238 Jacobs: Potential of Mulch to Transmit Pathogens Figure 1. Average growth increment of Austrian pine, eastern redbud, and red oak saplings planted in heat-treated and fresh mulch derived from diseased trees. Means represent the average of 15 trees for each treatment, and they are shown with standard error bars. of all fungi were more resilient to heat than mycelia, but only slightly so in the case of Armillaria rhizomorphs. Five cm (2 in.) long rhizomorph pieces were killed after 24 h at 40°C (104°F), while the control rhizomorphs incubated at room temperature remained viable. In contrast to Armillaria, conidia of B. ribis, protected within pycnidia embedded in bark pieces, remained viable and germinated after being incubated 2 days at 55°C (131°F). Conidia of S. sapinea, similarly protected in pycnidia embedded in pine needles, were extremely tolerant to heat. Its spores remained viable at 55°C (131°F) for 6 weeks, the longest incubation period tested. DISCUSSION Infected needles and shoot tips collected from diseased Austrian pines and then used as mulch transmitted S. sapinea to healthy saplings in the field. This finding is not surprising, as the primary inoculum (conidia) of S. sapinea develop and overwinter in pycnidia formed in these tissues (Hansen and Lewis 1997). This is the first report that quantifies transmis- ©2005 International Society of Arboriculture sion of the disease from materials used as mulch and, importantly, indicates that pre-treatment of infected needles and shoot tips with high temperature can reduce the threat. Steam heating of the needles and shoot tips to 60°C (140°F) was associated with a twofold reduction in the level of tip blight, compared with pines mulched with untreated (fresh) mulch. Tip blight was not eliminated completely due to background levels of disease that occur most anywhere the highly susceptible Austrian pine is planted. Botryosphaeria canker and Armillaria root rot diseases did not develop in redbuds and oaks, respectively, that were mulched with bark and wood chips derived from infected trees. Considering that both diseases may have extensive incubation periods sometimes lasting years (Sinclair et al. 1987; Wargo and Harrington 1991; Jacobs 2001), it is possible that some transmission occurred but went unde- tected. However, the small size of the saplings tested and multiple years of observation would seem to negate this possibility. Instead, other factors may explain the apparent absence of disease. First, the vitality of the redbud and oak saplings was high as evidenced by the rapid growth rates measured. High vitality and low stress may have impeded disease in redbud and red oak hosts, as stress is a well- documented predisposing factor in both pathosystems (Sinclair et al. 1987; Wargo and Harrington 1991; Jacobs 2001). While stress also enhances Sphaeropsis tip blight (Sinclair et al. 1987; Stanosz et al. 2001), the high level of susceptibility of Austrian pine to the disease may render stress unnecessary. A second explanation for the absence of canker and root rot is that there was insufficient viable inoculum present in the bark and wood chips to begin with. The amount of inoculum present was not quantified in this study. Instead the focus was on testing whether transmission might occur when mulches are derived from chipping diseased trees in the field. Still, the trees selected as sources of mulch were severely diseased by normal standard, and inoculum, be it rhizomoprhs and fan in the oaks, or cankers with pycnidia in the redbuds, was ample. Further, redbud bark chips utilized for the laboratory tests of temperature sensitivity had plenty of B. ribis pycnidia. Nonetheless, inoculum present in mature, diseased trees did not appear to be transmitted to healthy hosts when the trees were chipped or ground and used as mulch. Temperature sensitivity experiments in the laboratory revealed differences in heat tolerance among the pathogens that could influence their survival in mulch, as well as offer insight for potential treatments of diseased mulch. Natu- rally occurring reproductive structures (pycnidia) of S. sapinea and B. ribis embedded within host tissues withstood far higher temperatures and longer exposure times than mycelia of any fungus tested. Sphaeropsis sapinea was especially tolerant of heat, with conidia able to germinate
September 2005
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