236 Jacobs: Potential of Mulch to Transmit Pathogens improper composting may not fully eliminate pathogens from wood chips, as revealed by Foreman et al. (2003), studying V. dahliae. The objective of this study was to determine whether three chronic tree diseases could be transmitted, under field conditions, through mulch comprised of naturally infected bark and wood chips, needles, and shoot tips. A heat treatment of the mulch was tested as a possible means of reducing or eliminating the threat of transmission by killing the pathogens. Finally, heat sensitivity and inactivation temperatures of the three pathogens were determined in vitro to help guide recommendations for eliminating pathogens in mulch. MATERIALS AND METHODS Disease Transmission Study Thirty, 2-year-old potted seedlings each of red oak (Quercus rubra), eastern redbud (Cercis canadensis), and Austrian pine (Pinus nigra) were purchased from reputable nurseries in spring 1997 and planted in the field. Each species was divided into two groups (those to be mulched with heat- treated or fresh materials) and planted out at a spacing of about 1.52 m (5 ft). The plants were drip-irrigated for the 4 years of the study. All plants were in good health and showed no symptoms or signs of disease at the onset of the study. Diseased materials to be used as mulch were collected from mature trees during late spring 1997. Trunks and branches were collected from mature redbuds with severe Botryosphaeria canker at the Morton Arboretum in Lisle, Illinois, U.S., and chipped (Vermeer model 1800). A stump grinder (Vermeer model SC502) was used to likewise create wood chip from three mature red oak trees that had been recently killed by Armillaria root rot at the Morton Arbore- tum. The stumps had visible rhizomorphs and mycelial fans. Diseased needles and shoot tips were collected from mature Austrian pines with severe Sphaeropsis tip blight at the Chicago Botanic Garden in Glencoe, Illinois. The materials from each tree species were kept separate and divided into two groups: one to be heated and partially composted and the other to remain untreated (fresh). The partial composting treatment consisted of mixing the mulch materials with spent cow manure at a 1:3 ratio, manure to mulch, volume to volume, and building a pile 1 to 1.2 m high × 1 to 1.2 m (3.25 to 3.9 ft) wide. Each pile was turned biweekly and temperatures monitored daily for 4 weeks using a Windrow Profiling Probe (Pike Agri-labs Supplies, Inc.). The pile temperatures failed to exceed 42°C (108°F), well below the 55°C (131°F) needed for killing microbes. Therefore, a different heat treatment was substituted. Mulch materials were spread thinly in a 1.8 m long × 1.2 m wide (6 × 4 ft) soil cart, covered with a tarp, and heated at 60°C (140°F) for 45 min. After cooling to ambient temperature, the heat- treated mulch materials were applied to saplings of the ©2005 International Society of Arboriculture corresponding host species to a depth of approximately 7.6 cm (3 in.) and diameter of 0.5 m (1.6 ft). The corresponding fresh mulch materials were applied likewise to saplings in an adjacent section of the plot, separated by a 1.2 m (3.9 ft) high barricade intended to stop wind movement (and spores) between treatments. Again in 1998, diseased redbud chips, oak wood chips from stump grindings, and pine needles and shoot tips were collected from mature trees, treated as described, and reapplied to the saplings in the plot. In addition, the redbuds were pruned each year in an effort to create wounds that might serve as potential infection courts for B. ribis. Tree height and caliper were measured and disease development assessed at planting time and periodically for the next 4 years. Sphaeropsis tip blight was evaluated by counting the number of blighted tips and total tips per tree at the end of each growing season. Armillaria root rot and Botryosphaeria canker were evaluated by rating canopy symptoms including dieback, flagging, and branch wilt. The presence or absence of redbud pruning wounds with cankers containing B. ribis fruiting bodies was evaluated annually. Oak root systems were excavated in 2003, 6 years after the study was initiated, and checked for signs of A. gallica infection. Temperature Sensitivity Study Growth and viability of vegetative and reproductive struc- tures of the pathogens were quantified at temperatures from 25°C (77°F) to 55°C (131°F), under laboratory conditions. One strain each of Armillaria gallica, Botryosphaeria ribis, and Sphaeropsis sapinea that had been isolated from diseased trees and stored at 10°C (50°F) were grown at room temperature in Petri dishes containing potato dextrose agar (PDA). Temperature sensitivity of vegetative mycelium was tested by transferring 5 mm (0.2 in.) mycelial agar plugs from the periphery of each colony to each of five replicate dishes for each temperature–fungus combination. The dishes were sealed with Parafilm® and placed in an incubator (Isotemp® , Fisher Scientific) set to the target temperature. Two addi- tional dishes were made for each pathogen and were kept at 25°C (77°F) to serve as controls. Beginning after 24 h, and continuing for 6 weeks, dishes were removed from the incubators, and colony diameters were measured and averaged. Growth of at least 2 mm (0.08 in) was considered minimal for viability. The same temperatures and incubation periods were used to test the viability of naturally occurring inoculum of S. sapinea (pycnidia embedded in pine needles), B. ribis (pycnidia embedded in redbud bark pieces), and A. gallica (5.0 cm [2 in.] long pieces of rhizomorphs) collected from a recently killed red oak stump. Two replicate glass dishes of the needles, bark pieces, or rhizomorphs were removed from the incubators, when appropriate, and tested for
September 2005
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