Occurrence and Nature of Ice Nucleation-Active Strains of Pseudomonas syringae on Apple and Peach Trees in Georgia. J. W. Olive, Graduate Student, Department of Plant Pathology, University of Georgia, Athens 30602. S. M. McCarter, Professor, Department of Plant Pathology, University of Georgia, Athens 30602. Plant Dis. 72:837-843. Accepted for publication 9 May 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0837.

Bacteria that nucleate ice crystal formation (INA) were isolated from 60 and 80% of tissue samples collected from each of two apple orchards at approximately monthly intervals from November 1984 until October 1985. Populations varied from 2 × 10² to > 1.0 × 10⁴ cfu/g of green tissue. In a peach orchard sampled similarly, INA bacteria were detected in 20% of the samples at populations of 5.5 × 10³ cfu/g and higher. Populations of INA bacteria ranging from 10² to > 4.0 × 10⁴ cfu/g of tissue were isolated from 13 of 24 apple flower samples collected in 1985. In 1986, four of 13 samples had populations of INA bacteria ranging from 10² to more than 3.9 × 10⁴ cfu/g. Only one of 13 peach flower samples collected in 1985 had detectable levels of INA bacteria, but levels of 1.4 × 10⁵ cfu/g or higher were detected in seven of eight similar samples collected in 1986. Although most of 286 representative INA strains from the apple and peach trees tested were Pseudomonas syringae pv. syringae, they were diverse in pathogenicity and virulence on green tomato fruit, green bean pods, and peach seedlings. About 61% of 140 INA strains from apple grew on casitone-yeast extract-glycerol (CYE) medium containing 200 ?g/ml of streptomycin sulfate, and resistance apparently was related to streptomycin use for fire blight or blister spot control. Less than 2% of 119 strains from peach trees grew when exposed to 200 ?g/ml of streptomycin. About 37% of the apple strains—but none of the peach strains—grew on CYE medium with 60 ?g/ml of copper.