First Report of Root Rot Caused by Pythium myriotylum on Lablab bean ( Lablab purpureus ) in China

Lablab bean (Lablab purpureus L.), a variety of legumes (family Fabaceae), is an important summer-growing annual or occasional short-lived perennial (Naeem et al., 2023). Lablab bean has received relatively little attention from a research standpoint, especially in plant diseases. In July 2025, root rot was observed on Lablab bean in Zhaoyuan county (45.53°N, 125.28°E), Heilongjiang province, China. Diseased plants appeared brown lesions on the lower stems and root rot at the seedling stage (Figure. 1A, B). Approximately 5.7 to 9.4% of plants were affected in two fields, 0.4 ha in total, and disease severity in affected plants was 70 to 90%. To confirm the pathogen, six diseased plants were collected. Diseased roots or stems were cut into 2- to 5-mm-long fragments, surface sterilized with 75% ethanol for 1 min and 5% NaOCl for 2 min, rinsed with sterilized water six times (1 min per rinse). Fragments were blotted dry and put onto PDA medium (potato infusion 200 g/L, glucose 20 g/L, agar 18 g/L). After incubation at 28°C for 48 h, white, fluffy mycelium grew from plant fragments. Six morphologically similar strains were transferred to PDA plates by hyphal tip transfer. The edge of the colony was branch-shaped, and the growth rate was extremely fast for 3 days to reach a full dish (Fig. 2A). Hypha produce filamentous or digitated protrusions, which gradually developed into oospores or swollen sporangia (Fig. 2D-F). Each sporangium releases approximately 30~40 zoospores that encyst into cystospores. Zoospores are mostly aplerotic, lemon-shaped, biflagellate, and 14.57~16.48 μm × 10.55~10.84 μm in size (n=30, Fig. 2G-H). Genomic DNA was extracted from representative strain PmZY1 using the CTAB method. The internal transcribed spacer (ITS) and cytochrome oxidase subunit Ⅱ gene (COXⅡ) which are useful barcodes for identification of many oomycetes were amplified with the primer pairs ITS1/ITS4 (Riit et al., 2016) and P-cox2F/P-cox2R (Hudspeth et al., 2000), respectively. A BLAST search identified the sequences as P. myriotylum ITS (PZ374764) and COXⅡ (PZ381141) sequences (e.g., KY019263.1 and PP209157.1, respectively) with 100% coverage and 100% identity (Fig. 3). In order to evaluate pathogenicity, Lablab bean seeds (cultivar: Purple lentils) were planted in 10-cm-diameter plastic pots with a 1:1 mixture of sterilized grass charcoal soil and vermiculite. Sterilized tap water was added to cover the hyphae P. myriotylum which grown on PDA medium for 2 days, followed by further incubation at 28℃ in the dark for another 2 days to induce zoospore release. And then Zoospores were collected following with minor modifications. Using the method of root dipping, lablab bean roots were soaked with 1 ml of oospore suspension at 104 zoospores/mL of the strain; control plants were inoculated with sterilized water (n=20). All plants were kept in a greenhouse (28 ± 2°C, light/dark cycle of 12 h:12 h, >80% RH). The experiment was repeated with three replications. Plants inoculated with P. myriotylum showed brown lesions on the lower stem and growth restriction on 7 days after inoculation, while control plants were symptomless. Two-three weeks after inoculation, the plants showed root tissue necrosis, root rot, and dwarfing symptoms similar to those observed in the field, while control plants were healthy (Figure. 1C, D). P. myriotylum was re-isolated from inoculated plants and the morphology was the same as the original strain (Figure. 2B, C). These results suggest that P. myriotylum is the causal agent of Lablab bean root rot. Previous studies have found that P. myriotylum can cause root rot in Capsicum annuum, Glycine max, Phaseolus vulgaris and green bean. To the best of our knowledge, this is the first report of P. myriotylum causing root rot on Lablab bean of China. This pathogen can infect plant roots and develop rapidly if not effectively controlled. Once the disease breaks out in a large area, Lablab bean yield will be seriously threatened. The results will support the prevention and management of this disease.