Journal of Arboriculture 31(5): September 2005 235 THE POTENTIAL OF MULCH TO TRANSMIT THREE TREE PATHOGENS By Karel A. Jacobs Abstract. Field transmission of Sphaeropsis tip blight, Botryos- phaeria canker, and Armillaria root rot was evaluated in a 6-year study of 30 saplings each of Austrian pine (Pinus nigra), eastern redbud (Cercis canadensis), and red oak (Quercus rubra) mulched with diseased needles, bark chips, and wood chips collected from mature trees. Half of the saplings were mulched with fresh mulch materials; half with materials first heated to 60°C (140°F). Sphaeropsis tip blight was the only disease that developed during the study, and pines mulched with heat-treated materials devel- oped significantly fewer (P ≤ 0.001) blighted tips than those mulched with fresh materials (6.8% versus 15.1%). Naturally occurring inoculum of S. sapinea and B. ribis were highly tolerant of heat in laboratory tests and remained viable after 6 weeks and 48 h exposure, respectively, to 55°C (131°F). Armillaria gallica mycelium and rhizomorphs did not withstand temperatures above 35°C (95°F) and 37°C (98.6°F), respectively. The heated mulch treatment was associated with a significant (P ≤ 0.001) growth boost in redbuds and oaks during the first two growing seasons. Redbuds averaged two times more height and diameter increment than plants mulched with fresh bark and wood chips. Heating diseased mulch to 60°C (140°F) diminished the threat of tip blight transmission and likely killed all forms of the pathogens. Key Words. Armillaria gallica; Botryosphaeria ribis; Cercis canadensis; composting; disease transmission; fungi; heat inactivation; pasteurization; Pinus nigra; Quercus rubra; Sphaeropsis sapinea. The practice of mulching trees and shrubs with wood chips and other organic materials is commonplace in landscapes. In addition to aesthetic benefits, organic mulches benefit woody plants by helping to regulate soil temperatures and moisture levels throughout the year, increasing root surface area and tree growth and establishment rates (Watson 1988; Green and Watson 1989; Greenly and Rakow 1995; Smith et al. 2000; Herms et al. 2001). Mulched shrubs develop a root:shoot ratio more than twice that of unmulched shrubs (Wood et al. 1994). Significant gains in soil organic matter content and biomass of beneficial soil microbes in the rhizosphere and improved uptake and availability of N (Lloyd et al. 2001) and P (Smith et al. 2000) have been measured when trees are mulched. Additional indirect effects of mulching include greater tolerance of environmental stresses, including drought (Wood et al. 1994; Hoitink et al. 2001). Yet not all mulches are alike, as discussed by Herms et al. (2001). There is a general concern among tree pathologists and arborists that fresh organic mulches may harbor patho- gens and, perhaps, transmit disease. Herms et al. (2001) and Hoitink and Changa (2004) recommend avoiding fresh wood chips and similar mulch materials in favor of partially composted (heated) materials in order to reduce problems caused by, among other things, pathogenic microbes and unfavorable C:N ratios. To my knowledge, there are no published reports quantifying tree disease transmission in the landscape originating from mulches. There is anecdotal evidence of this for Armillaria root rot of Rosa spp. (Perez Sierra 2004), albeit transmission was apparently negligible because the fungus dried out rapidly in wood chips. In contrast, transmission of disease through mulch has been documented in greenhouse and pot studies. For example, Verticillium dahliae was transmitted to eggplant, maple, ash, and redbud seedlings that were planted in potting mix amended with naturally infected maple wood chips (Ash 1999; Foreman et al. 2003). Likewise, data describing extended survival of tree pathogens in mulch are accumulating. Verticillium dahliae, pinewood nematode (Bursaphelenchus xylophilus), and the canker-causing fungus Thyronectira austroamericana were found to survive periods of at least 17, 80, and 98 weeks, respectively, in wood chips in the field (Panesar et al.1994; Foreman et al. 2002; Koski and Jacobi 2004). Panesar et al. (1994) noted that the size and moisture content of wood chips were important determinants of pinewood nematode longevity. Additional reports describe mulch-induced changes in edaphic conditions that have proven conducive to soilborne pathogens (Faber et al. 1995; Hoitink and Changa 2004). Mulch recommendations now suggest that wood chips and other organic materials derived from diseased trees not be used as mulch immediately, but only after partial composting, or curing, to rid the materials of pathogens (Herms et al. 2001; Hoitink et al. 2001). Thermophillic composting of yard waste is believed to be effective at eliminating weed seeds, insects, and pathogen spores so long as piles reach temperatures of at least 55°C (131°F) and are mixed thoroughly (Richard 1992; Bollen 1993). Ash (1999) found in her studies of V. dahliae that within a week of incorporating infested maple wood chips into compost piles, the pathogen could not be recovered. Similarly, Jacobs (1995) found that Botryosphaeria dothidea could not be recovered from artificially inoculated redbud twigs 48 h after incorporation into compost piles. However, ©2005 International Society of Arboriculture
September 2005
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