First report of pistachio root rot caused by Fusarium solani in China

Pistachio (Pistacia vera L.) root rot disease caused by fungi, such as Fusarium solani, has resulted in huge economic losses (Nouri et al. 2020). In August 2024, root rot symptoms were observed in 1 to 6-year-old pistachio trees grown in two orchards located in the towns of Tuokezhake (75°86’E, 39°38’N) and Liaoyuan (75°81’E, 39°38’N) in Shufu County, Kashgar Prefecture, Xinjiang Uyghur Autonomous Region, China, where the disease incidence was measured as about 18.5%. The infected trees exhibited browning, drooping, and wilting of leaves, dieback of shoots and branches, and necrotic root rot. Symptomatic roots from 20 diseased trees were cut into about 1-cm-long segments, surface-sterilized in 75% alcohol and 1% NaClO, then washed with sterile distilled water, dried and placed onto potato dextrose agar (PDA) plates incubated at 25 ℃ for 5 d (Zhang et al. 2023). Among the 20 isolates with similar morphological features, KS1 showing the typical morphology was selected for identification. It produced fluffy aerial mycelia, with colonies appearing white and exhibiting an average radial growth rate of 6 mm per day at 25 ℃. The hyaline macroconidia measuring 17.6–30 × 4–6.2 µm (n = 50) were produced on PDA plates, which were moderately curved and possessed 3–5 septa, whereas microconidia with no septum measuring 7.3–16.9 × 4.3–6.4 µm (n = 50) were cylindrical to oval and slightly curved. The globose or subglobose and transparent chlamydospores (4.5–9.5 µm) were singly or in pairs in intercalary (within hyphae) positions. KS1 was identified by sequence analyses of the nuclear ribosomal internal transcribed spacer (ITS) region as well as genes encoding translation elongation factor (TEF-1α) and the second largest subunit (RPB2) of RNA polymerase II by using primer pairs ITS1/ITS4, EF1/EF2, and 5f2/7cR, respectively (O'Donnell et al. 2012; White et al. 1990). The alignment of KS1 nucleotide sequences against the Fusarioid-ID database displayed 100% identities with ITS (GU170639), TEF-1α (MW620195), and RPB2 (PX130505) of F. solani. DNA sequences generated in this study were deposited in Genbank under PX557947, PX600426, and PX600427 for ITS (577 bp), TEF-1α (676 bp), and RPB2 (1069 bp), respectively. Phylogenetic tree was inferred from the nucleotide sequences of ITS, TEF-1α, and RPB2 from KS1 and 19 Fusarium strains stored in Genbank, where KS1 was clustered with 3 F. solani strains and identified as F. solani. Pathogenicity tests were performed on the seedlings of pistachio cultivar Dichan7. Briefly, seeds were soaked in distilled water, followed by dark incubation on moistened sterile gauze. Each of the germinated seeds with 3–5 cm long primary roots was planted into a pot containing sterilized potting mix. F. solani KS1 spores (10 ⁶ spores/mL; 30 mL) were poured onto the pot soil, whereas the control was treated with sterile distilled water. Ten plant replicates were used for each treatment. Two independent experiments were performed. Three weeks after inoculation, over 70% of KS1 inoculated seedlings developed root rot symptoms, including stunted growth, leaf browning and scorching, root necrosis and biomass loss; however, controls remained asymptomatic. F. solani (identified as described above) was consistently isolated from symptomatic plants but never from controls, thereby completing Koch’s postulates. This is the first report of F. solani causing pistachio root rot in China, which will serve as a pivotal foundation for efficient disease control.