MICROBIAL SUPPRESSION OF IN-VITRO GROWTH OF PYTHIUM-ULTIMUM AND DISEASE INCIDENCE IN RELATION TO SOIL C AND N AVAILABILITY

Experiments were designed to examine effects of the soil microbial com munity, C and N availability on in vitro growth of Pythium ultimum and its infection of cotton seedlings by manipulating the stage of cellul ose decomposition, size and activity of microbial populations, and N a vailability. In comparison to the untreated control (CONT), cellulose addition alone (CELL) reduced soil nitrate by 35-80 fold, but had no s ignificant effect on soil ammonium. Soil microbial biomass C (SMBC) in creased over 2 fold in 14 days following cellulose addition, but signi ficantly decreased in the following 10 days due to N limitation. Addit ion of both cellulose and N (NCELL) resulted in sustained SMBC for 24 days and significantly reduced in vitro P. ultimum growth and disease incidence. In vitro growth of P. ultimum and disease severity were con sistently reduced in the order: CONT > CELL > NCELL. In vitro growth o f P. ultimum was lower in soils previously incubated for 24 days than in those incubated for 14 days, and was most closely correlated to cum ulative soil CO2 evolution (CO2T). Correlations between P. ultimum gro wth rates and NO3-N or total available N were substantial (p < 0.05), but much less significant than those between the growth rates and SMBC , microbial activity measured as CO2 evolution rates or CO2T (p < 0.00 01). Addition of available N (NH4NO3) and C (glucose) just before the assays did not increase the in vitro growth of P. ultimum or disease s everity on cotton seedlings, suggesting that time-dependent microbial processes or microbial metabolites significantly contributed to suppre ssion of P. ultimum growth.