First Report of Stem Rot of Pennycress ( <i>Thlaspi arvense</i> ) Caused by <i>Fusarium acuminatum</i> in China

Pennycress (Thlaspi arvense L.), a Eurasian annual herb widespread in China, is a promising winter oilseed cover crop. It controls soil erosion, runoff, nutrient leaching and weeds, supports insects and pollinators, and its oil is ideal for renewable jet fuel production (McGinn et al. 2019). In 2023 and 2024, a 30%–40% incidence of stem rot in pennycress was observed in Minzhu Town (45°49' N, 126°48' E), Harbin City, China. Initially, the leaves of affected plants exhibited yellowing during the flowering stage. Then, bottom part of stem turned brown and the whole plant lodging, and then died. 5 randomly selected typical disease plants were collected. Diseased tissue at the stem bottom was cut into small pieces, surface sterilized with 70% ethanol for 30 s and 1% NaClO for 5 min, and rinsed three times in sterile H2O. Subsequently, the pieces were plated onto the potato dextrose agar (PDA) medium at 28℃ for 5 days. A single-spore culture was obtained by monosporic isolation. After incubation on PDA at 28℃ for 7 days, the single-spore isolates initially formed abundant white aerial mycelium, then gradually developed a rose pigmentation with a brownish color in the center and grayish rose at the periphery of the colony (Li et al. 2019). The macroconidia of the isolates were slender, falcate, distinctively curved in the bottom half of the apical cell, and had 3 to 5 septa. Their size ranged from 29.8–49.5 µm × 4.9–7.1 µm (n = 50). Microconidia were irregularly oval and with zero to one septa. The morphological characteristics of the isolates were consistent with Fusarium acuminatum (Leslie and Summerell 2006). One representative isolates (MZT-2) identified putatively as F. acuminatum based on morphology. To confirm the species identity, the partial sequence of translation elongation factor 1 alpha (TEF1-a) gene, rDNA internal transcribed spacer (ITS) and RNA polymerase II second largest subunit (RPB2) region were amplified and sequenced (O’Donnell et al. 2015; White et al. 1990; Ponts et al. 2020). The sequences were deposited in GenBank under accession numbers PV067929, PP794646, and PQ656806, showing 100%, 97%, and 98% identity with F. acuminatum isolates MW423623, OQ704301, and OP838086, respectively. For pathogenicity test, a conidial suspension (1 × 106 conidia per mL) was prepared from a 7-day-old culture of the representative isolate MZT-2 grown in potato dextrose broth. Ten plants were drenched with 200 mL of conidial suspension every two days. As control plants were drenched with 200 mL of sterile water every two days until disease symptoms appeared. All pots were maintained in a greenhouse (26℃ ± 2; 60%–70%; 16 h light/8 h dark). The experiment was repeated three times with three replicates in each. The leaves of the inoculated plants gradually turned yellow and wilted within 10–15 days; the epidermal tissue of the stem showed brown discoloration. Eventually, the plants died within 28–35 days. The control plants exhibited no symptoms. The F. acuminatum pathogen was re-isolated from the diseased stem bottom of inoculated plants, and its morphology and molecular characteristics were identical to those of the original isolate, thus fulfilling Koch's postulates. Previous studies reported that F. acuminatum causes root rot in Schisandra chinensis , leaf spot in Atractylodes lancea, and white fruit disease in Morus alba (Chen et al. 2022; Zheng et al. 2023; He et al. 2025). However, to our knowledge, this is the first report of F. acuminatum causing stem rot in pennycress in China.