Validating and Optimizing a Method for Detecting Phytophthora Species by Baiting Leachate from Arrays of Container Nursery Plants

Abstract

In 2014, Phytophthora root rot was identified as a serious problem in nursery plants grown for habitat restoration in California. To support efforts to produce restoration nursery stock free of Phytophthora species, we developed a standardized leachate baiting method that can be conducted by nursery staff. Each plant in an array of up to 42 container-grown nursery plants is irrigated six times at 15-min intervals with a standardized water volume. Irrigation leachate is collected in a specialized vessel containing a green (unripe) pear bait. Test sensitivity was assessed using arrays of Phytophthora-free plants containing varying numbers of plants from which Phytophthora had previously been baited. The percentage of tests with Phytophthora detections increased as the proportion of infested plants in the array increased. In tests of individual inoculated plants, Phytophthora detection from a given plant varied over time, especially for less susceptible hosts. This variability likely limits test sensitivity in arrays with few infested containers. Direct probability and Monte Carlo simulation models showed that testing 2 to 3 arrays per block and 20 to 40 plants per array provided the greatest increases in detection probability for blocks of 200 to 1000 plants in which 1% to 5% of the plants were infected. Random sampling had lower detection success than sampling biased to increase the odds of selecting infected plants. Results of extensive testing at a case study nursery were consistent with model predictions. The leachate baiting method has wide applicability for detecting and identifying Phytophthora species in nursery stock.