Temporal analysis of charcoal root rot in forest nurseries under different pathogen inoculum densities and soil moisture content

The progress of charcoal root rot (CRR) disease in Pinus radiata seedlings caused by Macrophomina phaseolina was evaluated in greenhouse and field trials. In one greenhouse trial P. radiata was sown in a sandy soil with pathogen inoculum densities (ID) ranging from 25 to 1000 cfu.g-1. After two months, the seedlings were established under three soil moisture content (SMC) levels corresponding to 100%, 75% and 50% of field capacity (FC). In one field trial, P. radiata was sown on microplots infested with ID of 50 to 250 cfu.g-1. In the greenhouse trial the disease incidence was related with the pathogen ID and SMC. At 100% FC, the seedlings remained asymptomatic until the experiment ended, however M. phaseolina was able to infect the roots. In the field trial, the disease progress curves were similar to those of the greenhouse trial in response to ID and SMC but the plant mortality due to CRR incidence did not exceed 50%. The monomolecular model was the one that best fitted the disease progress curve data for both trials and most treatments. Knowledge of the existing ID of M. phaseolina in the soil, combined with information on SMC, may be important for predicting CRR epidemics in P. radiata nurseries.