Evaluation of selective media and bait methods for estimating <em> Phytophthora cactorum</em> in apple orchard soils.

Soil plating, with a specially devised selective medium, gave estimates of Phytophthora cactorum in an East Malling Research Station apple orchard soil up to three times those obtained by dilution and baiting with apple seedlings or cotyledons and using the most probable number analysis. When the same techniques were applied to a range of soils from apple orchards in south-east England with a history of P. cactorum disease the plating method failed in most instances, mainly because Phytium spp. rapidly swamped the plates. The dilution/baiting method was applicable to all soils though there was a tendency to underestimate because of anomalous results at lower soil dilutions. Oospores were the only propagules which could be confirmed as sources of P. cactorum colonies on soil isolation plates. [ABSTRACT FROM AUTHOR]