EVALUATION OF PYTHIUM-NUNN AS A POTENTIAL BIOCONTROL AGENT AGAINST PHYTOPHTHORA ROOT ROTS OF AZALEA AND SWEET-ORANGE

Pythium nunn parasitized the hyphae, sporangia, chlamydospores, and sexual organs of five isolates of P. cinnamomi, P. citrophthora, and P. parasitica in vitro, and caused inhibition of mycelial growth of these isolates. Population densities of P. nunn in a peat/sand mix, monitored up to 8 wk, declined gradually unless 1% ground rolled oats were added to the mix at 2 wk. Population densities of all three Phytophthora spp. also increased after 1% ground rolled oats were added. Population densities of P. cinnamomi, P. citrophthora, and one isolate of P. parasitica in oat-amended treatments were reduced in the presence of P. nunn, but no reduction in population densities of the other isolate of P. parasitica occurred in the presence of P. nunn, with or without oats. The effectiveness of P. nunn in suppressing root rot of azalea (Rhododendron spp.) caused by P. cinnamomi or P. parasitica, and root rot of sweet orange (Citrus sinensis) caused by P. parasitica, was evaluated in the peat/sand mix amended with 1% ground rolled oats in greenhouse tests. P. nunn at 300 propagules per gram did not suppress azalea or sweet orange root rot. At 1,000 propagules per gram, it significantly suppressed sweet orange root rot caused by P. parasitica. P. nunn did not affect the growth of azalea but slightly reduced sweet orange seedling growth.