First Report of Leaf Rot on Catalpa bungei caused by Fusarium verticillioides in China.

Abstract

Catalpa bungei originates from China. Because of its well-developed root system and strong resistance to wind and soil, it is one of the top ten recommended species of ecological management (Jian et al., 2022). In September 2023, a severe leaf rot of C. bungei was observed at Lanlake farm (500 acres) in Nanyang (33°3'23" N, 112°28'50" E), China. The incidence rate of leaf rot reached 45% (n = 100). The pale-yellow spots initially appeared on the adaxial surface of leaf margins, subsequently enlarging to form irregular black rot lesions, with the yellow halos around the necrotic area of the lesion, ultimately causing the entire leaves to wither. Diseased leaves (20) were collected, cut into pieces, sterilized, and then placed on potato dextrose agar (PDA). A total of 25 purified fungal strains were isolated, and three strains (QS2-1, QS2-2, QS2-3) from distinct areas were selected for further analysis. Each strain produced abundant aerial mycelium, initially white, which later developed purple pigments. The aerial conidiophores were sparsely branched, ending with verticillate phialides. The strains generally produced many more microconidia than macroconidia on PDA media. Microconidia were clavate and measured 3.9 to 6.6 × 1.1 to 2.4 μm (n = 50). To produce macroconidia, we used YPG liquid medium (0.3% yeast extract, 1% peptone, and 2% glucose) with shaking (200 r.p.m.) for 5 days. Macroconidia were slender, straight, and measured 19.5 to 27.1 × 1.9 to 3.5 μm, with 3 to 5 septa (n = 50). The morphological characteristics matched the species description of Fusarium verticillioides (Sacc.) Nirenberg 1976 (Leslie and Summerell, 2006). The rDNA internal transcribed spacer (ITS), β-tubulin gene (tub2), translation elongation factor 1-alpha gene (tef1), calmodulin (cmdA), RNA polymerase II largest subunit (rpb1), and RNA polymerase II second largest subunit (rpb2) were amplified for molecular identification (O'Donnell et al., 2022). The sequences were submitted to GenBank with accession numbers OR741762, OR741763, OR741765 (ITS), OR762222, OR762223, OR939807 (tub2), OR939799, OR939800, PQ035927 (tef1), OR778611, OR939808, OR939809 (cmdA), PQ035921, PQ035922, PQ035923 (rpb1), and PQ035924, PQ035925, PQ035926 (rpb2). BLASTn analysis of QS2-1 sequences exhibited 99% similarity with F. verticillioides sequences (strains CBS 576.78) MT010888 of cmdA (711/713, 99%), MT010956 of rpb1 (1790/1791, 99%), and MT010972 of rpb2 (868/870, 99%). A phylogenetic tree was constructed using concatenated sequences along with the sequences of the type strains employing the neighbor-joining method, showing the three strains formed a clade with the type strain CBS 576.78. Pathogenicity was tested on 10 healthy potted seedlings by spraying them with a conidial suspension (10⁶ conidia ml^-1), while 5 seedlings were sprayed with sterilized water as a control. The plants were placed in climate incubators. Ten days after inoculation, typical lesions were observed on the treated plants, but not on the control group. The reisolated strains were identified as F. verticillioides through morphological characterization, thus fulfilling Koch's postulates. F. verticillioides is known to cause Fusarium ear rot on maize, and other plants including Brassica rapa (Akram et al., 2020) and Schizonepeta tenuifolia (Li et al., 2024). This is the first documented instance of F. verticillioides causing leaf rot on C. bungei globally. Identifying the pathogen is critical to implementing effective disease management strategies, especially in choosing proper pesticide agents and screening disease-resistant varieties.