Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Temperature, humidity, carbon dioxide and light play a key role in the viability of conidia of Peronospora hyoscyami f.sp. tabacina.;Conidia lodge in the depressions between the epidermal cells above the anticlinal walls, and commence to produce short germ tubes. An appressorium forms immediately at the top of the germ tube above or near an anticlinal wall.;A spherical vesicle about 12 um in diameter, forms inside the epidermal cell. An electron-transparent plug is formed in the inner end of the penetration tube.;Once the intracellular hypha leaves the epidermal cell, it develops in the intercellular spaces of the host tissue. The intercellular hyphae grow between the host cells, follow their contour and send haustoria of various shapes and sizes into adjoining cells. Between 5 and 6 days after inoculation, knots of intercellular hypha occupy the stomatal chamber. Juvenile conidiophores are evident emerging from the hyphal knots. Five to 6 dichotomies occur prior to the initiation of conidial formation.;Haustorial formation in P. hyoscyami f.sp. tabacina passes through three stages, bulb, cane and coil. After host wall penetration, the haustorium becomes bulb-shaped and grows into a straight hypha about 7-10 um in length that bends at its tip to produce a cane-like handle. The tip of the handle grows into a torulose coiled structure up to 50 um in length. A papilla or collar is never formed. The young haustorium is surrounded by extrahaustorial matrix composed of a thin inner electron-opaque layer on the haustorial wall and a much thicker electron-transparent layer, which extends to the extrahaustorial membrane. Haustoria occur in the mesophyll, palisade, lower and upper epidermis.;Starch accumulation / formation in infected tissue is different than in the healthy controls. Starch degradation was retarded as early as 2 h after infection and this retardation continued throughout the latent and sporulation period. Starch formation in infected tissue was retarded 31 h after inoculation and thereafter.



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