First Report of Vine Wilt Disease Caused by Fusarium solani on Sacha Inchi (Plukenetia volubilis) in China

Sacha inchi (Plukenetia volubilis L.) is native to the Amazon rainforest, where it has been cultivated by indigenous people for centuries. Because of its high protein and oil content and general nutritional quality, it is regarded as a promising oil crop. Sacha inchi was introduced to Xishuangbanna by Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, China, in 2006, and about 13.5 ha has been cultivated as under-forest economy crop under Areca catechu trees in Lingshui, Hainan Province, China, since 2014. In June 2016, ∼85% of plants in a field of sacha inchi grown for seeds in Lingshui exhibited symptoms of chlorosis, defoliation, wilting, and/or death. Dissection of roots and vines of symptomatic plants revealed discolored vascular tissue. Symptomatic roots and vines’ pieces (∼5 × 5 mm) from four randomly selected plants were surface-sterilized with 70% ethanol solution treatment for 30 s, washed in sterile water three times, and placed on potato dextrose agar (PDA) medium. Resulting fungal colonies were subcultured using the single-spore method. The colonies on Spezieller Nährstoffarmer agar medium (SNA) plates had creamy white colored growth. Based on observations of 1-week-old isolates, microconidia were fusiform to oval with zero or one septum, dimensions of 7.6 to 17.9 × 2.7 to 5.5 μm (n = 20), and formed at the tips of long unbranched monophialides. Macroconidia were two to four septate, spindle to crescent-shaped, hyaline, and blunt on both ends, 12.2 to 49.4 × 3.2 to 7.5 μm (n = 50). Genomic DNA was extracted from representative isolate YJG3-2. The internal transcribed spacer (ITS), β-tubulin (TUB2), and translation elongation factor 1 (eEF-1α) sequences were amplified using primer pairs of ITS1/ITS4, Bt2a/Bt2b, and EF1-728F/EF1-986R, respectively. PCR products were sequenced and deposited in GenBank with accession numbers KX757042 (ITS), KX757044 (TUB2), and KX757046 (eEF-1α), respectively. A BLAST search revealed that all three sequences of isolate YJG3-2 showed 99% sequence homology with the ITS, TUB2, and eEF-1αsequences of Fusarium solani (KF999012.1, GQ121902.1, and KX454503.1, respectively) represented the best BLAST hit in GenBank. Based on morphological characteristics and sequences analysis, isolate YJG3-2 was identified as F.solani. Pathogenicity of this isolate was confirmed on sacha inchi plants in the greenhouse. Surface disinfected seeds of sacha inchi were immersed in sterilized water overnight and then cultivated in sterile soil in seedling tray until the four leaf stage. Healthy seedlings were selected and transplanted into plastic pots (15 cm diameter, 15 cm height) with sterilized soil after 2 weeks of growth. Ten 45-day-old healthy sacha inchi plants were directly inoculated by irrigating plants with a 50 μl drop of a mixed conidial suspension (106 conidia/ml) of isolate YJG3-2 into each pot near the plants’ roots without wounding after 7 days without watering, and control plants were inoculated with 50 μl of sterile distilled water (Uppala et al. 2013). Symptoms of chlorosis and wilt similar to those in the commercial field were observed over 6 months (September 2016 to April 2017) on 7 of 10 inoculated plants, while control plants were symptomless. Fungal isolates identical to F. solani were reisolated from the symptomatic plants. Sacha inchi is known to be extremely sensitive to Fusarium spp. and Meloidogyne spp. (Mendoza 2008, Senino et al. 2008), but to our knowledge, this is the first report of F. solani causing sacha inchi wilt disease in China. © 2017, American Phytopathological Society. All rights reserved.