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BIOLOGICAL CONTROL OF FIRE BLIGHT USING ERWINIA HERBICOLA EH252 AND PSEUDOMONAS FLUORESCENS A506 SEPARATELY OR IN COMBINATION
In the hope of reducing this variation, we used a combination of biological control agents.
For this study we chose two of the most effective biocontrol agents: E. herbicola Eh252 and P. fluorescens A506 (Beer et al., 1984; Johnson et al., 1992; Lindow, 1982, 1992; McLaughlin and Roberts, 1992; Stockwell et al., 1991; Vanneste and Yu, 1990, 1994; Vanneste et al., 1995). These two strains offer the advantage of inhibiting Erwinia amylovora the fire blight pathogen, by different mechanisms.
Eh252 produces an antibacterial compound which inhibits E. amylovora (Vanneste et al., 1990, 1992), whereas A506 reduces fire blight incidence by displacing populations of the pathogen following competition for nutrients and space (Wilson and Lindow, 1993).
The level of control provided by Eh252 or A506 is related to their level of population on flowers, it was, therefore, essential to determine whether these two strains inhibit each other on flowers.
We compared on apple flowers the population dynamics of Eh252 and A506 when sprayed together or separately.
Since E. amylovora multiplies on the stigma and most probably enters the plant tissue by the nectaries (Vanneste, 1995), we followed populations on the pistils and on the rest of the flower separately.
The day following application of the biocontrol agents, five flowers per day and per treatment were dissected; pistils and nectaries were washed separately in a 10 mM MgSO4 solution.
This was continued daily for a further seven days.
Serial dilutions were sprayed on selective media allowing determination of the populations of Eh252 and A506 on pistils and nectaries.
Populations of A506 or Eh252 on pistils or nectaries were similar whether they were sprayed alone or in combination (Fig. 1), indicating that on flowers these two strains are not antagonistic to each other.
Similar results were obtained on Asian pear flowers.
The challenge experiments were conducted in two research orchards: apple (Malus pumila cv. Granny Smith) orchard in Pukekohe, and Asian pear (Pyrus pyrifolia cv.
Kosui) orchard in Blands Hamilton, both in New Zealand.
Bacterial strains of Eh252 and A506 were grown separately at 25°C in Roux bottle on Luria medium for 24 hr and were diluted with deionized water before application (Table 1). On apple, each treatment was applied to at least one tree randomly selected in each of five blocks.
Bacterial suspensions were sprayed to run off onto 9 to 26 newly open blossom clusters, with an average of 16 clusters per tree.
In some cases, to increase the number of clusters treated, two trees in the same block were used for the same treatment; this has been taken into consideration in the data analysis.
On Asian pear, each treatment was applied to one tree randomly selected in each of three blocks.
Bacterial suspensions were