Optimal, lower and upper temperature thresholds for uredospore germination and germ tube growth of Phakopsora pachyrhizi

ABSTRACT In laboratory experiments, the effects of temperatures and exposure times on uredospore germination and germ tube growth of Phakopsora pachyrhizi were measured. The study was conducted in biological oxygen demand (BOD) growth chambers, and the effects of the temperatures (factor ‘a’) 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 31, 32, 33, 34, 35 and 36ºC, combined with the exposure times (factor ‘b’) 3, 6, 12 and 24 hours, were tested in a randomized block design and four replicates. After each exposure time, germination and germ tube length were determined. The experiments were repeated twice and the data underwent regression analysis. The generated equations were used to calculate the lower and upper thermal thresholds and the optimal temperature, which were 6.5ºC, 34.2ºC and 22.3ºC, respectively, for spore germination, and 5.8oC, 34.2oC and 22.2oC, respectively, for germ tube length. The temperature thresholds for spore germination are used to calculate the thermal constant K so that rust occurrence in an area cultivated with soybean can be forecasted based on the accumulated heat.