Biological traits of anthracnose on Plumeria rubra in Hainan and identification of its pathogen

Plumeria rubra, an important ornamental plant in the Apocynaceae family, is severely impacted by anthracnose disease, which threatening its growth and economic value. In Guangxi, the disease is caused by Colletotrichum gloeosporioides and C. siamense, leading to black spots on the leaves and petals. However, in Hainan, the disease manifests distinct green island symptoms. To explore this difference, pathogen strain JDHTJ2020 was isolated from infected Plumeria and identified as Colletotrichum tropicale based on morphological and multi-gene identification, including the Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH), Chalcone Synthase (CHS), Actin (ACT), and Internal Transcribed Spacer (ITS) genes. Subsequently, pathogenicity was confirmed through Koch’s postulates, and the strain’s growth characteristics were assessed under varying media, light conditions, pH values, and temperatures. The study showed that the optimal growth temperature for Plumeria rubra was 28 °C, the optimal pH was 6, and the optimal light condition was alternating light and darkness, with PDA medium being the most effective. On PDA medium, the colony appeared dark green in the center with dense wool-like aerial hyphae, and the reverse side was dark green in the center with white edges. Conidia were colorless, unicellular, short-cylindrical with rounded ends, containing two oil droplets, and measured 11.71–16.02 μm×4.52–7.17 μm. Based on this, a principal component analysis model was used to evaluate the variables affecting plant growth (light, temperature, pH, and culture medium). The results showed that temperature had the highest principal component loading value (0.91) and contribution rate (0.66), making it the most influential factor in plant growth.