INVESTIGATIONS ON SOME OF THE MECHANISMS BY WHICH BIOENHANCED MULCHESCAN SUPPRESS PHYTOPHTHORA ROOT-ROT OF AVOCADO

In previous studies, bioenhanced yardwaste (a mulch colonized with bio control agents) showed an ability to reduce or suppress populations of Phytophthora cinnamomi, resulting in decreased root rot of avocado tr ees. 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 yea rs. Soils that originated from underneath mulch treatments in the gree nhouse were also used in this study. Soils treated with mulches for lo nger periods accumulated higher levels of organic matter. Sudangrass r esulted in more accumulation of organic matter than did yardwaste. Bio assays using tension plates and soil columns indicated that organic ma tter levels were correlated positively with the ability of mulches to trap zoospores and reduce the disease incidence in Persea indica seedl ings placed 35 mm from a source of zoospore-producing inoculum. Yardwa ste mulches slightly enhanced sporangia production under field and gre enhouse conditions. However an average of 20% of the sporangia produce d in the yardwaste treatments resulted in aborted sporangia which is s porangia that did not differentiate or release zoospores. Only 9% of t he sporangia aborted in the control treatment. Hyphal lysis could not be correlated to disease suppression, since it exhibited high variabil ity. Yardwaste infested with Gliocladium vir ens increased significant ly the levels of CO2 under field conditions. All mulch treatments infe sted or not with biocontrol agents increased total microbial activity, as measured by fluorescein diacetate, after 24 months of treatment. S oil dilution plate assays indicated that soils with high microbial act ivity can reduce the survivability of P. cinnamomi zoospores.