Elucidating Etiological Complexity and Predicting Disease Predisposition Factors of Alternaria Leaf Blight in Groundnut

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Alternaria leaf blight (ALB) has emerged as a serious foliar disease limiting groundnut productivity in the semi-arid region of North Gujarat, India. The present investigation was conducted for three consecutive kharif seasons (2016–2018) to elucidate pathogen diversity and epidemiological factors governing disease development under natural epiphytotic conditions. Symptomatic leaf samples were collected and the causal organisms were isolated, purified, and identified through morphological and molecular characterization. Two species, Alternaria alternata and A. tenuissima, were confirmed based on cultural traits, conidial morphology, and a multi-locus sequence typing (MLST)-based phylogenetic tree constructed using concatenate sequence of the internal transcribed spacer (ITS) region of 18s ribosomal RNA (18S rRNA) gene, partial translation elongation factor 1-alpha (TEF-1α), and RNA polymerase II second largest subunit (RPB2) gene. Epidemiological studies revealed that disease initiation occurred 30–35 days after sowing, followed by rapid progression up to 55–60 days. Correlation and regression analyses indicated that maximum temperature (31–34 °C) and sunshine hours showed a positive association with disease intensity, whereas minimum temperature, relative humidity, rainfall, and number of rainy days exhibited variable associations across seasons. Suitable weather parameters including 31–34 °C maximum temperature, 21–25.5 °C minimum temperature, 85–95% maximum relative humidity, 65–75% minimum relative humidity, and 50–100 mm weekly rainfall favor severe infection of groundnut with Alternaria. The apparent infection rate (r) was highest during the early crop growth stages (>0.1) and declined gradually thereafter. The findings provide a region-specific epidemiological framework for timely intervention and sustainable management of Alternaria leaf blight in groundnut under semi-arid agro-climatic conditions.