First report of a new leaf blight caused by Phacidiopycnis washingtonensis on Pacific Madrone in Western Washington and Oregon

Authors: ELLIOT, M - Washington State University; CHASTAGNER, G - Washington State University; COATS, K - Washington State University; SIKDAR, P - Washington State University; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: 9/16/2014
Publication Date: 12/1/2014
Citation: Elliot, M., Chastagner, G.A., Coats, K.P., Sikdar, P., Xiao, C. 2014. First report of a new leaf blight caused by Phacidiopycnis washingtonensis on Pacific Madrone in Western Washington and Oregon. Plant Disease. 98:1741.

Interpretive Summary: In recent years, we have observed an unknown leaf blight disease on Pacific madrone, consisting of browned, desiccated leaves occurring mainly in the lower parts of the canopy in western Washington and Oregon. In May 2009 and 2011 severe outbreaks occurred, and a fungus was isolated from symptomatic leaves from Pacific madrone trees growing in the region. The fungus was identified as Phacidiopycnis washingtonensis based on its morphology and DNA sequence. In tests conducted in the greenhouse, the fungus was able to cause disease on leaves that had been cold injured prior to inoculation. This fungus has previously been reported to cause a postharvest fruit rot disease on apple fruit and a canker and twig dieback disease of apple and crabapple trees in Washington State. To our knowledge, this is the first report of P. washingtonensis causing a leaf blight disease on Pacific madrone in North America.

Technical Abstract: In recent years, an unknown leaf blight disease, consisting of browned, desiccated leaves occurring mainly in the lower parts of the canopy, has been observed during periods of wet springs on Pacific madrone (Arbutus menziesii) in western Washington and Oregon. In May 2009 and 2011 severe outbreaks occurred and symptomatic leaves from madrones growing in the region were sampled to determine the causal agent. Two types of symptoms, leaf necrosis or blotching along the edges and tips of the leaves, and leaf spot, were observed. Isolations from diseased tissues indicated that 50% of the leaf blotch and 30% of leaf spot samples yielded a fungus that was fast-growing and produced colonies that were a pale grey with dark grey reverse and a felty texture. On potato dextrose agar (PDA), pycnidia formed and exuded conidia in peach-colored droplets after two weeks at room temperatures under alternating light and dark. Pycnidia were spherical and 12.5 to 39.8 µm, average 24.2 µm in diameter. Conidia were hyaline, ovoid, and 5.8 to 8.5 × 3.1 to 4.7 µm (average 7.0 × 3.7 µm). The fungus was identified as Phacidiopycnis washingtonensis based on its morphology. To confirm the identity, the internal transcribed spacer (ITS) region of the rDNA was amplified with ITS1/ITS4 primers and sequenced. BLAST analysis showed that the sequence of the fungus had 100% nucleotide identity with the sequences for P. washingtonensis in GenBank. To test pathogenicity, 3-year-old Pacific madrone seedlings were inoculated with 5 isolates of the fungus recovered from Pacific madrone in WA. Three replicate tree seedlings were used for each isolate and the experiment was conducted twice. Five leaves from each replicate tree were cold injured at a marked 5 × 5 mm2 area with a commercial aerosol tissue freezing product prior to inoculation and 5 leaves were not cold injured. A 5 mm diameter mycelial plug cut from the margin of 6-days-old PDA culture was applied to the marked areas on the upper leaf surface. The inoculated area was covered with moist cheese cloth and wrapped with Parafilm. Leaves treated with PDA plugs without the fungus served as control. Leaves were enclosed in plastic bags to maintain moisture for the first 15 h post inoculation and the cheese cloths were removed 15 days post inoculation. All cold-injured inoculated leaves showed symptoms of leaf blight starting at two weeks after inoculation, and no symptoms appeared on the controls. On non-cold injured inoculated leaves, only one isolate caused symptoms (to 80% leaves). The fungus was re-isolated from diseased leaves. These results suggest that P. washingtonensis is able to cause foliar blight on Pacific madrone when leaves are subjected to cold stress. This fungus has previously been reported to cause a postharvest fruit rot disease on apple fruit and a canker and twig dieback disease of apple and crabapple trees in WA. To our knowledge, this is the first report of P. washingtonensis causing a leaf blight disease on Pacific madrone in North America.