Stem infection of tobacco (Nicotiana tabacum L. cv. Virginia Gold) with Peronospora tabacina Adam was established and its development studied. Mycelium of the pathogen spread from parenchyma tissue of the infected leaf blade into the vascular system of midrib and petiole and thence into the stem. In small seedlings mycelium was usually associated with all tissues and often killed the plant. In plants at the flowering stage necrosis was chiefly confined to the periphery of secondary xylem, the cambium, and the inner part of the external phloem region. As xylem aged it acted as a barrier to the spread of mycelium from external to internal phloem and to the pith. Cortex was not penetrated readily by the pathogen. When the pathogen became established in the cambium region of the stem at an early stage of plant growth, development of xylem was often inhibited. Such plants, when full grown, were brittle at the base and snapped off readily. Development of the pathogen in the stem was favoured by high humidity but was arrested by exposing the plants to benzol vapour, or to a daily temperature range of 75-105°F when humidity was not higher than 70 per cent. High temperature during the day followed by low temperature at night did not prevent spread of the pathogen from the leaves into the stem. Mycelium sometimes spread from infected stems into new axillary shoots with subsequent sporulation on leaves.
Population structure and migration of the Tobacco Blue Mold Pathogen,Peronospora tabacina,into North America and Europe