COMPOSTED MUNICIPAL WASTE REDUCES INFECTION OF CITRUS SEEDLINGS BY PHYTOPHTHORA-NICOTIANAE

Most citrus nurseries and orchards in Florida are infested with Phytop hthora nicotianae, the causal agent of Phytophthora root rot. Although fungicides control the disease and increase seedling growth and tree yields, they are not always economically or environmentally sustainabl e. Amendment of citrus soils with composted municipal waste (CMW) may provide an alternative to fungicides for disease management. Citrus se edling growth decreased with increasing proportions of one CMW source, indicating the potential for phytotoxicity from soluble salts and ace tic acid when the proportion of CMW in soil exceeded 20% (vol/vol). Wh en a citrus soil was amended (20% vol/vol) with certain sources of CMW , the incidence of infection of 5-week-old susceptible citrus seedling s by P. nicotianae was reduced from 95% to as low as 5%. Addition of f resh CMW to two different citrus soils reduced colony growth of P. nic otianae after the fungus was incubated in the amended soil for 6 days and then recovered on PARPH selective medium. If CMW was stored for mo re than 3 months before amendment, the soil mixtures did not suppress colony development. In general, extracts of fresh CMW reduced mycelial growth in vitro; whereas suppressiveness of CMW from the same batch w as lost after storage. Acetic acid was detected at higher concentratio n in fresh CMW that suppressed colony growth than in CMW that lacked a ctivity. A species of Acremonium was isolated from another source of C MW that was suppressive to P. nicotianae. This fungus parasitized hyph ae of P. nicotianae in vitro. Thus, CMWs have the potential to at leas t temporarily suppress P. nicotianae through chemical and microbial ag ents depending on source and age of the CMW.