OCCURRENCE OF INDOLE-3-ACETIC ACID-PRODUCING BACTERIA ON PEAR TREES AND THEIR ASSOCIATION WITH FRUIT RUSSET

A relatively high percentage of epiphytic bacteria on pear leaf and fr uit surfaces had the ability to produce indole-3-acetic acid (IAA) in culture media supplemented with tryptophan. While over 50% of the stra ins produced at least small amounts of IAA in culture, about 25% of th e strains exhibited high IAA production as evidenced by both colorimet ric and highperformance liquid chromatography analysis of culture supe rnatants. A majority of the strains that produced high amounts of IAA were identified as Erwinia herbicola (Pantoea agglomerans), while some strains of Pseudomonas syringae, Pseudomonas viridiflava, Pseudomonas fluorescens, Pseudomonas putida and Rahnella aquaticus that produced high amounts of IAA also were found on pear. Fruit russeting was signi ficantly increased in 39 out of 46 trials over an 8-year period in whi ch IAA-producing bacteria were applied to trees compared with control trees. A linear relationship was observed between fruit russet severit y and the logarithm of the population size of different IAA-producing bacteria on trees in the 30 days after inoculation, when normalized fo r the amount of IAA produced by each strain in culture. On average, th e severity of fruit russet was only about 77% that on control trees wh en trees were treated at the time of bloom with Pseudomonas fluorescen s strain A506, which does not produce IAA. Both total bacterial popula tions on pear in the 30-day period following full bloom and fruit russ et severity varied greatly from year to year and in different commerci al orchards over a 10-year period. There was a strong linear correlati on between the logarithm of total bacterial population sizes and fruit russet severity.