Microbial suppression of in vitro growth of Pythium ultimum and disease incidence in relation to soil C and N availability

Abstract

Experiments were designed to examine effects of the soil microbial community, C and N availability on in vitro growth of Pythium ultimum and its infection of cotton seedlings by manipulating the stage of cellulose decomposition, size and activity of microbial populations, and N availability. In comparison to the untreated control (CONT), cellulose addition alone (CELL) reduced soil nitrate by 35‐80 fold, but had no significant effect on soil ammonium. Soil microbial biomass C (SMBC) increased over 2 fold in 14 days following cellulose addition, but significantly decreased in the following 10 days due to N limitation. Addition of both cellulose and N (NCELL) resulted in sustained SMBC for 24 days and significantly reducedin vitro P. ultimumgrowth and disease incidence. In vitro growth of P. ultimumand disease severity were consistently reduced in the order: CONT > CELL > NCELL. In vitro growth of P. ultimum was lower in soils previously incubated for 24 days than in those incubated for 14 days, and was most closely correlated to cumulative soil CO2 evolution (CO2T). Correlations between P. ultimum growth 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<0.0001). Addition of available N (NH4NO3) and C (glucose) just before the assays did not increase the in vitro growth of P. ultimum or disease severity on cotton seedlings, suggesting that time-dependent microbial processes or microbial metabolites significantly contributed to suppression ofP. ultimum growth. Abbreviations: CELL ‐ cellulose treatment, CO2R ‐ CO 2 evolution rate, CO2T ‐ cumulative CO2 evolution, CONT ‐ controls, GRP ‐in vitro growth rate of P. ultimum, NCELL ‐ cellulose plus N treatment, ORIG ‐ original soils (pre-incubated), SAFS ‐ Sustainable Agriculture Farming Systems project, SMBC ‐ soil microbial biomass C, SMBN ‐ soil microbial biomass N, SOC ‐ soluble organic C, TAN ‐ total available N.