Leaf Blight of Fatsia japonica caused by Phytophthora cactorum

Abstract

A leaf blight caused by a species of Phytophthora was found on fatsia plants (Fatsia japonica Decne et Planch.) growing in an apartment garden in Daegu, Korea in late April to May, 2003. The species of Phytophthora isolated from the diseased plants produced sporangia and sex organs on V8 juice agar medium. Sporangia were papillate, ovoid to subspherical, and caducous with a pedicel. The dimensions of the sporangia were <TEX>$31.2-46.8\times23.4-33.2{\mu}m$</TEX> in range, <TEX>$39.6\pm4.1\times28.3\pm2.8{\mu}m$</TEX> in <TEX>$average{\pm}standard$</TEX> deviation, I/b ratio approximately 1.40, with papillae about <TEX>$3.6{\mu}m$</TEX> high, and pedicels <TEX>$0.9­5.8{\mu}m$</TEX> long. Oogonia were spherical, <TEX>$25.0-32.5{\mu}m$</TEX> in range with an average of <TEX>$28.2\pm2.3{\mu}m$</TEX> in diameter. Antheridia were predominantly paragynous, globose to ovoid, <TEX>$8.8-13.8\times7.5-10.0{\mu}m$</TEX> with an average of <TEX>$10.9\pm1.2 \times9.2\pm1.1{\mu}m$</TEX>. Oospores in the oogonia were aplerotic or plerotic, and <TEX>$20.0-25.0{\mu}m$</TEX> in diameter with an average of <TEX>$23.5\pm1.5{\mu}m$</TEX>. Pathogenicity of the isolate was confirmed on detached leaves of fatsia. The disease was observed only in April and May of 2003 when the weather was unusually wet. It then diminished with increase of temperature in the year and did not appear again on the same plants in 2004. Thus, the fungus appeared to be a relatively weak pathogen of fatsia.