Potentiation of plant defenses with organic soil amendments

As part of a collaborative research effort, my goal was to understand the processes by which organic soil amendments derived from paper mill residuals (PMR) influence plant disease. A series of field experiments examined the effects of annual additions of non-composted PMR, PMR composted alone (PMRC), or PMR composted with bark (PMRBC) on soilborne and foliar plant diseases compared to a non-amended control. As expected, all amendments effectively reduced symptoms of Pythium damping-off of cucumber caused by Pythium ultimum and common root rot of snap bean caused by P. ultimum and Aphanomyces euteiches. However, only the PMRC amendments consistently suppressed foliar and pod symptoms of brown spot of snap bean caused by Pseudomonas syringae pv. syringae and foliar symptoms of angular leaf spot of cucumber caused by P. syringae pv. lachrymans. Snap beans grown on PMRC-amended soils also exhibited milder symptoms of anthracnose caused by Colletotrichum lindemuthianum in a greenhouse experiment.;In growth chamber experiments, Arabidopsis thaliana and tomato plants grown in stored PMRC- or PMRBC-amended soil collected from field experiments exhibited reduced symptoms of bacterial speck when inoculated with P. syringae pv. tomato than those plants grown on non-amended soil or PMR-amended soil. The stored PMRC- and PMRBC-amended soils consistently suppressed bacterial speck of Arabidopsis in consecutive disease assays by 34% to 65% for nearly 18 months before losing activity. The reduction of bacterial speck symptoms of Arabidopsis was associated with lower in planta bacterial titers and increased expression of pathogenesis-related genes prior to inoculation. An Arabidopsis npr1 mutant and NahG transgenic line, both of which disrupt systemic acquired resistance (SAR), failed to respond to PMRC- and PMRBC-amended soils. These findings indicate that the foliar disease suppression associated with the composted PMR amendments is an induced form of resistance exhibiting molecular features characteristic of SAR. The exact identity of the PMR elicitor(s) is unknown, but was disrupted by autoclaving. Of the field soils re-amended in 2000 and 2001, only the PMRC-amended soil from 2000 exhibited foliar disease suppression activity, while all amended soils were effective at suppressing diseases caused by soilborne oomycetes in field and greenhouse experiments.