Investigations on some of the mechanisms by which bioenhanced mulches can suppress Phytophthora root rot of avocado

In previous studies, bioenhanced yardwaste (a mulch colonized with biocontrol agents) showed an ability to reduce or suppress populations of Phytophthora cinnamomi , resulting in decreased root rot of avocado trees. Some of the possible mechanisms of suppression were investigated by collecting avocado field soils from underneath bioenhanced and non-bioenhanced mulches at intervals of 12 months for a period of four years. Soils that originated from underneath mulch treatments in the greenhouse were also used in this study. Soils treated with mulches for longer periods accumulated higher levels of organic matter. Sudangrass resulted in more accumulation of organic matter than did yardwaste. Bioassays using tension plates and soil columns indicated that organic matter levels were correlated positively with the ability of mulches to trap zoospores and reduce the disease incidence in Persea indica seedlings placed 35 mm from a source of zoospore-producing inoculum. Yardwaste mulches slightly enhanced sporangia production under field and greenhouse conditions. However an average of 20% of the sporangia produced in the yardwaste treatments resulted in aborted sporangia which is sporangia that did not differentiate or release zoospores. Only 9% of the sporangia aborted in the control treatment. Hyphal lysis could not be correlated to disease suppression, since it exhibited high variability. Yardwaste infested with Gliocladium virens increased significantly the levels of CO 2 under field conditions. All mulch treatments infested or not with biocontrol agents increased total microbial activity, as measured by fluorescein diacetate, after 24 months of treatment. Soil dilution plate assays indicated that soils with high microbial activity can reduce the survivability of P. cinnamomi zoospores.