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POPULATION DYNAMICS OF ERWINIA AMYLOVORA AND A BIOLOGICAL CONTROL AGENT, ERWINIA HERBICOLA, ON APPLE BLOSSOM PARTS
The trees then were transferred to a controlled environment chamber maintained at 22°C and 75 to 80% relative humidity. Erwinia herbicola, strain Eh 252, was grown in nutrient-yeast-extract-glucose broth, centrifuged, resuspended to 108 colony forming units (cfu)/ml, and sprayed on blossoms with a compressed-air-powered atomizer.
Other blossoms were sprayed with sterile water as a check.
After 24 hours, all blossoms were sprayed with a suspension of E. amylovora, strain Ea 273 Rp (resistant to rifampicin), prepared to 107 cfu/ml.
Blossom samples were taken two hours after spraying, when the blossom had dried.
In the first experiments, E. amylovora was used alone, with no E. herbicola treatment In those experiments, the E. amylovora concentation was 108 cfu/ml.
Six to eight replicate blossoms were used per treatment.
After sampling, blossoms were dissected and the parts ground in buffer and plated on nutrient medium.
The stamens were removed first.
Next, in most experiments, the stigmas were removed by cutting off the top 2 mm of the pistil.
Then the styles were removed just below the point at which they were fused.
In some experiments, the stigmas were not separated from the styles, in which case this portion was referred to as the "pistil". In one experiment, the styles were further dissected into halves.
The half at the stigma end is called the "upper style", and the half at the calyx end is called the "lower style".
In the first experiment, only E. amylovora was applied.
The pistils, stamens, and calyx were plated 2 hours, 24 hours, and 4 days after spraying.
During the first 24 hours, pistil populations increased 100-fold, but stamen populations remained constant, and calyx populations dropped.
After four days, the stamen populations were still unchanged, and pistil populations were no higher than they had been after 24 hours.
However, the calyx populations had risen five orders of magnitude, and symptoms were visible in about half of the blossoms.
In the second experiment, the pistils were dissected into stigmas, upper styles, and lower styles.
As in the first experiment, only E. amylovora was applied, but sampling times were 0, 1, 2, and 3 days.
The stigmas were colonized rapidly in the first 24 hours; they were clearly the site of the rapid multiplication as in the first experiment, since style populations did not begin to rise until 2 or 3 days after inoculation.
By 3 days, populations of upper styles ranged