Verticillium wilt of tobacco: IX. The relationship between leaf symptoms and levels of infection in the organs of mature tobacco plants

Mature plants of six tobacco cultivars (Nicotiana tabacum L.) with a range of resistance to Verticillium dahliae Kleb. were studied to determine the amount of this fungus present in the different plant organs. Significant differences were found between the cultivars. There was a significant correlation between resistance as evaluated by leaf symptoms and the amount of fungus in the plants. Resistant plants had higher dry matter content than susceptible ones. At the end of the growing season there was a higher concentration of Verticillium near the top of the stems than at the base. An estimate of the amount of fungus returned to the soil in the crop remains indicated that tolerant cultivars would contribute less than susceptible varieties to the inoculum potential of the soil. The evidence suggests that cell vitality is of prime importance in symptom expression, and that functioning cells in both resistant and susceptible tobacco cultivars suppress the growth of this fungus. It is proposed that s...