Optimal environmental conditions for infection and development of Puccinia purpurea on sorghum

Although rust can reduce grain yields of late-planted sorghum crops in Queensland, little research has been conducted on environmental parameters affecting infection and development. The effects of temperature, leaf wetness period, plant growth stage, urediniospore concentration and darkness period on the development of rust (Puccinia purpurea) on the inbred Sorghum bicolor line IS8525 were evaluated separately by artificial inoculation of plants under controlled conditions. Rust developed between 16 and 28 °C, with the optimum temperature being 20 °C. Disease severity (pustules cm−2) increased as the length of leaf wetness increased from 4 to 24 h. Infection occurred when plants were exposed to full light, full darkness and varying periods of darkness for the first 24 h after inoculation; 16 h of darkness resulted in the highest rust severity. Plants of IS8525 and of the more resistant line IS12539 inoculated 21–49 days after sowing developed higher levels of rust than others inoculated at, or close to, flowering (63 days after sowing). Rust severity also increased with increasing urediniospore concentrations, but leaf death occurred on young plants inoculated with the highest concentration (50 mg 100 mL water−1). The findings of this study have been used to develop an inoculation technique to detect putative pathotypes of P. purpurea in Australia.