First report of basal stem rot caused by Geotrichum candidum on Aloe vera in China

Aloe vera is a perennial evergreen herb belonging to Liliaceae Aloe, which is widely used in beauty, food, healthcare, and medicine (Kumar et al. 2019) and extensively cultivated worldwide due to its enormous commercial value. However, a stem rot disease severely restricts the development of A.vera cultivation in China (Ji et al. 2007). In 2023, the average disease incidence of A.vera Mill. with basal stem rot in Dajie Town, Daming County, Handan City, Hebei Province, was 20％. The disease initially appeared as translucent, water-soaked patches on the epidermis of basal stems. These lesions progressed from light brown to dark brown, with vascular bundles turning brown. Internal tissues softened into a soggy decay and emitted a foul odor. Nine diseased plants showing representative symptoms were collected from three greenhouses. The pathogen was isolated from diseased stems using a tissue separation method. The stem tissues were surface sterilized with 75% ethanol for 30 s, then rinsed three times with sterile distilled water, dried, transferred to potato dextrose agar (PDA) supplemented with 100 μg/ml of streptomycin sulfate, and incubated at 25℃ for 2 days. A total of six isolates with similar morphology on PDA were consistently isolated, and purified by single-spore isolation (Qiu et al. 2011). Colonies showed round, flat surfaces, white downy or powdery mycelium, clear on the reverse side. Hyphae were hyaline, septate and branched microscopically. Arthrospores were hyaline, cylindrical or oval-shaped, ranging in size from 2.5 to 7.2 μm × 0.1 to 6.2 μm (n=50). These morphological features matched those of Geotrichum candidum (Kai et al. 2021). To further confirm the identification, the internal transcribed spacer (ITS) and translation elongation factor 1 -alpha (TEF1-α) regions were amplified and sequenced from two randomly selected isolates (BAI, BAI2) using the primers ITS1/ITS4 (White et al. 1990), EF1-1018/EF1-1620R (Stielow et al. 2015). BLAST analysis revealed that the ITS sequences (PV018630, PV168341) had 100% identity to that of G. candidum and TEF1-α sequences (PV469904, PV469905) had 100% and 99% identity to that of G. candidum. A neighbor-joining phylogenetic tree based on ITS and TEF1-α indicated that BAI and BAI2 clustered with G. candidum (Saitou and Nei, 1987). BAI and BAI2 were deposited in the Life Sciences Experimental and Practical Training Center. For pathogenicity testing, healthy 1.5-year-old A.vera Mill. plants were surface disinfected with 75% ethanol, and then two 2-mm-depth wounds were punctured on each basal stem using a sterilized toothpick. Ten plants were inoculated with 10 μl of a conidial suspension (5 × 10 5 conidia per ml) obtained from 7-day-old PDA cultures and eluted with sterile distilled water (Ding et al. 2015). Ten control plants were inoculated with 10 μl of sterile distilled water. All plants were incubated at 25℃ under a 12/12-h light/dark cycle. After 10 days, all plants inoculated with BAI and BAI2 developed basal stem rot symptoms that were similar to those observed in the greenhouses. While control plants remained healthy, G. candidum was reisolated from the inoculated stems and confirmed by morphological and molecular analysis, fulfilling Koch’s postulates. Tests were repeated with consistent results. To the best of our knowledge, this is the first report of G. candidum causing basal stem rot on A. vera in China. These findings will aid the cultivation of this economically important crop by enabling early diagnosis and targeted control strategies.