First Report of Fusarium redolens Causing Root Rot of Carthamus tinctorius in China

Carthamus tinctorius, commonly known as “safflower” in China, is an herbaceous plant known for its medicinal properties and is distributed throughout China (Zhai et al. 2013). In August 2024 root rot was observed on C. tinctorius plants in Handan (36.55°N, 113.26°E), Hebei Province, China. Plants in 200 ha were observed to be severely affected by a disease, causing an estimated yield loss of 85%. Leaves of infected plants gradually turned yellow and withered from bottom to top. Vascular bundles at the base of the rhizome turned brown, progressing to deep brown rot, ultimately leading to plant mortality. To isolate the causal agent, tissues (5×5 mm) from four symptomatic plants were removed from the border of lesions, surface sterilized in 75% ethanol for 30 s and 0.1% HgCl2 for 1 min, then rinsed three times with sterile distilled water, plated on potato dextrose agar (PDA) at 25℃, and incubated in the dark for 7 days. Two representative isolates, 202566 and 202567, were cultivated on PDA. Macroconidia of the isolates were three to five septate, were robust and thick walled, and the upper third of the conidia were the widest. The apical cell were hooked, while the basal cell were foot shaped. Microconidia were oval to cylindrical with 0 or 1 septa and often pointed on one end. These were produced in the aerial mycelium in false heads with conidia borne or monophialides (Leslie and summerell 2006). The translation elongation factor (TEF), and partial RNA polymerase second largest subunit (RPB2) were amplified (accession nos. TEF: PX966586 and RPB2: PX966588) (O'Donnell et al. 2022). When compared with sequences of other Fusarium species in GenBank, the isolate exhibited 100% (TEF, MK353340) and 100% (RPB2, MT305165) similarity with F. redolens (query coverage=100%, per-centage identity=100%). A phylogenetic tree was constructed in MEGA software (version 11.0.10) (Tamura et al. 2021) using the partial gene sequences. Maximum likelihood analysis revealed that the isolates shared 100% identity with reference sequences of F. redolens. To test pathogenicity, a conidial suspension of 202566 and 202567 with a concentration of 1×106 conidia/mL was inoculated via root wounding separately on 36 plants. For the control treatment, 36 plants were treated with an equivalent volume of sterile water. The experiment was repeated three times. The plants were placed in a greenhouse from 26~30℃ and 95% relative humidity with a 12-h light/12-h dark photoperiod. Typical symptom were observed 10 days after inoculation, except on the control samples where no symptoms were observed. The same fungus was successfully reisolated from the symptomatic plant tissue and was reidentified as F. redolens through morphological characteristics and TEF and RPB2 sequence analysis (accession nos. TEF: PX966587 and RPB2: PX966589), these strains sequences exhibited 100% similarity with the original isolated strains sequences, thereby fulfilling Koch’s postulates. While F. redolens has been reported to cause tobacco Fusarium root rot, Polygonatum sibiricum root rot, Ginseng root rot and rice seedling blight in China (Gai et al. 2023; Liu et al. 2026; Guan et al. 2014; Wang et al. 2019), this is the first study to report that F. redolens can infect C. tinctorius in China. This report will help growers recognize this disease and take appropriate measures to minimize or prevent economic losses.