First Report of <i>Dactylonectria pauciseptata</i> Causing Root Rot on <i>Eucalyptus cinerea</i> in China

Eucalyptus cinerea is an evergreen tree in the Myrtaceae. It is native to southern and eastern New South Wales and northern and eastern Victoria, Australia. It was introduced into China in the 1980s (Silva et al. 2011). Because of its unique shape, flexible stems, and rapid growth characteristics, it is widely used in the pulp industry and in decorative materials such as flower bouquets. In July 2022, 5- to 10-year-old E. cinerea showing symptoms of dehydration, withering and yellowing leaves, were found in forests and nurseries in Kunming and Songming, China. More than 37% of the trees showed these symptoms at each location, and disease severity was about 30%. Sixty symptomatic plants were collected from five tree nurseries. Diseased roots with 2-cm-long lesions were soaked in 75% ethanol for 15 s, 0.1% mercuric chloride for 2 min, rinsed with sterilized water, and placed on potato dextrose agar (PDA) at 25℃ for 3 days. Thirty samples were plated, and 21 isolates (YJLGF01 to YJLGF21) obtained, 11 strains with similar colony morphology (including representative strains YJLGF03 to YJLGF05). Three isolates (YJLGF03 to YJLGF05) were obtained by single-spore purification. On PDA, the colonies were circular with fluffy white to light yellow mycelium; the underside was yellowish brown. Conidiophores were bifurcated, with macroconidia borne terminally. The macroconidia were cylindrical with rounded, blunt ends, yellow to transparent, 1 to 3 septate (22.5 to 47.6 × 4.5 to 7.1 µm); microconidia were 0 to 1 septate (12.5 to 19.6 × 4.7 to 6.4 µm). Chlamydospores were spherical, rosary-like, and light yellow. Morphological characteristics were consistent with published descriptions of Dactylonectria pauciseptata (Piperkova et al. 2017). For molecular identification, the internal transcribed spacer (ITS), translation elongation factor 1- alpha (ef1-α) gene, and the beta-tubulin 2 (β-tub2) gene were amplified and sequenced (ITS accessions OR735053, OR735054, OR735055; β-tub2 accessios OR757447, OR757448, OR757449; ef1-α accessions OR757450, OR757451, OR757451) using published primers (White et al. 1990; Carbone et al. 1999). A phylogenetic tree was developed by Maximum Parsimony (MP) and Maximum Likelihood (ML) methods. These three isolates fell into the D. pauciseptata clade and were distinguished clearly from other species. Pathogenicity tests were performed using the same three isolates. Each isolate was cultured on PDA, and then subcultured in V8 juice broth on an orbital shaker at 180 RPM for 5 days. Conidia were collected by centrifugation at 6,000 RPM for 5 min, and then resuspended in sterilized distilled water (1×106 conidia/ml). Injured roots of one-year-old E. cinerea were soaked in the spore suspension for 1 h before being transplanted in sterile vermiculite. The plants were incubated at 25℃ with a 12 h photoperiod and 90% humidity. Five plants were inoculated as a group for each treatment and the entire experiment was completed three times. Among the inoculated plants, the incidence of disease development was 100%. A small sot appeared after 4 days, with a water-soaked lesion appearing and gradually expanding during days 5 to 7. After 10 days symptoms of root necrosis were similar to the those observed in the nursery, and aboveground plant parts had yellow, withering leaves and defoliation after 10 to 15 days. Control plants treated with sterile water showed no disease symptoms. The three strains were successfully reisolated from inoculated seedlings and confirmed them using DNA sequencing. No isolates were obtained from the control plants, thus fulfilling Koch's postulates. Dactylonectria pauciseptata was first reported from necrotic tissue of infected grape roots (Schroers et al. 2008). So far, it has been reported in Turkey, Canada, Brazil, Italy, and other countries (Erper et al. 2013; Úrbez-Torres et al. 2014; Santos et al. 2014). Based on our results, E. cinerea is a new host plant of D. pauciseptata in China. This disease is a threat to the nursery production of E. cinerea, potentially leading to a reduction in yields and economic losses.

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