Monitoring the epiphytic population of Erwinia amylovora on pear with a selective medium.
A selective medium for the growth of Erwinia spp. but inhibiting that of most other micro-organisms was prepared by adding (in the order listed) to 1l distilled water: mannitol (10g), nicotinic acid (0.5g), L-asparagine (3g), K2HPO4 (2g), MgSO4.7H2O (0.2g), sodium taurocholate (2.5g), tergitol anionic 7 (0.1 ml), nitrilotriacetic acid (10 ml 2% aqueous sol.), bromothymol blue (9ml, 0.5%) and neutral red (2.5 ml, 0.5%). The agar medium was adjusted to pH7.2-7.3 and autoclaved at 121 deg C for 15 min after which actidione (50 mg) and thallium nitrate (1.75 ml 1% sol.) were added. Each Erwinia sp. tested could be distinguished on the medium by its colony morphology and colour. The few other bacteria which also grew on it were blue or green in contrast to the red to orange characteristic of the E. spp. The substitution of sorbitol (10g) for mannitol was found later to restrict growth of E. herbicola.E. amylovora was present as an epiphyte on pear flowers and other plant parts in spring but was not detected in leaf or flower buds during winter. The pathogen was found in flowers and on the surface of cankers before flower infection was evident. The epiphytic population varied among orchards and trees, and was related to disease severity. Each flower tested during an epidemic contained populations of 104-105 cells/flower, although only a few flowers (100/tree) became infected. Neither the pathogen population nor disease incidence was affected by the presence of saprophytic bacteria in flowers, while 4-33% of flowers were colonized by E. amylovora alone. Surface populations of 10-105 cells/insect were carried by Pegamya and Minettia. The first naturally occurring, streptomycin resistant str. of E. amylovora was found in a severely diseased orchard and 200 mu g/ml streptomycin did not affect its growth.