First Report of Fusarium Crown and Root Rot on Torreya grandis Caused by Fusarium oxysporum Species Complex in China.

Torreya grandis cv. Merrillii is an important economic forest crop with a price as high as $75/kg in nut markets throughout humid regions of South China. In 2009, sporadic occurrence of crown rot disease was recorded in Shaoxing City, which produces 80% yields of T. grandis in China. In 2014, nearly 37% of orchards and 4.5% of trees were affected in Shaoxing. Symptoms were crown rot characterized by light-brown discoloration of the cambium, with brownish black necrotic areas which appeared on the roots and often advanced to the collar on most plants. Diseased trees showed reduced vigor and chlorosis on the foliage and eventually died. Infected root and crown samples collected from 9 orchards in Shaoxing were surface sterilized with 1.5% sodium hypochlorite for 3.0 min, rinsed in water, plated on 2% potato dextrose agar (PDA), and incubated at 25°C in the dark for 1 week (Kirk et al. 2008;Snyder and Hansen 1940). Single conidium cultures were consistently isolated, and cultured on acidified PDA (APDA) for morphological characterization. Colonies with purple mycelia and beige or orange colony colors developed after 7 days incubation at 25°C. Colonies produced abundant microconidia and macroconidia. Microconidia were produced on false heads on short phialides formed on the hyphae. Microconidia were hyaline and oval-ellipsoid to cylindrical (3.6 to 9.1 × 1.1 to 3.4 μm). Macroconidia had 3 to 5 septate and were fusoid-subulate with a pedicellate base (27.3 to 38.1 × 3.6 to 4.4 μm). Morphology and development of macroconidia and microconida were consistent with a description of Fusarium oxysporum Schltdl (Kirk et al. 2008; Leslie and Summerell 2006; Snyder and Hansen 1940). The ribosomal internal transcribed spacers ITS1 and ITS2 of 10 isolates were amplified using primers ITS1/ITS4 on DNA extracted from mycelium. The nucleotide sequences were a 100% match to that of F. oxysporum (GenBank Accession No. AF069310.1). The translation elongation factor 1-alpha gene (EF1α) of 12 isolates were amplified using primers EF-1 and EF-2 (O’Donnell et al. 1998). The nucleotide sequences were 99% match with EF1α Accession No. KP674229 at GenBank and had a 99.9% homology with the F. oxysporum sequence FD_00202_EF-1a at the Fusarium-ID database. To confirm pathogenicity, 40 three-year-old, 0.8-m-high T. grandis seedlings were inoculated by dipping the roots into a conidial suspension (107 conidia/ml) for 30 min. The inoculated plants were transplanted into pots containing sterilized peat and maintained at 25°C and 100% relative humidity for 24 h and thereafter at 90% relative humidity in a growth chamber with a daily 12-h photoperiod of fluorescent light. The pathogenicity test was conducted three times. Within 50 days, all inoculated plants developed symptoms similar to that observed in the fields. No symptoms were observed on 10 plants dipped into distilled water. The fungus was successfully reisolated from roots cultured on APDA, exhibiting morphological characteristics identical to F. oxysporum (Kirk et al. 2008; Snyder and Hansen 1940), confirming Koch’s postulates. To our knowledge, this is the first report of T. grandis crown and root rot caused by fungus belonging to the F. oxysporum species complex worldwide. Further work is needed to determine the clade of F. oxysporum to which the isolates pathogenic to T. grandis belong. © 2016 The American Phytopathological Society.