Two immunodiagnostic detection assay procedures were compared with two conventional assays for their sensitivity in detecting propagules of Pythium ultimum var. sporangiiferum , Pythium Group F, Phytophthora cactorum and P. cryptogea in dilution series in sterile distilled water. The most sensitive assay for all four species was the zoospore trapping immunoassay (ZTI). Conventional membrane filtration-dilution plating gave similar results to ZTI with the two Phytophthora spp., but was less sensitive in Pythium detection. Immunodiagnostic dipstick assays and conventional bait tests showed similar sensitivities in the dilution series, and were generally about two orders of magnitude less sensitive than ZTI. The four techniques were also compared for their detection efficacy with water samples collected from horticultural nurseries and in in situ tests of infected root zones of Chamaecyparis , tomato and Chrysanthemum . In these comparisons, ZTI was again the most sensitive test for water samples, although membrane filtration-dilution plating proved to be a more consistent test. Dipstick and baiting assays were the best techniques for in situ testing, and dipsticks provided epidemiologically valuable, quantitative data on pathogen propagule numbers.

A detached leaf assay was developed to determine the pathogenicity of Pythium isolates to cut-flower chrysanthemum roots. Leaves from young plants were excised and inoculated by insertion of a plug of mycelium into a slit cut in the excised petiole. After incubation leaves were assessed for presence and extent of necrosis. Necrosis indicated pathogenicity and was consistently confirmed by comparisons with whole plant inoculations. The rate of necrosis spread also gave some indication of virulence. Isolates of Pythium sylvaticum, P. ultimum and HS group were the most virulent, with a mean rate of spread of 14AE6 mm per day, significantly (P < 0AE05) faster than the mean rate of spread, 1AE6 mm per day, of less virulent isolates. Less virulent isolates included P. irregulare, P. oligandrum and P. aphanidermatum. The latter was unexpected, as P. aphanidermatum is an important species in pythium root rot epidemics in chrysanthemums elsewhere. The value of the detached leaf assay for screening large numbers of isolates was demonstrated in a survey of isolates from clinic samples from chrysanthemum nurseries and in a series of dilution-plating experiments looking at numbers of Pythium propagules in commercial chrysanthemum beds showing root rot. In the survey, the predominant pathogenic species was identified as P. sylvaticum and the most likely source of infection was contaminated soil as opposed to blocking media or irrigation water, whilst in soil colonization studies the use of detached leaf assays demonstrated a relationship between pathogenic inoculum concentration in soil and the expression of root rot symptoms.