In August 1998, severe leaf spot, resulting in extensive defoliation, was observed on cv. Gala apple trees in two orchards in eastern Tennessee. Symptoms were similar to those reported in Brazil for Glomerella leaf spot on Gala (1), and Glomerella cingulata (Stoneman) Spauld. & H. Schrenk was observed fruiting in lesions. Single-ascospore isolates were obtained by placing individual perithecia from different lesions on microscope slides in a drop of sterilized distilled water. Perithecia were crushed, and the spore and mycelial suspension was distributed on the surface of petri dishes containing acid-water agar (AWA). Petri dishes were incubated at 24°C in light for 24 h. Germinated ascospores were transferred to petri dishes containing AWA. Cultures were transferred to potato dextrose agar and grown at 24°C in light for 14 days to induce sporulation. Four isolates (TN-1A, TN-1B, TN-2A, and TN-2B) were selected for pathogenicity tests. Three virulent isolates from Brazil (FK6, R-11, and 2VGE) also were included. Trees of apple cvs. Gala and Golden Delicious were placed in humidity chambers before inoculation. After 24 h, shoots on three trees of each cultivar were sprayed with an aqueous spore suspension of each isolate (1× 105 spores per ml) and maintained at 100% relative humidity (RH) and ≈22°C for 2 days. Shoots on three trees sprayed with sterilized distilled water and maintained at 100% RH served as a control. All isolates from Brazil and isolates TN-1A and TN-1B caused symptoms characteristic of Glomerella leaf spot on both cultivars after 2 days. Five days after inoculation disease severity on each leaf was visually rated on a scale of 0 to 5 (where 0 = no lesions and 5 = 25 to 50% of leaf surface covered with lesions). FK6 and R-11 were the most aggressive isolates on both cultivars. 2VGE and TN-1B were the least aggressive isolates.
Reference: (1) T. B. Sutton and R.M. Sanhueza. Plant Dis. 82:267, 1998.
