STRUCTURAL ASPECTS OF ASCOCHYTA BLIGHT OF LENTIL

Ascochyta fabae Speg. f.sp. lentis (Gossen et al. 1986) causes lesions on the leaf, stem, and pod of lentil (Lens culinaris Medik.), thereby reducing seed quality and yield. Lesion formation was studied in two cultivars, Laird and Invincible, using light and electron microscopy o f intact and excised leaves and stems inoculated with spore suspension . Spores germinated usually within 6 h of inoculation and germ tubes g rew for varying distances along the leaf surface before forming an app ressorium, sometimes within less than 10 h. A penetration peg then eit her directly entered the underlying epidermal cell, or grew as a subcu ticular hypha for a short distance before entering the cell. The first response of epidermal cells to presence of the fungus was an aggregat ion of cytoplasm abutting the site of infection. This was followed clo sely by deposition of a papilla. Some relatively thick papillae were s een at 29 h postinoculation. The fungus then grew into the papilla and formed an infection vesicle. In susceptible host cells, the protoplas m became necrotic before hyphae grew into the lumen of the cell from t he infection vesicle. In more resistant cells, the infection vesicle o ften became surrounded by electron-dense wall material developed by th e host. The fungus remained in susceptible epidermal cells for up to 4 days, amongst remnants of the protoplast, before spreading to the adj acent mesophyll. Hyphae grew into intercellular spaces of the mesophyl l and remained there for 2-3 days before penetrating the cells. The me sophyll reacted in a similar way to infection as did the epidermis, wi th only host cells close to the fungus becoming affected. Cultivar Lai rd was found to be less susceptible to infection than cv. Invincible. At the structural level, the infection process was found to be similar except that in cv. Laird the infection vesicle more frequently became surrounded by electron-dense wall material formed by the host. In ste m tissue of cv. Laird the middle lamella was also occasionally thicken ed with electron-dense material deposited on either side of it. After the degeneration of host tissue, pycnidia-bearing spores were formed 1 0-14 days after inoculation of the leaf.