Corn (Zea mays L.) is the most important forage crop in Japan. It was cultivated on 92,000 ha in 2011 and was mainly used as whole crop silage for cattle feed. In September 2009, a root and stalk rot disease was detected on corn plants cultivated in Tochigi, located in the central region of Japan. The symptoms of the disease included wilting of whole plants after the R5 (dent) stage (2) with drooping ears. Roots turned black and their number decreased. Further, the stalks became hollow and soft and harbored white hyphae. This tissue deterioration made machine harvest difficult. We obtained seven isolates of a Pythium-like organism by single hypha isolation from surface-sterilized pieces of diseased roots and stems on water agar and deposited one of the isolates at the NIAS genebank, Japan, under the accession no. MAFF511547. The isolate was grown in the dark on V8 juice agar medium for 10 days to produce oogonia. The oogonia were globose, light brown to yellow, smooth, 23.9 to 30.5 μm in size, and had 1 to 8 antheridia. Oospores were mostly plerotic, and oogonia walls were 1.3 to 2.7 μm thick. The morphology of the isolates was similar to that of Pythium arrhenomanes Drechsler and consistent with the species description (3). We analyzed the rDNA-ITS region sequences of the isolate as described by Kageyama et al. (1). The sequence (GenBank Accession No. AB903904) showed 99.1% (783/790 bp) similarity with that of P. arrhenomanes (AY598628). On the basis of morphological and rDNA sequence similarities, we identified the isolates obtained from corn as P. arrhenomanes. The pathogenicity of the isolate was confirmed by planting corn seedlings of the commercial Pioneer Brand hybrid 36B08 immediately after germination in five replicate pots containing soil mixed with 5% boiled barley grain by weight, incubated with or without the isolate for 7 days. After 10 days of incubation in a greenhouse at 20 to 25°C, only the inoculated plants exhibited symptoms of root and stalk rot. Since the inoculated organism was readily re-isolated from the diseased stems and roots, the pathogenicity of the isolate was confirmed. For field observation, the same hybrid of forage corn was sown in the fields in Nasushiobara, Tochigi, on 16 May 2011. The hybrid was sown in a row of 2 m, with 20 seeds planted at a distance of 10 cm with two replicates. For inoculum, the isolate was cultured on 5-cm-long wooden toothpicks, previously soaked in potato dextrose broth and placed on a V8 agar plate for 7 days at 25°C in the dark until covered by hyphae. The toothpicks were pierced into wounds made on the stems of corn plants, approximately 10 cm above the ground, using a thin iron needle. The wounds were about 2 mm in diameter and 2 cm deep. Field inoculation was conducted in late July at the R1 (silking) growth stage. Disease symptoms were observed in mid-September at R5, and only those plants that were inoculated with the toothpicks harboring the hyphae exhibited the typical stem rot symptoms. To our knowledge, this is the first report of root and stalk rot caused by P. arrhenomanes in forage corn in Japan.
References: (1) K. Kageyama et al. J. Phytopathol. 151:485, 2003. (2) S. W. Ritchie et al. Spec. Rep. 48. Iowa State Univ. Coop Ext. Serv., Ames, 1993. (3) A. J. Van der Plaats-Niterink. Stud. Mycol. 21:1, 1981.
