First Report of Phytopythium helicoides Causing Rhizome Rot of Asian Lotus in China.

Asian lotus (Nelumbo nucifera Gaertn. ssp. nucifera), also called sacred lotus, is an economically important aquatic plant in China. It is a well known and popular vegetable for its nutritious rhizomes and seeds. Rhizome rot is a prevalent disease threatening lotus production and can cause yield loss of up to 80%, or even total loss (Li et al. 2013; Ding et al. 2002). The diseased lotus showed root necrosis, rhizome rotting, and leaf wilting. To determine the pathogen present, six symptomatic rhizomes of No. 4 variety Hubei Lotus were collected from Yaoqiang Town, Jinan City (36°40′ N; 117°00′ E), in December 2013. The collected rhizomes were excised, surface-sterilized, and placed on potato dextrose agar (PDA) medium. After incubation at 28°C for one day, white fungal colonies were isolated. To confirm the pathogenicity, 2-week-old lotus seedling plants and 1-month-old asexual plants were inoculated with mycelial agar disks of the isolates. Three seedling plants and eight asexual plants’ rhizomes were inoculated. The fungus-free agar disks were used as the negative control. Inoculated seedling plants were placed in the growth chamber, and small water tanks with inoculated rhizomes in soil were placed outdoors in the summer months in Jinan. One week postinoculation, the inoculated seedling plants showed root necrosis and rhizomes showed symptoms of rot. The same fungus was recovered from symptomatic roots and rhizomes. The inoculation experiments were repeated twice and showed the same results. The roots and rhizomes inoculated with the control agar disks remained symptomless, and no fungi were recovered. The pathogenic fungal isolates were identified based on morphological characteristics and DNA sequence analysis. After incubation at 28°C for one day on PDA, colonies were cottony to felty in texture and always white in color with average diameters of 90 ± 3 mm. Hypha can grow at a temperature range from 15°C to 40°C. In the typical isolate, Jnyq1, sporangia were ellipsoidal, papillate, proliferating, and 20.2 to 31.2 × 22.8 to 34.6 µm in diameter. Zoospores were formed in vesicles. Oogonia were terminal, smooth, and 23.3 to 28 μm in diameter. Antheridia were lobulate, elongate, monoclinous or diclinous, and 1 to 3 per oogonium. Oospores were aplerotic and spherical. Jnyq1 was further classified by sequencing the ITS1-5.8S rRNA-ITS2 region (the rDNA of the internal transcribed spacer regions 1 and 2 and the 5.8S gene, GenBank Accession No. KT258905), co I (the cytochrome oxydase I gene, Accession No. KT258906), and co II (the cytochrome oxydase II gene, Accession No. KT258907) using primer pairs ITS1/ITS4 (White et al. 1990), FM35/FM59, and FM66/FM58 (Martin 2000), respectively. BLAST results revealed that the ITS1-5.8S rRNA-ITS2 region, co I, and co II from Jnyq1 shared nucleotide identities of 97%, 99%, and 99% with corresponding segments of Phytopythium helicoides (Accessions Nos. HQ643382, FR774199, and AB108014), respectively. To our knowledge, this is the first report of P. helicoides causing rhizome rot of lotus in China. Rhizome rot of lotus caused by Fusarium oxysporum is well known in China (Liu 1992). © 2016 The American Phytopathological Society.