Effect of Elevated Temperature on the Epidemiology of Phytophthora Root Rot of Cowpea in South West Nigeria Under Projected Climate Change Scenarios

Phytophthora root rot of cowpea, caused by Phytophthora vignae, is a significant disease constraint in humid cowpea production systems. Climate change projections indicate mean temperature increases of 1.5 to 2.5 degrees Celsius for South West Nigeria by 2050, with uncertain implications for Phytophthora disease epidemiology. This study empirically tested the effects of elevated temperature on P. vignae growth, sporangium production, infection efficiency, and disease progress rate in controlled environment experiments using temperature regimes representing current ambient (27 degrees Celsius) and projected future conditions (30 and 33 degrees Celsius). Additionally, a field study in Ogun State examined the interaction between natural seasonal temperature variation and root rot epidemics in 40 cowpea farms over three seasons. In vitro experiments showed that P. vignae mycelial growth peaked at 30 degrees Celsius and declined at 33 degrees Celsius, while sporangium production and germination were significantly reduced at 33 degrees Celsius. Controlled inoculation studies at 30 degrees Celsius produced 23 percent higher disease incidence compared to 27 degrees Celsius, while 33 degrees Celsius reduced incidence relative to 30 degrees Celsius. Field data confirmed that seasons with mean temperatures between 29 and 31 degrees Celsius during the vegetative stage had 31 percent higher root rot incidence than cooler seasons. These empirical relationships suggest that moderate warming below 32 degrees Celsius will increase Phytophthora root rot risk in South West Nigerian cowpea systems, while extreme heat may partially self-limit epidemic development. Keywords: Phytophthora vignae, cowpea root rot, climate change, temperature, epidemiology.