CHRONOLOGY OF GAS-EXCHANGE EFFECTS AND GROWTH EFFECTS OF INFECTION BY VERTICILLIUM-DAHLIAE IN POTATO

Growth-chamber and field experiments were conducted to study the chronology of effects on gas exchange, visible symptoms, defoliation, and stunting caused by Verticillium dahliae in potato. Use of gas exchange measurements for detecting individual diseased leaves was successful in growth chambers and the field but was sometimes confounded by water stress. Gas exchange effects often were detectable before visible symptoms for individual leaves. Defoliation was roughly concurrent with these effects, but stunting occurred later, if at all. Disease development could be divided into three phases. In the latent phase, the fungus was present in stems, but no symptoms or gas exchange effects were detectable. In the local phase, there was an acropetal procession of wilting, chlorosis, or senescence of leaves or parts of leaves, but young leaves had normal gas exchange and no visible symptoms. Acropetal defoliation continued in the systemic phase, and young leaves had reduced gas exchange and often were stunted. Reduced radiation use efficiency accounted for about one third and reduced radiation interception accounted for about two thirds of the yield loss caused by V. dahliae in two field seasons.