This study is concerned with the resistance of tobacco seedlings to a second attack of downy mildew caused by Peronospora tabacina Adam. Observations were made on the development and duration of resistance following recovery of tobacco seedlings from downy mildew. The duration of the period of resistance is variable and the observations indicate that plants in the greenhouse recovering during the summer remain resistant longer than do plants recovering from an initial attack during the winter. Samples of leaf tissue from recovered (resistant), infected and previously uninfected (susceptible) plants grown in soil in the greenhouse were analyzed for ammonia-nitrogen, nitrate-nitrogen, reducing sugars and total sugars. Similar analyses were made of samples of leaf tissue from tobacco plants grown in crocks of quartz sand and irrigated with nutrient solutions and from plants grown in crocks of sand-peat mixture to which various mixed fertilizers were added. There was no correlation between the ammonia-nitrogen content of tobacco plants, as determined in these experiments, and the response of the tobacco plant and its susceptibility to attack by P. tabacina, but there was a strong indication that such a relationship exists. Recovered plants contained a higher ratio of total sugar to nitrate-nitrogen than did comparable plants which had never been infected. Furthermore the ratio of total sugar to nitrate-nitrogen was lower in plants infected for a second time than it was in plants from the same lot shortly after they had recovered from the initial attack. It seems probable however that the changes in the nitrate-nitrogen and sugar content in recovered tobacco leaves are an indication of recovery and are not responsible for the resistance possessed by such plants. A series of experiments were performed to test the immediate effect of several nutrient salts and of sugar upon the sporulation of the fungus on the leaves of diseased tobacco plants. There was a very definite response in sporulation to some of these treatments, especially to potassium nitrate, calcium nitrate, and sucrose, but it is not known whether the response of sporulation of the fungus to such treatment is related to the type of resistance possessed by recovered plants. Sodium chloride and calcium chloride were dissolved in water and applied to the soil in which young tobacco plants were growing in the greenhouse. Although increased chloride in one of the sand cultural experiments seemed to protect the plants somewhat against downy mildew, application of solutions of these salts to the soil at the rates used (100 lbs. And 138 lbs. Per acre respectively) produced no noticeable effect on the severity of the disease or subsequent recovery of the plants. The leaves of recovered (resistant) tobacco plants are usually reduced in number, the lower ones having been killed by the initial attack of the fungus. Removal of the leaves from healthy tobacco plants in imitation of this condition did not produce resistance to an attack by P. tabacina. The artificial production of necrotic areas on healthy tobacco leaves in simulation of those usually following sporulation of P. tabacina on diseased leaves had no effect either on the susceptibility of the plants to infection or upon sporulation of the fungus on leaves already infected at the time the necrotic areas were induced. Results of attempts to confer immunity from downy mildew upon an entire plant by infection of a single leaf of the plant were inconclusive, since under the conditions of the experiment it was impossible to be sure that the single leaf was infected. However, the indication is that infection of a single leaf will not render the entire plant immune from a second attack by the fungus. / Master of Science