First Report of Cephaleuros virescens Causing Algal Leaf Spot of Manilkara zapota in Thailand

During September 2015, we observed orange to dark-brown algal leaf spots on leaves of Manilkara zapota (L.) P. Royen (sapodilla, lamoot) in Chiang Mai Province, Thailand. The disease was found on 20% of surveyed M. zapota, caused necrotic leaf lesions. Transverse sections of the spots collected from symptomatic leaves revealed that algal thalli occurred beneath the cuticle of upper leaf surfaces and caused host-cell necrosis beneath most of the thalli. The thalli of Cephaleuros virescens were pseudoparenchymatous and predominantly ramulose, which distinguished them from other species in this genus. Algal thalli were more or less circular, composed of short cylindrical filamentous cells that measured 12.5 to 30 × 7.5 to 12.5 μm with a length/width ratio of 1:1.25 to 4. Sporangiophores penetrated the cuticle, grew perpendicular to the thallus, and measured 112.5 to 337.5 × 10 to 17.5 μm. Elliptical sporangia measured 17.5 to 30 × 15 to 25 μm and attached to the terminal cell of the sporangiophore by a unique appendage, the suffultory cell. Gametangia were spherical to elliptical, measured 25 to 40 × 20 to 37.5 μm, and produced by filaments on the surface of thalli. Setae were three- to five-celled filaments that measured 70 to 257.5 × 5 to 7.5 μm. These morphometric traits were consistent with those for C. virescens (Thompson and Wujek 1997). Zoospores and gametes were not observed in this study because reproductive structures were too old at the time of observation. Fresh thalli were cultured on Bold’s basal medium (BBM) and incubated at 25 ± 2°C with a 12-h photoperiod to purify the culture, and then subjected to PCR amplification. PCR was conducted to amplify a portion of 18S rDNA by using primer pair PNS1/NS41. The nucleotide sequence analysis of the alga’s 18S rDNA using a BLAST search revealed that our partial sequence was 1,610 bases long. This nucleotide sequence was deposited in GenBank with accession number (LC175220), and was compared with known Cephaleuros and other algal genera in the NCBI databases. A 99% sequence identity confirmed the isolate as C. virescens. The pathogenicity of Cephaleuros as a plant parasite has not been clarified experimentally; however, this is probably due to difficulties with producing zoospores on synthetic media for inoculation tests. However, leaves of healthy M. zapota inoculated with fragments of algal filaments (Suto and Ohtani 2011) produced algal thalli after 4 months. Reisolates of the alga were morphologically consistent those originally isolated. To our knowledge, this is the first report of algal leaf spot on M. zapota caused by C. virescens in Thailand. © 2017, American Phytopathological Society. All Rights Reserved.