Development of Phakopsora pachyrhizi on soybean at controlled temperature, relative humidity and moisture periods

Abstract

The influence of temperature, relative humidity (RH) and moisture period (MP) was studied in relation to infection of soybean plants (Glycine max (L.) Merr.) by Asian Soybean Rust (ASR) caused by Phakopsora pachyrhizi Syd. Disease development was investigated at 15, 19, 21, 24, 26, 28 and 30°C and 6, 9, 12, 14 and 16 h MP at 75–80%, 85–90% and 95–100% RH. Following treatments in an electronically controlled dew chamber, plants were placed in a growth chamber (21-22°C, 80% RH). Number of pustules per lesion (abaxial leaf surface) and lesion size (abaxial and adaxial leaf surfaces) were calculated at 21 days after inoculation. The expression of disease was greatly influenced by post-inoculation temperatures, RH and MP. The highest number of pustules per lesion occurred at 21–24°C at all RH and MP levels tested. However, the number of pustules per lesion was significantly higher at both 85–90% and 95–100% than at 75–80% RH. On the adaxial leaf surface, lesion size only increased significantly at 75–80% RH after a 16 h MP reaching a maximum length at 24°C. At 85–90% and 95-100% RH, lesion size increased significantly after a 14 h MP with the largest lesion size produced at 21°C and 24°C, respectively. On the abaxial leaf surface, lesion size increased significantly after a 16 h MP at 75–80%, whereas at 85-90% and 95–100% RH, an increase in lesion size was found after a 14 h MP and was largest at 21°C after a 16 h MP. Infection did not occur on plants incubated at 15°C and 30°C at 85–90% and 95–100% RH whereas at 75–80% infection did not occur on plants incubated at 15°C, 19°C and 30°C, regardless of MP. No infection occurred with a MP of 6 h at any temperature and RH level tested. The results suggest that ASR is unlikely to occur at high temperatures (>28°C) and short MP (<6 h). However, in soybean production areas of South Africa where moisture is provided by mist, light rain, irrigation or dew, with temperatures of 21–28°C and RH of 75–100%, infection can occur after a 6 h MP. Success of infection is enhanced if plants are exposed to these conditions for 16 h.