Etiology, Pathogenicity, and Fungicide Management of Pigeonpea Shoot Dieback Caused by Lasiodiplodia theobromae in China

Pigeonpea (Cajanus cajan) shoot tip dieback was recently observed in Danzhou, Hainan Province, China, with disease incidence of 30–100% in affected fields, but its causal agent and management options remained unclear. This study aimed to isolate and identify the fungal agent associated with the disease, verify its pathogenicity, characterize its biological traits and effects on host growth, and screen candidate fungicides. Fungal isolates were obtained from symptomatic shoot margins and purified by hyphal-tip and single-spore isolation. Representative isolates LYZ0717, LYZ0718, and LYZ0719 were characterized by colony morphology, pycnidial and conidial traits, and combined phylogenetic analysis of ITS and TEF1-α sequences. These analyses identified the isolates as Lasiodiplodia theobromae. Spray inoculation of 20-day-old pigeonpea seedlings reproduced typical shoot tip dieback symptoms within 6 days, and the inoculated fungus was reisolated from symptomatic tissues, fulfilling Koch’s postulates. The pathogen grew over a pH range of 3–11 and at temperatures of 5–35 °C, with optimal mycelial growth at 25–30 °C and maximum sporulation under acidic conditions. At 20 days after inoculation, infection reduced plant height, dry weight, and total flavonoid content by 45.21%, 75.88%, and 42.19%, respectively. Among the seven fungicides tested, trifloxystrobin–tebuconazole and thiophanate-methyl showed the strongest in vitro inhibition, with EC50 values of 0.0270 and 0.0614 ppm, respectively. In greenhouse pot experiments, thiophanate-methyl achieved the highest disease control efficacy, reaching 64.35% after the second application. These findings demonstrate that L. theobromae is the causal agent of pigeonpea shoot tip dieback in China and provide a basis for disease diagnosis, biological understanding, and fungicide-based management.