First report of Lasiodiplodia theobromae causing branch canker and dieback of cashew in Panama

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Cashew (Anacardium occidentale) is an important subsistence crop cultivated widely in Panama. In August 2020, 20-year-old cashew trees showing branch canker (Figure 1) and dieback symptoms were observed in a 0.5-ha orchard located at Río Hato, Coclé Province, Panama, with a disease incidence of 70%. Ten symptom-bearing branches were collected arbitrarily from affected trees and washed under tap water, surface disinfected for 1 min in 1% sodium hypochlorite solution, and rinsed twice in sterile distilled water. Small pieces (5 × 5 mm) were excised from the lesion margins and plated onto potato dextrose agar (PDA) supplemented with 0.5 g/l of streptomycin sulphate and incubated at 25°C in dark for four to seven days. Fungal isolates were grown on PDA and 2% water agar containing pine needles and incubated at 25°C with a 12-h photoperiod of near-UV/darkness for four weeks. Colonies initially appeared greyish-white, later turning grey-black with aerial mycelium (Figure 2). Dark brown and globose pycnidia were formed. The mature conidia were one-septate, ovoid to ellipsoid, dark brown, with longitudinal striations (mean ±SD = 27.69 ±2.46×13.48 ±1.04 μm, n = 50) (Figure 3). The cultural and morphological characteristics of the isolates were similar to those reported for the genus Lasiodiplodia (Phillips et al., 2013). For molecular identification, genomic DNA was extracted from one isolate (MAR-2), and ITS1-5.8S-ITS2 (ITS) region, fragments of translation elongation factor 1-α (TEF1-α) and β-tubulin (TUB2) were amplified and sequenced using the primers ITS1/ITS4 (White et al., 1990), EF1-688F/1251R (Alves et al., 2008) and Bt2a/Bt2b (Glass & Donaldson, 1995), respectively. BLASTn analysis of the resulting ITS, TEF1-α, and TUB2 sequences of isolate MAR-2 (GenBank Accession Nos. OL454783, OL544943, and OL544944, respectively) showed 100%, 99.3%, and 100% identity with sequences of Lasiodiplodia theobromae ex-type strain CBS 164.96 (AY640255, AY640258, and EU673110, respectively) (Phillips et al., 2013). The pathogenicity of isolate MAR-2 was evaluated by inoculating four three-month-old cashew plants. In each plant, a 5-mm-diameter wound was made in the stem with a scalpel. A 5-mm-diameter PDA plug retrieved from the colony margin was placed on each wound and sealed with plastic Parafilm. Control plants were inoculated with PDA plugs. Plants were maintained in a greenhouse at 20 to 35°C until symptom expression. After 21 days incubation, necrotic cankers developed on all the inoculated plants, whereas the control plants remained asymptomatic (Figure 4). Lasiodiplodia theobromae was consistently reisolated from stem lesions. Lasiodiplodia theobromae has been reported on various hosts in Panama (Farr & Rossman, 2022) but to our knowledge, this is the first report of this pathogen causing branch canker and dieback disease of cashew in this country. Previous reports from Brazil have described L. theobromae causing branch dieback in cashew (Cardoso et al., 2002). The disease substantially reduces cashew production and thus proper control measures should be implemented. This work was funded by IDIAP 501.B.1.15 and SENACYT, through the National Research System (SNI).