Effect of chitin waste-based composts produced by two-phase composting on two oomycete plant pathogens

Biphasic composts were prepared by first mixing peat moss and sawdust with a nitrogen-rich biomass such as chitinous waste or cow manure and compostin g them until termination of the thermophilic phase. These partially stabili zed composts were then amended with shrimp waste inducing a second thermoph ilic phase. Filter-sterilized water extracts obtained from two mature bipha sic composts (SP2W2+S and MPW+S) reduced the growth of two oomycete plant p athogens, Phytophthora fragariae var. rubi and Pythium ultimum. Both SP2W2S and MPW+S composts significantly reduce the incidence of cucumber damping -off caused by Pythium ultimum as compared to a commercial brand of compost made from shrimp waste and peat moss. Hydrolysis products of chitin were u nlikely to be responsible for growth inhibition since no oligomeric forms o f chitin were detected in SP2W2+S. The shrimp waste amendment carried out a fter the first thermophilic phase modified the microbial populations of bip hasic composts. Following the amendment, the proportion of branched-chain m icrobial fatty acids typical of Gram-positive bacteria increased considerab ly suggesting that this group of bacteria became more prevalent within the total microbial population. These data suggests that the two-phase composti ng process promotes the proliferation of Gram-positive bacteria antagonisti c to oomycete plant pathogens.