First Report of Fusarium avenaceum Causing Root Rot of Maca (Lepidium meyenii) in China

Maca (Lepidium meyenii) is an herbaceous biennial plant native to the high Andes of Peru. The production area and market for maca are expanding rapidly in China. During the period from August to October in 2014 and 2015, it was observed that ∼10% of the plants in a 33.33 ha area (26°46′N; 100°04′E) in Lijiang County, Yunnan Province, showed abnormal symptoms that included yellowing, wilting, and collapsing of the plants. The root symptoms included wet rot and pith hollowing with brown discoloration of the internal vascular tissue. Five 3 × 3 mm pieces of root tissue were excised from the margin of individual lesions from each of five representative symptomatic 7-month-old maca plants. They were surface disinfested using 0.1% HgCl2for 3 min, then rinsed three times with sterilized H2O, plated on potato dextrose agar (PDA), and incubated at 25°C (Robak et al. 2014). Three fungal isolates with consistent morphology were obtained. Morphological characteristics of the fungal isolates cultured on PDA for 7 days are described as follows: colonies were cotton-like and white to pale orange in color with a red or deep amber reverse; the hyphae were achromatic, branching, and septate, with an average diameter from 2.5 to 5.6 μm; macroconidia were scattered, hyaline, falciform, slightly curved, with 3 to 5 septa, the dimensions of which ranged from 1.5 to 6.0 μm × 8.2 to 35.5 μm (100 macroconidia were measured) (Booth 1977). DNA was extracted from 100 mg of freeze-dried mycelium using a cetyl trimethyl-ammonium bromide protocol (O’Donnell et al. 2010). Translation elongation factor (TEF-1 alpha) was amplified by ef1/ef2, and RNA polymerase II second largest subunit (RPB2) was amplified by 5f2/7cr and 7cf/11ar (O’Donnell et al. 2010). The TEF sequence (GenBank accession no. KU999088) and RPB2 nucleotide sequence (KX579897) showed that isolate ROT-2 had 99% TEF sequence similarity with F. avenaceum (KP400697 and EU744841) and 100% RPB2 sequence similarity with F. avenaceum (JX171663 and HQ728167). Based on the above morphological characteristics and related nucleotide sequence analysis, this pathogen was identified as F. avenaceum. Koch’s postulates were fulfilled on ten 7-month-old maca, inoculated with 10 ml of spore suspension (1.0 × 106 conidia/ml) into rhizosphere by drench method, using sterilized H2O as control on additional 10 maca plants. The experimental and control groups were incubated in an artificial climate incubator, at 25°C with 12 h light/dark. Symptoms similar to the original plants were observed in the inoculated group after 14 days, while the control group remained healthy. F. avenaceum was consistently reisolated from the inoculated symptomatic roots, which were identified through TEF and RPB2 nucleotide sequence analysis. To our knowledge, this is the first report of F. avenaceum causing root rot disease on maca in China. The disease is expected to have a significant economic impact on maca processing industry; therefore, pathogen detection during the early stages of maca growth is important. © 2017, American Phytopathological Society. All rights reserved.