young leaves. Light microscopic studies have revealed proliferation of fungal hyphae within spongy parenchyma and rupturing of the epidermis overlaying the developing hymenial layer in susceptible interactions; and degeneration of hyphae, lack of hymenial development and a halo of lignin-rich cells around necrotizing, self-delimiting, transluscent infection spots in resistant interactions [2]. However, the infection process is poorly understood. The ultrastructure of host-parasite interactions in tea blister-blight pathogens is reported at different stages of disease development.

Materials and methods
Tea leaves were cut into peices 2x3 mm through inoculated/blistered spots and uninoculated/uninfected portions, and fixed at 4C overnight in 2.5% gluteraldehyde buffered with 0.1 M sodium cacodylate (pH 6.8). For scanning electron micoscopy, samples were dehydrated in graded acetone series, critical point-dried in liquid carbon dioxide with LADD system, mounted on aluminium stubs, coated with gold (Joel Sputter Coater JFC-100), and examined in the Joel-1200 EX electron microscope. For transmision electron microscopy, specimens were post-fixed in 2% OsO4 in sodium cacodylate buffer for 1 h, after three buffer rinsings following gluteraldehyde fixation. The material was dehydrated in graded ethyl alcohol series and embedded in araldite-epon-epoxy resin. The resin medium was polymerized at 70C. Sections were cut on an Reichert-Jung Ultracut microtome, stained in uranyl acetate and lead citrate, and viewed in the electron microscope.

Results and conclusions
Ultrastructural observations on penetration, colonization and cytological modifications in blister blight-infected tea leaves showed penetration of host tissue by germinating basidiospores was direct and preceded by appressorium formation; a fine infection hypha (penetration peg) developed from the lower surface of the appressorium and penetrated through the host epidermis; the colonization hyphae spread intercellularly through the adjacent tissues. The haustoria were produced on intercellular fungal hyphae; the haustorium consisted of three parts: haustorial mother cell, haustorial neck, and haustorial body. The haustorial mother cell, closely appressed to the host cell, produced a penetration peg and invaginated the plasma membrane of the host cell; the host-pathogen interface exhibited elaborate modifications at the penetration site, the haustorial neck flared out, forming the haustorium; occasional ensheathing of the invading haustorial body occurred by thick cytoplasmic matrix; infected tissue showed a loss (disorganization and complete olbiteration) of cytoplasmic organelles; basidia emerged through stomatal apertures or rupturing the host epidermis; and each basidium produced two or four basidiospores on sterigmata. Studies showed development of the haustorium to be a cardinal feature of the host-parasite relationship in tea blister blight. Observations will be discussed in relation to the fine structure of the host-parasite relationship in other biotrophic fungal pathogens and their hosts.

1. Gulati A, Gulati A, Ravindranath SD, Chakrabarty DN, 1993. Indian Phytopathology 46, 155-159.
2. Gupta AK, Gulati A, Maitra A. Kottaisamy K, 1998. Annual Meeting of the Phytopathological Society (in press).