THE ROLE OF MUSCOID INSECTS IN THE RAPID COLONIZATION OF JONATHAN APPLE FLOWERS WITH ERWINIA AMYLOVORA
SV THOMSON1, AC WAGNER1 and SC GOUK2
1Department of Biology, Utah State University, Logan, UT 84322-5305, USA;2 HortResearch, Ruakura Research Centre, Hamilton, New Zealand
Background and objectives
Epiphytic colonization of pear and apple flowers is a critical and limiting step in the initiation of fire blight infections. Splashing rain and insects have been implicated but the details of the primary infestation are not well understood. This study investigates the role of overwintering fire blight cankers and insects in the primary epiphytic colonization of apple flowers.
Materials and methods
Stigma imprints are a very simple and useful technique and often more sensitive than flower washing when used to detect the presence of Erwinia amylovora on flowers [1,2]. The sensitivity of the technique was evaluated by making imprints of the stigmas of Bartlett pear flowers after spray inoculating with a range of E. ;amylovora concentrations. The imprint technique was used daily during bloom for two seasons to monitor for the presence of E. ;amylovora in healthy flowers in a Jonathan apple orchard with numerous overwintering cankers. Yellow sticky traps were placed adjacent to cankers on Jonathan apple trees and changed weekly. Insects on these traps were identified to family and compared with those on traps mounted on adjacent cherry trees as controls. Insects caught in a malaise trap were removed daily and washed to check for the presence of E. ;amylovora.
Results and conclusions
The fire blight pathogen was detected with the stigma imprint technique on flowers sprayed with lower concentrations of E. ;amylovora when compared with the standard blossom washing technique. The stigma imprint technique was also a good tool for predicting future infections. Flowers were free of E. ;amylovora through full bloom on Jonathan apple flowers but then over a period of only 2-3 ;days, in both years, nearly every flower became colonized. This occurred without rain and coincided with a significant increase in the populations of muscoids (Muscidae, Calliphoridae, Anthomyiidae, Sarcophagidae plus Syrphidae) caught on yellow sticky traps adjacent to overwintering fire blight cankers. There was no correlated increases in populations of muscoids on control trees nor of other insect families on the traps near cankers. E. ;amylovora was also isolated simultaneously from bulk washes of insects caught in a malaise trap. Muscoid and syrphid flies are attracted to the ooze on overwintering cankers and their presence is correlated with the rapid colonization of flowers with E. ;amylovora in the absence of rain.
1. Thomson SV, 1992. Phytopathology 82, 1077.
2. Gouk SC, Hutchings SO, and Voyle MD. 1993. Proceedings 46th New Zealand Plant Protection Conference 1993, pp. 177-178.