URBAN AND AGRICULTURAL WASTES FOR USE AS MULCHES ON AVOCADO AND CITRUS AND FOR DELIVERY OF MICROBIAL BIOCONTROL AGENTS

Urban and agricultural waste products generally available to avocado a nd citrus growers in southern California were analyzed for their suita bility for use as bioenhanced mulches on citrus and avocado. Of the mu lches tested only yard waste (consisting of wood chips, grass and leav es), rice hulls and rice hulls-and-paper were not harmful to any growt h parameter of citrus or avocado and were also adequate substrates for the growth of three biocontrol agents: Trichoderma harzianum, Gliocla dium virens, and Pseudomonas fluorescens. Several mulches such as mill ed peanut hulls, milled almond hulls, chicken manure, a horse/cow manu re mixture, cow manure and alfalfa hay were poor substrates for growth of the biocontrol agents and were damaging to at least one growth par ameter of avocado and citrus. These mulches released toxic amounts of ammonia upon degradation, some in excess of 1000 mu g NH3-1 dry wt. Th e percentage of healthy citrus roots, percentage of healthy avocado ro ots and growth of T. harzianum and P. fluorescens were negatively corr elated with both ammonia evolved and total nitrogen content of the mul ches. Citrus grew better with mulches having lower carbon/nitrogen rat ios than did avocado; organic matter and organic matter/nitrogen ratio were negatively correlated with citrus shoot weight and the percentag e of healthy citrus roots but were not negatively correlated with avoc ado growth parameters. Growth of all three biocontrol agents was posit ively correlated with the organic matter/nitrogen ratio and negatively correlated with the pH of the mulches. Growth of G. virens was negati vely affected by the sodium concentration of the mulches. For the vari ous mulches, the percentage of healthy citrus roots, percentage of hea lthy avocado roots, avocado root weight, avocado height increase and a vocado shoot weight were positively correlated with the growth of P. f luorescens. In addition, the growth of T. harzianum on mulches was cor related positively with the percentage of healthy avocado roots mulche d with the same material. This indicates that mulch characteristics wh ich favour healthy roots of citrus and avocado also favour the growth of P. fluorescens and T. harzianum.