Infection Biology and Sporulation of Phakopsora pachyrhizi Causing Asian Soybean Rust Under Humid Tropical Conditions

Abstract

Asian soybean rust caused by Phakopsora pachyrhizi is one of the most devastating foliar diseases of soybean, particularly under humid tropical conditions where epidemics develop rapidly. The present study investigates the infection biology and sporulation dynamics of P. pachyrhizi under controlled and field-simulated humid environments. Diseased soybean leaves exhibiting characteristic rust pustules were collected and used for microscopic examination of infection structures, including germ tube formation, appressorium development, and uredinia initiation. Results revealed that spore germination was highly dependent on prolonged leaf wetness periods and elevated relative humidity. Infection initiated within a few hours of spore deposition under optimal humidity, followed by rapid penetration through stomatal openings and colonization of leaf mesophyll tissues. Uredinia development was observed as early as 5–7 days post-inoculation under favorable conditions. Sporulation intensity was significantly enhanced under continuous high humidity and moderate temperature regimes, leading to dense urediniospore production and secondary infection cycles. Variations in microclimatic conditions influenced both infection efficiency and sporulation rates, with intermittent drying periods reducing disease severity. Histological observations confirmed extensive intercellular mycelial growth and host tissue degeneration during advanced infection stages. The study highlights the critical role of humidity in governing the infection cycle and epidemic development of P. pachyrhizi. These findings emphasize the importance of microclimate management and timely fungicidal interventions in soybean rust management programs. Overall, the results provide detailed insights into the infection biology and sporulation behavior of P. pachyrhizi under humid tropical conditions and contribute to improved understanding of disease epidemiology in soybean production systems.