Eucalyptus, native to Australia, is widely distributed in India and other parts of the world and is valued for its rapid growth, adaptability and high wood yield (Dasgupta et al. 2020; Yang et al. 2023). In 2025, a severe disease occurred in the leaves of Eucalyptus in Liumo Village, Wuwei Town, Qipo Branch, Qipo Forest Farm, Nanning City in China. More than 200 Eucalyptuses were investigated and it was found that the incidence rate was 50% and the severity of the disease was moderate. The symptoms are polygonal or irregular purple spots on the leaves. In severe cases, the spots are connected in patches. The spots on the back of the leaves are grayish-white. When conditions are suitable, small granular or powdery protrusions, soot, are conidiomata of the pathogen on the back of the spots. Take leaves with typical symptoms, cut them into 5mm squares at the junction of the diseased and healthy parts, disinfect them with 3% sodium hypochlorite for 1 min, then with 75% alcohol for 15 sec, and finally wash them three times with sterile water. The leaf discs were then placed on PDA plates. After five days of incubation at 25℃, the marginal hyphae were extracted with a 7 mm sterile punch and subculture three times to purify the fungal isolate. The strain was initially grayish-white and grew slowly. Later, the color darkened and the back was brown. The hyphae and conidia stalks are brownish green with septa. The conidia are inverted and rod-shaped, with transverse and mediastinal septa on the surface. The transverse septa are relatively thick, 2 to 8, and the mediastinal septa are 2 to 4, and the size is relatively uniform, about 15 to 42 μm x 5 to 15 μm, arranged in long straight or oblique chains. It is preliminarily judged to be a fungus of the genus Cyclospora. To identify the pathogen, the mycelium of the fungus was scraped and the fungal genome was extracted using a fungal genome extraction kit. The fungal DNA was amplified with the ITS1/ITS4（White et al. 1990.） primers and primers for the Alt a1 gene (Hong et al. 2005). The sequences of PCR products were deposited in GenBank (accessions PX735823 and PZ189099). Nucleotide BLAST shows the best match with Alternaria burnsii, with 91% coverage and 99.6% identity for ITS (NR136119), and 94% coverage with 99.79% identify for Alt a1 (KP123967). A Phylogenetic tree was constructed using the neighbor-joining method with MEGAl1 software. Pathogenicity tests were conducted on 3-month-old Eucalyptus trees. The leaves were gently scratched with sterilized blades, and then four 7-mm mycelial discs were placed on top of each leaf and fixed with plastic wrap. In the control, same sized PDA discs were used. The same parts of the leaves of each plant were treated. Each treatment had 10 replicates. The Eucalyptus seedlings were placed in greenhouse at 25±2℃ with a relative humidity of 60%. Five days later, the infected plants showed the same symptoms as the original diseased ones. The strain was re-isolated from diseased leaves of artificially inoculated plants and sequenced as the original strain. The results indicated that A. burnsii was the main pathogen causing Eucalyptus purple spot disease. Previous studies have shown that A. burnsii causes Cumin Blight (Feng et al. 2021) and Bletilla striata leaf spot (Yin et al. 2023). This is the first report of it causing purple spot-on Eucalyptus in China. Identification of the fungus will contribute to the prevention and control of purple spot disease.
Zhang et al. (Sun,) studied this question.