Epidemiology of target spot of cotton and tomato

Abstract

Target spot is a foliar disease of cotton caused by Corynespora cassiicola. This disease was first reported in Alabama in 2011. The overall goal of this study was to address the epidemiology of the target spot of cotton and tomato. The specific objectives were: 1) to determine the effect of the temperature on conidial germination of C. cassiicola causing target spot of cotton; 2) to determine the effect of temperature and leaf wetness duration on target spot of cotton caused by C. cassiicola; and 3) to evaluate the if there is a difference in epidemiology of tomato and cotton isolates of C. cassiicola causing target spot of tomato. For the first objective, spores of cotton isolate of C. cassiicola were incubated in wet water agar at six different temperatures (12, 16, 20, 24, 28, and 32°C) and conidial germination was counted at 4, 8, and 12 hrs. Highest conidial germination percentage occurred at 24°C with 8 and 12 hrs incubation in water. To address the second objective, cotton plants were inoculated with each of three cotton isolates of C. cassiicola (CA1, CC1, and CM18). The inoculated plants were incubated at different temperatures (16, 20, 24, 28, and 32°C) with varying leaf wetness durations (8, 12, 16, 24, 32, 40, and 48 hrs). Early onset, i.e., 1 day after inoculation, was observed at 28°C with ≥24 hr leaf wetness. Lengthening leaf wetness durations lead increased numbers of lesions at all tested temperatures (16, 20, 24, 28, and 32°C). Highest lesion numbers were observed at 28°C with 48 hr leaf wetness for all three isolates used in this study. Low lesion numbers were observed at 16 and 32°C for all three isolates (CA1, CC1, and CM18). For the third objective, cotton and tomato isolates of C. cassiicola were inoculated on tomato plants. These inoculated plants were incubated at three temperatures (20, 24, and 28°C) with varying leaf wetness durations (8, 12, 16, 24, 32, 40, and 48 hrs). Highest lesion numbers on tomato was observed at 28°C with 48 hr leaf wetness for cotton and tomato isolates of C. cassiicola. Low temperature (20°C) required ≥40 hr leaf wetness for onset of disease on tomato caused by cotton isolate, while tomato isolates required ≥16 hr wetness for disease onset on tomato. Lowest lesion numbers among the three tested temperatures were observed at 20°C for tomato and cotton isolates. Lengthening leaf wetness duration led to increase in lesion development for both cotton and tomato isolates on tomato.