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Ecology and Epidemiology

Relationships Among Population Levels of Pseudomonas syringae, Amount of Ice Nuclei, and Incidence of Blast of Dormant Flower Buds in Commercial Pear Orchards in Catalunya, Spain. E. Montesinos, Department of Crop Sciences, Polytechnic School of Girona, Polytechnic University of Catalunya, Avgda. Lluis Santal?, s/n, Giroua, Spain; P. Vilardell, Department of Crop Sciences, Polytechnic School of Girona, Polytechnic University of Catalunya, Avgda. Lluis Santal?, s/n, Giroua, Spain. Phytopathology 81:113-119. Accepted for publication 19 June 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-113.

Flower bud necrosis of commercial pear trees during winter dormancy is the most destructive phase of bacterial blast of pear caused by Pseudomonas syringae in the fruit tree-growing area of Catalunya, Spain. Forty percent of strains of P. syringae isolated from buds were ice nucleation active (INA) and 70% were phytopathogenic to pear. Thirty-nine percent were both INA and phytopathogenic. They were capable of developing blast of dormant buds in the absence of cold depending on the inoculum dose. However, cold temperatures increased disease incidence in inoculated flower buds from cut branches forced to bloom. Winter populations of P. syringae in dormant flower buds ranged from less than 10³ to 10⁸ colony-forming units (cfu) per gram fresh weight. Ice nuclei (i.n.) active at ?5 C were associated with populations of P. syringae during 1988 and 1989 and ranged from undetectable to about 10⁵ i.n./g fresh wt. Mean nucleation frequency was 3.2 ? 10³ cfu/ice nucleus. Mean nucleation temperatures (NT₅₀) of detached flower buds during tree dormancy ranged from ?2.7 to ?6 C and depended upon population levels of P. syringae and amount of ice nuclei. A significant relationship was found among the population levels of P. syringae, the amount of ice nuclei in dormant flower buds at the start of winter, and subsequent disease incidence measured as bud failure assessed in early March after several winter frosts. The threshold level producing a 5% disease incidence in six orchards and three cultivar types was 9.3 ? 10² cfu/bud or 1 i.n./bud, and the slope of the log dose-log response curve was 0.2.