First record of leaf rust caused by Melampsora populnea on Populus alba x glandulosa in Korea

A poplar hybrid (Populus alba x glandulosa), which was developed in 1965 by the Korean Institute of Forest Genetics, has been planted widely and used successfully for both hillside afforestation and urban landscaping in Korea. It is known for heterosis with advantages of fast growth, resistance to diseases and insects, and tolerance to acid soil. In addition, it has been used for phytoremediation of soils contaminated with heavy metals in abandoned mines.

In September 2006, leaf rust was found in a plantation of the poplar hybrid at Anyang, Gyeonggi province, Korea. Initial symptoms were small orange pustules (uredinia) of about 2 mm in diameter on the undersides of leaves, which enlarged and became darker in colour (Fig. 1A). Tiny yellow specks of about 2 mm in diameter appeared concurrently on the upper sides of leaves, merging to become brown lesions of about 2-5 mm in diameter (Fig. 1B). The disease spread from tree to tree during the summer, and the infected trees defoliated early. The uredinial state of this rust under light and scanning electron microscopy had the morphology of Melampsora populnea (Fig. 2) (Hiratsuka et al., 1992): uredinia hypophyllous, yellow or orange; urediniospores light yellow, globose or ellipsoid, 19.4 μm (16.3-21.8 µm) x 13.5 µm (11.7- 16.6 µm) in size, wall 1.0-1.5 μm thick, surface structure echinulate. No known Melampsora species other than M. populnea matched these structural characters. A sample was deposited in the local herbarium (ME 200601). In Korea, neither the telial nor aecial stages of this rust have been observed as yet on either the poplar hybrid or alternate host plants (Larix species) (Hiratsuka et al., 1992).

Leaf rust causing premature defoliation and weakening of Populus species can be caused by many Melampsora species (Ostry et al., 1988). However, no rust disease has been recorded previously on Populus alba x glandulosa, and this is the first record of M. populnea as a pathogen of the poplar in Korea.