Lesion and epidemic development of Alternaria longipes (Ell. & Ev.) Mason on tobacco

Lesion development of A. longipes was studied in the glasshouse and in the field. The incubation period on glasshouse plants varied from 3 to 8 days. Some pinpoint lesions failed to develop, and lesion development on the upper leaves of pot plants was less than on the lower ones. The maximum estimated lesion multiplication rate was 2.1 daughter lesions/mother lesion/day, but was generally ≤ 0.6. Epidemic development was studied in four seasons (1966 to 1969) by weekly assessment of infection levels on alternate leaves. Leaf area, airborne inoculum concentrations, air, leaf and soil temperature, soil moisture, saturation deficit, rainfall, duration of leaf wetness, and wind speed were also measured. Relative infection rates varied from 0.17-0.26 units/day. Slow disease development in 1968 (r = 0.17 units/day) could be related to the adverse effects of climate on host development and on infection processes. The relative rate of infection varied widely at different stalk position (0.16-0.29 units/day in 1966) and at different times at the same stalk position (0.17=0.48 units/day at position 25 in 1969). There was no consistent relationship between the stalk position of leaves and the rate at which they were infected. Similarly there was no consistent relationship between the rate of infection of whole plants or of leaves at a particular stalk position and climatic or inoculum factors, although during limited periods, changes in one or other of these factors did coincide with an alteration in the relative rate of infection. The interpretation of rates of infection and the theoretical results of greater field hygiene are discussed.