First Report of <i>Pseudocercospora norchiensis</i> Causing Leaf Blight on <i>Persea americana</i> in China

Avocado (Persea americana) is a highly nutritious fruit. Menglian County in Yunnan province has emerged as the largest avocado cultivation in China. With a planted area of 8,051 hectare, it plays a pivotal role in driving the local economy. Leaf diseases can directly damage leaves, thereby impairing photosynthesis and nutrient uptake and reducing the quality of avocado. In November 2024, leaf blight were observed on 4-year-old avocado trees in Menglian Dai and Lahu Autonomous County, Pu'er City, Yunnan Province (22°19′43.6″N, 99°35′30.2″E), with disease incidence rate of 30% in the field. Small brown spots appeared on the diseased leaves, then gradually expanded to form round or irregularly shaped lesions. The lesions have dark brown edges and a grey center. Under a microscope, 10 conidia were isolated and transferred onto potato dextrose agar (PDA) medium by the single-spore separation method. Colonies exhibited extremely slow growth on PDA plates at 25℃, nearly 60 mm diameter after 35 days. The colonies were erumpent, short villous, and irregularly shaped. The swfu15 surface of the colonies was pale olive gray, while the reverse side was iron gray with an irregular texture. Conidiophores arranged in moderately dense fascicles, arising from stromata, through stomata or erumpent through the cuticle, brown, smooth, unbranched, straight to curved, subcylindrical, regular in wide, 95.5- 149.3× 8.2- 13.5 μm (n=50). Conidia solitary, pale brown, subcylindrical to obclavate, apex subobtuse to subacute, base obconically truncate, straight to gently curved, 1- 8 septate, 62.2- 98.1× 3.05- 4.3 μm (n=50). The morphological features were consistent with those of Pseudocercospora norchiensis. The internal transcribed spacer regions (ITS), translation elongation factor 1-alpha (TEF1-α), and the partial actin (ACT) were amplified from DNA (swfu15, swfu16) with the primers ITS1/ITS4, EFl-728F/EFl-986R, and ACT512F/ACT783R. All sequences were deposited in GenBank (ITS, PV052387-PV052388; TEF1-α, PV085847– PV085848; ACT, PV085845– PV085846). BLAST analysis of the obtained sequences showed homology (100.00% (492/492 bases), 94.74% (307/323 bases), and 99.55% (221/222 bases)) with P. norchiensis (EU853474 (UY1436, Acca sellowiana), PP404879 (CBS:118413, Rubus sp.), and GU320455 (CBS:120738, Eucalyptus sp.)). Phylogenetic analysis of multiple genes was conducted with the maximum likelihood method using MEGA7. Based on the phylogenetic and morphological analyses, the strain was identified as P. norchiensis. For the pathogenicity test, the strain swfu15 spore suspension (1 × 106 spores/mL) was dropped onto twenty leaves of 2-year-old seedlings, while twenty control leaves were inoculated with ddH2O for 20 days at 25°C with 70% relative humidity and under natural light. All inoculated leaves exhibited symptoms consistent with those observed in the field, whereas the control leaves remained symptom-free P. norchiensis (strain swfu16) was reisolated from the lesions on inoculated plants but not from the control plants. This is the first report of Pseudocercospora norchiensis causing leaf blight on Persea americana in China. This fungus currently infects mainly Myrtaceae and Lauraceae plants. This work provides crucial information for epidemiological studies and the development of appropriate control strategies for the Chinese avocado industry.