Reduction in Tomato Yield Due to Septoria Leaf Spot. Francis J. Ferrandino, Assistant Scientist, Department of Plant Pathology and Ecology, Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, CT 06504. Wade H. Elmer, Assistant Plant Pathologist, Department of Plant Pathology and Ecology, Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, CT 06504. Plant Dis. 76:208-211. Accepted for publication 17 September 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0208.

In 1988 and 1989, unpruned tomato plants (cv. Better Boy) were grown on black plastic mulch in plots (four to six plants per plot). In early July, plots were artificially infested with tomato residues colonized by Septoria lycopersici. Control plots were not infested with residues and were sprayed weekly with mancozeb. The number of empty leaf nodes, attached dead leaves, diseased leaves, and healthy leaves were counted throughout the season. The rate of disease progress was 0.05–0.18 per day, and yields from diseased plots were 40–95% of the yields obtained from sprayed controls. In both years, final yield was negatively correlated with the square of the area under the disease progress curve (AUDPC) (1988, R² = 0.70; 1989, R² = 0.80–0.83) and positively correlated with healthy leaf area duration (HAD) (1988, R² = 0.76; 1989, R² = 0.83–0.89). The equations that best described the relation between final yield (Y) and AUDPC and HAD were: Y = 9.9 – 0.0028 (AUDPC)², (R² = 0.80, df = 47) and Y= 1.3 + 0.10 HAD (R² = 0.82, df = 47). Manual defoliation experiments conducted concurrently with these studies demonstrated that removal of 25 or 50% of the total number of leaves from sprayed plants did not significantly reduce yield, whereas removal of 75% of the leaves lowered yield by 34–53%. The relation between HAD and yield from manually defoliated plots was similar to that with nonsprayed plants that were defoliated by Septoria leaf spot. Measurements of HAD may be a better predictor of final yield than AUDPC because of its linear relation to yield.