Hybrid Poplar Stem Cankers Caused by Mycosphaerella populorum in Kentucky, USA

Cankers on two year-old stems of a hybrid poplar clone ((Populus trichocarpa x P. deltoides) x P. maximowiczii) were noted in two plantations established for clonal evaluation in Livingston County, Kentucky USA in January 2001. These plantations are separated by approximately 35 km and located on upland hardwood sites, dominated by Quercus spp., where no extensive native poplar stands are found. In one plantation 30% of stems had cankers; few stems were affected in the second plantation.

The cankers were flattened or sunken, discolored and resembled those of septoria canker, a severely damaging disease caused by Mycosphaerella populorum (anamorph = Septoria musiva) (Waterman, 1954). Fungal isolates obtained from cankers collected from both plantation sites were identified as S. musiva (Sivanesan, 1990), based on morphological characteristics of pycnidia and conidia produced on V8 vegetable juice medium. Cuttings of the hybrid poplar clone NC11505, known to be susceptible to S. musiva, were rooted in pots in a greenhouse and maintained with ambient light and day and night temperatures of approximately 25 and 15°C, respectively for 3 months. Stems were then inoculated by placing an agar plug colonised by an isolate of S. musiva on a freshly made leaf-scar. Control trees were inoculated in a similar manner but with uncolonised agar plugs. Plugs were held in place with parafilm and removed after one week.

After four weeks cankers had developed on 9 of 10 stems inoculated with S. musiva. The fungus was successfully reisolated and its identify confirmed. Cankers did not develop on the control stems and S. musiva was not obtained from these trees.

Mycosphaerella populorum is assumed to be widely distributed in the eastern United States and Canada (Farr et al., 1989), and reports include mention of its occurrence in states adjacent to Kentucky, including Indiana (Farr et al., 1989), Missouri and Ohio (Thompson, 1941), and Tennessee (Waterman, 1954). However, this is the first confirmed report of septoria canker of poplars in Kentucky and this is noteworthy given the current local forest industry interest in establishing short rotation, intensive culture poplar plantations. The report underscores the need for disease resistance screening and long-term testing of clones derived from highly susceptible parental species such as P. trichocarpa and P. maximowiczii.