Articles
RELATIVE MERITS OF INDEXING NURSERY STOCK FOR THE PURPOSE OF THE FIREBLIGHT PATHOGEN
Basically the procedure utilizes evidence developed that Erwinia amylovora may overwinter in lateral and terminal buds of apple.
Secondly, the procedure relies on a selective medium developed by Crosse and Goodman for the positive identification of Erwinia amylovora.
Essentially the medium consists of 160 g sucrose, 12 g nutrient agar, 0. 8 ml crystal violet (in 0. 1% absolute ethanol) and 20 ml of 0. 1% cycloheximide in 380 ml of distilled water.
Basal and terminal buds from the budstick are macerated in nutrient broth and permitted to incubate for 4 hours at 28 C. Tripicate plates of nutrient yeast-glucose agar (nutrient agar 1%, yeast extract 0. 5%, and glucose 1%) are inoculated with 0. 1 ml of the bud homogenate which is spread over the agar surface.
Generally only two bacterial species persist in numbers in the medium and these are Erwinia amylovora and E. herbicola.
The latter grows more rapidly and the colonies have smooth surfaces.
E. amylovora colonies however, although somewhat smaller, will, after 60–72 hours of incubation at 28 C, reveal distinct cratering on their surfaces.
This cratering effect is exclusively a feature of E. amylovora on the Crosse-Goodman medium.
Examination of the plates with the aid of a dissecting microscope and with oblique lighting clearly reveal these distinctive craters.
Furthermore, plating efficiency of the medium is approximately 98%.