Abstract
Trees in the genus Populus and their interspecific hybrids are used across North America for fiber production and as a potential source of biofuel. Plantations of these species are severely impacted by a fungal pathogen, Sphaerulina musiva, the cause of leaf spot and stem canker. An inoculation protocol that does not rely on stem wounding to achieve infection was recently developed. Using this protocol two experiments were conducted to examine infection biology and disease development in the S. musiva-Populus interaction. In the first experiment non-wounded stems of one moderately resistant clone (NM6) and one susceptible clone (NC11505) were inoculated and examined by scanning electron microscopy at six different times (6 h, 12 h, 24 h, 72 h, 1 week, and 3 weeks) post-inoculation. The images indicate that the pathogen appears to enter host tissue through small openings and lenticels and that there are no significant differences in the penetration rate between the moderately resistant (NM6) and susceptible (NC11505) clones at 12 h post-inoculation. In a second experiment a histological comparison of stem cankers for resistant clone DN74 and susceptible clone NC11505 were conducted at three time points (3 weeks, 5 weeks, and 7 weeks) post-inoculation. Distinct differences in disease development were apparent between the resistant and susceptible clones at each time point, with the susceptible clone exhibiting a weak and delayed defense response. These results suggest, that following penetration, the pathogen may be able to interfere with the defense response in the susceptible host.
Highlights
Poplar trees, as foundation species [1] within many ecosystems, occur across most of North America in native populations hundreds- to thousands-of-years old
Blue autofluorescence viewed with ultraviolet light (Excitation filter G 365, Beam Splitter FT 395, Emission filter BP 445/50) and green autofluorescence viewed with blue-green light (Excitation filter BP 450–490, Beam Splitter FT 510, Emission filter BP 515–565) were used to visualize host responses. In both experiments necrotic lesions were first observed on the susceptible clone (NC11505) at 2 weeks PI and on the moderately resistant (NM6) and resistant clone (DN74) at 3 weeks PI
At 7 weeks PI necrotic lesions had coalesced completely girdling the stem of the susceptible clone (NC11505)
Summary
As foundation species [1] within many ecosystems, occur across most of North America in native populations hundreds- to thousands-of-years old. These species are ecologically and commercially important as a result of their broad geographic distribution and potential use as a bioenergy feedstock. The primary limitation to the use of Populus spp. for fiber, biomass, and bioenergy in central and eastern North America are the fungal diseases, Septoria leaf spot and stem canker, caused by Sphaerulina musiva (Peck) Verkeley, Quaedvlieg, & Crous Septoria canker has the greatest impact in pulp and bioenergy plantations due to the potential for a single infection to kill a tree [7]. Some progress has been made, the S. musiva-Populus interaction is still poorly understood
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