First Report of <i>Alternaria alternata</i> Causing Target Leaf Spot on <i>Thespesia populnea</i> in India

ABSTRACT Thespesia populnea (L.) Corrêa (Portia tree), known for its numerous ethnopharmacological uses and ecological significance in coastal and tropical forests of India, showed foliar disease in September 2024 in Karnataka and Andhra Pradesh, India. Alternaria alternata was found in all three sampled populations, and pure cultures were successfully obtained from 9 of the 18 trees analysed. Three representative fungal isolates (UHFTP001, UHFTP002 and UHFTP003) were isolated from infected samples using tissue isolation. The A. alternata pathogen was isolated from infected leaves showing target leaf spot symptoms, marked by circular necrotic lesions with purple‐edged margins and prominent concentric rings, especially evident on mature foliage. The isolates were confirmed as Alternaria alternata through their cultural and morphological traits, as well as a multi‐gene phylogenetic analysis with Maximum likelihood analysis using combined sequences of internal transcribed spacer ( ITS ), translation elongation factor 1‐alpha ( EF‐1α ), glyceraldehyde‐3‐phosphate dehydrogenase ( GAPDH ) and Alternaria alternata major allergen ( ALT a 1 ). The pathogenic nature of the isolates was confirmed on twelve T. populnea seedlings by applying Koch's postulates under polyhouse conditions, maintained at 75% shade and 85% relative humidity. Twenty‐eight days after artificial inoculation, characteristic symptoms appeared on the seedlings treated with UHFTP001. This study reports the first Indian report of the fungus Alternaria alternata causing target leaf spot disease on the tree species Thespesia populnea .