A Reassessment of the Etiology of Late Blight Epidemics in the Indian Plains Reveals that Asymptomatic Seed Potatoes Are the Principal Source of Disease Spread

India is the second largest potato producer in the world, with 80% of production on the Gangetic Plains from 3-month winter crops when cooler conditions are optimal for both potato growth and, at times, the concerning late blight disease that threatens food security. During the intercropping summer period, however, the average temperature remains above 35°C, which is unsuitable for the survival of the causal pathogen, <i>Phytophthora infestans.</i> This study investigated the source of potato late blight primary infections. Over 7,000 apparently healthy seed potatoes from a range of sources were planted under field and glasshouse conditions, and careful daily observation and sampling of the subsequent growing crops detected almost 120 primary infections. Overall, 1.6% of asymptomatic seed tubers resulted in late blight emerging as a stem lesion, with higher rates detected on blight-susceptible cultivars than on those more resistant. The rate also varied depending on the seed-production system, with formal systems generating fewer primary infections than seeds produced informally. Genotyping of <i>P. infestans</i> cultures revealed 31 multilocus genotypes of the locally dominant EU_13_A2 clonal lineage, with their tracking confirming overwintering of the pathogen in seed. This study provides clear evidence of seed tuber infection as the main source of primary inoculum driving late blight epidemics in Indian plain potato crops. Weaknesses in the seed-production systems are also flagged, which, if resolved through improved inspection and certification systems, would improve seed health, lower disease incidence, and improve socioeconomic outcomes in rural economies in India and neighboring regions.