Structural aspects of Ascochyta blight of lentil

Abstract

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 of intact and excised leaves and stems inoculated with spore suspension. Spores germinated usually within 6 h of inoculation and germ tubes grew for varying distances along the leaf surface before forming an appressorium, sometimes within less than 10 h. A penetration peg then either directly entered the underlying epidermal cell, or grew as a subcuticular hypha for a short distance before entering the cell. The first response of epidermal cells to presence of the fungus was an aggregation of cytoplasm abutting the site of infection. This was followed closely by deposition of a papilla. Some relatively thick papillae were seen at 29 h postinoculation. The fungus then grew into the papilla and formed an infection vesicle. In susceptible host cells, the protoplasm became necrotic before hyphae grew into the lumen of the cell from the infection vesicle. In more resistant cells, the infection vesicle often became surrounded by electron-dense wall material developed by the host. The fungus remained in susceptible epidermal cells for up to 4 days, amongst remnants of the protoplast, before spreading to the adjacent mesophyll. Hyphae grew into intercellular spaces of the mesophyll and remained there for 2 – 3 days before penetrating the cells. The mesophyll reacted in a similar way to infection as did the epidermis, with only host cells close to the fungus becoming affected. Cultivar Laird 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 stem tissue of cv. Laird the middle lamella was also occasionally thickened with electron-dense material deposited on either side of it. After the degeneration of host tissue, pycnidia-bearing spores were formed 10 – 14 days after inoculation of the leaf. Key words: Ascochyta, lentil, ultrastructure, infection process.