Evaluation location: Florida, United States
Study Name: Alternaria dauci tolerance evaluation Experiment Type: Field Exp. Design: RB Exp. Location: Sanford Soil class/texture: Myakka fine sand. Investigator: James Strandberg. Comment: Seeds of 'Apollo' were planted for infection rows which consisted of the outside rows and every third row within the plot area artificially inoculated on two occasions with spores of Alternaria dauci (Strandberg, 1987). Rows were 280 ft. long on 30 inch centers. Plants were hand-thinned to 1-2 inches per row. Plant Introduction accessions were hand planted 1-2 days later and also thinned as needed. Thus, accessions were infected by natural inoculum spread from the adjacent infection rows. Each accession plot consisted of a single row 15 ft. long. After disease symptoms appeared, accessions sampled weekly by collecting 20 leaves at random from each plot and compared with an Alternaria leaf blight damage key and classified as to the percent of leaf area damaged (Strandberg 1988). The average percent leaf area damaged (LAD) on each 20-leaf sample was calculated weekly. Subjective rating from 0 = no visible disease damage; 1 = few lesions mostly on older leaves; 2 = numerous lesions on older leaves, scattered on younger leaves; 3 = abundant lesions, some defoliation and leaf damage; 4 = abundant lesions, moderate leaf damage, defoliation; 5 = severe leaf damage and defoliation. Plots clearly judged to be intermediate between two classes were assigned fractional values in increments of 0.5 units (a plot intermediate between 3 and 4 was assigned a 3.5). For some very susceptible lines, subjective ratings were made before the end of the experiment because the plants were rapidly being killed by the disease. A special category called persistent leaves was used to describe lines where numerous leaves remained alive and persisted for several days or weeks in spite of high disease damage ratings which may express useful levels of disease tolerance.