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Cellulase Activity as a Mechanism for Suppression of Phytophthora Root Rot in Mulches

First and fourth authors: Department of Plant Pathology, North Carolina State University, Raleigh 27695; second author: Department of Plant Pathology, North Carolina State University, Mountain Horticultural Crops Research and Extension Center, Mills River 28759; and third author: Department of Soil Science, North Carolina State University, Raleigh 27695.

Wood-based mulches are used in avocado production and are being tested on Fraser fir for reduction of Phytophthora root rot, caused by Phytophthora cinnamomi. Research with avocado has suggested a role of microbial cellulase enzymes in pathogen suppression through effects on the cellulosic cell walls of Phytophthora. This work was conducted to determine whether cellulase activity could account for disease suppression in mulch systems. A standard curve was developed to correlate cellulase activity in mulches with concentrations of a cellulase product. Based on this curve, cellulase activity in mulch samples was equivalent to a cellulase enzyme concentration of 25 U ml^–1 or greater of product. Sustained exposure of P. cinnamomi to cellulase at 10 to 50 U ml^–1 significantly reduced sporangia production, but biomass was only reduced with concentrations over 100 U ml^–1. In a lupine bioassay, cellulase was applied to infested soil at 100 or 1,000 U ml^–1 with three timings. Cellulase activity diminished by 47% between 1 and 15 days after application. Cellulase applied at 100 U ml^–1 2 weeks before planting yielded activity of 20.08 μmol glucose equivalents per gram of soil water (GE g^–1 aq) at planting, a level equivalent to mulch samples. Cellulase activity at planting ranged from 3.35 to 48.67 μmol GE g^–1 aq, but no treatment significantly affected disease progress. Based on in vitro assays, cellulase activity in mulch was sufficient to impair sporangia production of P. cinnamomi, but not always sufficient to impact vegetative biomass.